By Vince Giuliano and Melody Winnig
This is the third blog entry related to the Xenohormetic live food hypothesis. In the first blog entry – Health through stressing fruits and vegetables – we laid out the basic hypothesis, cited a variety of supportive evidence for it, and discussed some of its widespread practical implications. The hypothesis is based on the fact that the cells in most fruits and vegetables stay alive when uncooked right up to the point of their digestion in human guts. As such, these cells retain their abilities to respond to stresses, and judicious utilization of such stress responses can be used to enhance the beneficial polyphenol content of consumed fruits and vegetables as well as beneficial plant-based mRNAs that can support our health. For example, the stress associated with cooking blueberries or tomatoes significantly multiplies their content of beneficial phytochemicals, anthocyanins or lycopene.
The second blog entry More on the xenohormetic live food hypothesis More on the xenohormetic live food hypothesia –- discusses synergies among polyphenols when consumed by humans, additional post-harvest plant stressors and stress responses, plant ethylene synthesis, root hypoxia, the Yang Cycle, plant polyphenol transcription factors, and other selected aspects of plant stressing and stress responses.
This third blog entry goes on to revisit xenohormesis in a little more theoretical depth and additional topics such as pre-harvest plant stressing, and how pre or post-harvest stressing may be used to induce additional benefits such as crop mold resistance and reduced food spoilage. We cite a number of other findings from the literature related to stressing plants. We describe a simple and practical approach that ordinary consumers could use to further preserve and enhance nutritional values of fruits and vegetables while they sit in the refrigerator.
While we initially thought the concept of post-harvest stressing to enhance nutritional benefits was original to us, we have come to realize that many of the concepts we put forward are already familiar to a number of researchers, and that there is a growing body of studies related to them. Still, we believe the central concept remains under-appreciated and could be of great practical importance if it were widely applied in agriculture, in the food industries, and in the chain of food delivery.
More on xenohormesis — inducible plant stress defenses and human-beneficial plant polyphenols
- A. inducible phytochemical plant stress defenses
Plants have in the course of evolution developed and amazing number of chemical as well as other mechanisms to protect themselves against the stresses they may be expected to encounter, be these draught, excessive heat or cold, UV exposure, fungi, hypoxia, bacteria or predatory insects or animals. For example, plants can produce a number of toxins to protect themselves against predators who might otherwise eat them. Those of us who live in the midst of Poison ivy plants know this too well, in this case the toxin being urushiol. And cattle growers fear toxic plants that contain prussic acid like sorghum and sorghum-sudangrass crosses(ref).
Predators and pathogens have adopted a numbers of clever strategies for attacking plants. “Many plant pathogens act like “silent thieves” who want to steal money locked inside of a bank vault. These thieves use specialized tools designed to disable the bank’s security system and unlock the vault without being detected. In a similar way, many pathogens establish intimate connections with their hosts in order to suppress plant defenses and promote the release of nutrients(ref).” And plants have likewise adapted a number of sophisticated strategies for fighting back. The life-survival dance between plants and attacking animals or pathogens is an elaborate and fascinating one, though not a central theme of this article. We are concerned here particularly with stress-inducible phytochemicals, which constitute a very important general plant approach to defense. “ — virtually all living plant cells have the ability to detect invading pathogens and respond with inducible defenses including the production of toxic chemicals, pathogen-degrading enzymes, and deliberate cell suicide. Plants often wait until pathogens are detected before producing toxic chemicals or defense-related proteins because of the high energy costs and nutrient requirements associated with their production and maintenance(ref).” Plant stress responses are also inducible by a host of other causes –virtually every stress the plant can be expected to encounter consistent with the general theory of multifactorial hormesis(ref). There are significant bodies of research literature related to inducible plant stress chemical defenses, some related to pathogens(ref), some related to plant predators and parasites(ref), and some related to the many other stresses plants endure and respond to in nature. As we write this in mid-October in New England, the leaves in the trees surrounding our house have turned into a dazzling array of colors. This is due to leaf phytochemical responses to the changing light and temperature stresses of the coming winter. The leaves are starting to die, and soon they will wither and fall off to protect the tree until Spring.
- B. Xenohormetic phytochemicals
Our specific focus here is with stress-inducible plant phytochemicals that can also induce beneficial stress-responses in humans, e,g,, those that satisfy the criteria for xenohormesis(ref)(ref)(ref). We have come back again to xenohormesis because our specific focus here is on stressing fruits and vegetables to accelerate the production of such phytochemicals before they are digested by humans. We are concerned centrally with making the foods we eat healthier.
Image source, the publication Xenohormesis: Sensing the Chemical Cues of Other Species
“Figure 2. The Xenohormesis Hypothesis We propose that the common ancestor of plants and animals synthesized polyphenols. Since the divergence of phyla, there has been selection such that heterotrophs (animals and fungi) detect chemical cues about their environment from plants and other autotrophs (that is, organisms that derive energy from light or inorganic chemical reactions). These chemical cues would give the heterotroph advance warning about the deterioration of the environment, allowing them to prepare while conditions are still relatively favorable. The theory predicts that many key mammalian enzymes and receptors will have evolved binding pockets that allow modulation by molecules produced by other species.”
Xenohormesis as related to sirtuins.
The 2011 publication Xenohormesis mechanisms underlying chemopreventive effects of some dietary phytochemicals summarizes the situation: “A wide variety of phytochemicals present in our diet, including fruits, vegetables, and spices, have been shown to possess a broad range of health-beneficial properties. The cytoprotective and restorative effects of dietary phytochemicals are likely to result from the modulation of several distinct cellular signal transduction pathways. Many dietary phytochemicals that are synthesized as secondary metabolites function as toxins, that is, “phytoalexins,” and hence protect plants against insects and other damaging organisms and stresses. However, at the relatively low doses consumed by humans and other mammals, these same toxic plant-derived chemicals, as mild stressors, activate adaptive cellular response signaling, conferring stress resistance and other health benefits. This phenomenon has been referred to as xenohormesis. This review highlights the xenohormesis mechanisms underlying chemopreventive effects of some dietary chemopreventive phytochemicals, with special focus on the nuclear transcription factor erythroid 2p45 (NF-E2)-related factor 2 (Nrf2) as a key player.” We also discussed xenohormetic roles of certain phytochemicals as related to Nrf2 expression in the blog entry The pivotal role of Nrf2. Part 2 – foods, phyto-substances and other substances that turn on Nrf2.
There are many examples of stresses leading to leading to upregulated production of xenohormetic phytochemicals. This observation applies both to naturally occurring stresses and stresses deliberately induced by humans.
Predator grazing is one of many naturally occurring stresses
Again, examples are many. To pick one, the 1988 publication Herbivore Grazing Increases Polyphenolic Defenses in the Intertidal Brown Alga Fucus Distichus reported “Although predator—induced defenses have been reported for several species of terrestrial vascular plants, they have not been previously described in aquatic or nonvascular plants. In this study, field manipulations were used to demonstrate the presence of inducible chemical defense production in the intertidal brown alga Fucus distichus. When experimentally damaged, Fucus increased its concentrations of polyphenolic compounds by °20% over uninjured control plants within 2 wk. These increases occurred in the area where the plant was injured and within adjacent undamaged branches. The increase in concentrations of polyphenolic compounds in clipped plants in these experiments corresponded well with differences in phenolic levels in naturally grazed and ungrazed algae. Herbivorous snails (Littorina sitkana) showed a preference for clipped plants immediately after they were wounded. However, over a 2—wk period the snails shifted their preference towards the uninjured control plants, corresponding with the increase in polyphenolic levels within the experimentally damaged plants. L. sitkana spent less time feeding on clipped plants, and these plants lost °50% less tissue (by surface area) to grazers than did uninjured algae. The presence of induced defenses in algae causes plant quality to vary spatially and temporally. This may result in variation in intra— and interspecific food preferences of herbivores, and ultimately may affect benthic algal community.”
Another publication relating plant phytosubstance production to predator stress is INDUCIBLE CHEMICAL RESISTANCE TO HERBIVORY IN THE BROWN SEAWEED ASCOPHYLLUM NODOSUM
A large number of human-inducible stress interventions are known to increase plant polyphenol content. Examples include:
Hypoxic and hyperbaric stress on lettuce
The 2013 publication Hypobaria and hypoxia affects phytochemical production, gas exchange, and growth of lettuce relates: “Hypobaria (low total atmospheric pressure) is essential in sustainable, energy-efficient plant production systems for long-term space exploration and human habitation on the Moon and Mars. There are also critical engineering, safety, and materials handling advantages of growing plants under hypobaria, including reduced atmospheric leakage from extraterrestrial base environments. The potential for producing crops under hypobaria and manipulating hypoxia (low oxygen stress) to increase health-promoting bioactive compounds is not well characterized. Here we showed that hypobaric-grown lettuce plants (25 kPa ≈ 25% of normal pressure) exposed to hypoxia (6 kPa pO2 ≈ 29% of normal pO2) during the final 3 d of the production cycle had enhanced antioxidant activity, increased synthesis of anthocyananins, phenolics, and carotenoids without reduction of photosynthesis or plant biomass. Net photosynthetic rate (P N) was not affected by total pressure. However, 10 d of hypoxia reduced P N, dark respiration rate (R D), P N/R D ratio, and plant biomass. Growing plants under hypobaria and manipulating hypoxia during crop production to enhance health-promoting bioactive compounds is important for the health and well-being of astronauts exposed to space radiation and other stresses during long-term habitation.”
Microbial polysaccharide stress on mung beans
The 2002 publication A BIOCHEMICAL ANALYSIS OF MUNGBEAN (VIGNA RADIATA) RESPONSE TO MICROBIAL POLYSACCHARIDES AND POTENTIAL PHENOLIC-ENHANCING EFFECTS FOR NUTRACEUTICAL APPLICATIONS reported: “Foods that contain plant phenolic secondary metabolites that are antioxidants are getting more attention due to an increase in experimental data suggesting health-promoting effects when such foods are ingested as part of a low-fat diet. As the synthesis of phenolic compounds in plants is known to increase during the defense response to microorganisms, we investigated specific food-grade microbial polysaccharides as potential elicitors of mungbean phenolic content. Mungbean (Vigna radiata) was pretreated with solutions of both xanthan and ge STREllan gums, as well as commercial yeast extract and purified yeast glucan (as potential fungal elicitors), and dark-germinated for 1–5 days. Tissue samples were assayed for enzymatic (glucose-6-phosphate dehydrogenase and guaiacol peroxidase) and antioxidant activity, and for proline and phenolic content. Microbial polysaccharide treatments were found to stimulate phenolic content and enzyme activity, as well as occasional cotyledon pigmentation. In addition, xanthan gum, yeast extract, and purified yeast glucan treatments stimulated antioxidant activity. Possible mechanisms linking acid-induced plant growth to growth induced by food-grade microorganisms (and related polysaccharides), such as yeasts and acid-producing bacteria, are hypothesized and discussed.”
Microwave stress on fava bean sprouts
The 1991 publication Microwave-induced stimulation of l-DOPA, phenolics and antioxidant activity in fava bean (Vicia faba) for Parkinson’s diet reported: “Fava bean sprouts are rich in levo-dihydroxy phenylalanine (l-DOPA), the precursor of dopamine, and are being investigated for use in the management of Parkinson’s disease. The phytopharmaceutical value was improved during germination by a microwave treatment of the seeds, the phenolic content of the germinated sprouts increasing 700% and l-DOPA content by 59% compared to control. A higher antioxidant activity that was observed correlated with total phenolics and l-DOPA contents. The glucose-6-phosphate dehydrogenase activity peaked on the seventh day of germination with a concurrent increase in phenolics indicating enhanced mobilization of carbohydrates. A higher guaiacol peroxidase activity was observed indicating an increased polymerization of phenolics. The elevated superoxide dismutase activity was proportional to the stimulation of antioxidant activity. The major implication from this study is that microwave treatment can significantly stimulate the phenolic antioxidant activity and Parkinson’s relevant l-DOPA content of fava beans sprouts.”
UV light stress on germinating fava beans
The 2002 publication Effects of UV treatment on the proline-linked pentose phosphate pathway for phenolics and L-DOPA synthesis in dark germinated Vicia faba reports: “Fava beans (Vicia faba) have a high phenolic content. Antioxidant properties of phenolics are extensively studied for their ability to inhibit lipid oxidation in food and for improved health. This legume has the potential to be an effective dietary source of polyphenolics for health. The rationale behind this study was to observe the stimulation of nutraceutically relevant phenolic metabolites when germinating fava beans are exposed to ultra violet (UV) and the potential role of the proline-linked pentose phosphate pathway (PPP). Studies have shown phenolic phytochemicals can be stimulated with UV light. The objective was to over-express the PPP through the regulation of the key enzyme, glucose-6-phosphate dehydrogenase (G6PDH), for enhanced phenolics and L-DOPA production and investigate the link to proline metabolism. Guaiacol peroxidase (GPX) activity was monitored to understand the extent to which the UV-stimulated phenolics could be converted to polymeric phenols. L-DOPA is the precursor of dopamine (DA), the neuromodulator and used for treatment of Parkinson’s disease. Seeds of fava beans were pretreated in water, exposed to 5, 10 and 15 h UV light and germinated for 8 days in dark. Total phenolics, proline levels and enzyme activities were measured spectrophotometrically. L-DOPA was measured by High-performance liquid chromatography (HPLC). A total of 15 h UV treatment stimulated L-DOPA on day 1, which reduced over next 8 days of germination. Antioxidant activity of the total phenolic acids was determined by inhibition of the β-carotene oxidation and there was stimulation of antioxidant activity in response to 10 h UV treatment on day 2. Total phenolic content increased 10-fold after 15 h UV light exposure on day 6 along with the highest activity of G6PDH. All treatments stimulated GPX activity indicating polymerization of free phenolics in the late stage of germination. Proline levels were also stimulated in response to all treatments. The stimulation of proline concurrent with stimulation of G6PDH and total phenolics in response to 15 h UV indicates a role of proline-linked PPP. Results indicate an increase in nutraceutically-relevant phenolic metabolites along with enhanced antioxidant activity in response to UV stress. This novel approach provides a mechanism to understand and enhance biosynthesis of important phenolic compounds in plant and legume systems for use in nutraceutical applications.”
Salt stress on tomatoes
The 2006 publication Controlled Environments for Production of Value-added Food Crops with High Phytochemical Concentrations: Lycopene in Tomato as an Example reports “– Our research at the University of Arizona Controlled Environment Agriculture Program has shown that application of moderate salt stress to tomato plants can enhance lycopene and potentially other antioxidant concentrations in fruit. The increase in lycopene in response to salt stress in the tomato fruit was shown to be cultivar specific, varying from 34% to 85%. Although the specific biological mechanisms involved in increasing fruit lycopene deposition has not been clearly elucidated, evidence suggests that increasing antioxidant concentrations is a primary physiological response of the plant to the salt stress. Another experiment showed that low temperature during postharvest increased antioxidant capacity of tomato fruit while it maintained the lycopene concentration. More detailed study in this area is needed including accumulation of antioxidant phytochemicals as affected by environmental conditions during the cultivation and the postharvest.”
UV stress on snow algae
UV induces a hormetic stress response in a wide variety of terrestrial as well as marine plant organisms. An example is snow algae. The 1999 publication Phenolic compounds and antioxidant properties in the snow alga Chlamydomonas nivalis after exposure to UV light reports: “The snow alga Chlamydomonas nivalis was collected from the Sierra Nevada, California, USA, and examined for its ability to produce phenolic compounds, free proline, and provide antioxidant protection factor in response to UV-A and UV-C light. Exposure of C. nivalis cells to UV-A light (365nm) for 5 days resulted in a 5–12% increase in total phenolics, where as exposure to UV-C light (254 nm) resulted in a 12–24% increase in phenolics after 7 days of exposure. Free proline was not affected by UV-A, but increased markedly after UV-C exposure. A three-fold increase in free proline occurred within two days after exposure to UV-C, but then dropped as cells became bleached. Antioxidant protection factor (PF) increased after treatment of cells with UV-A and remained constant throughout UV-C exposure. Spectral analysis of algal extracts revealed a decrease in absorption in the 215–225 nm region, short-term (2day) stimulation of pigment at 280 nm, and an increase in carotenoids (473 nm), after exposure to UV-A. Snow alga exposed to UV-C light had a different spectrum from that of UV-A exposed cells, i.e. an enhancement of three major peaks at 220, 260, and 280 nm, and loss of absorption in the carotenoid region.We report that UV light exposure, especially in the UV-C range, can stimulate phenolic-antioxidant production in aplanospores of C. nivalis effecting biochemical pathways related to proline metabolism.” While not to many of our readers may be into eating snow algae, this is another example of how many diverse species have the upgraded polyphenol production stress response
Stressing marijuana plants to increase THC content
A Google search on “stressing plants before harvest” produces a large number of citations. Almost all of these are short items and blog comments related to amateur marijuana growing. One thing can be said about marijuana growers – they have probably included some very creative people. THC (Tetrahydrocannabinol) is the primary psychoactive active ingredient that is responsible for getting smokers high. This phytochemical is produced by the plant in response to stress, probably primarily stress due to predators eating the plants. Therefore, marijuana growers have turned to stressing marijuana plants to increase THC content and thus the value of their dried leafy product. While there appears to be widespread agreement that plant THC production is enhanced by pre-harvest stress, there seems to be little agreement among grower gurus as to what the best approach is. Approaches to such pre-harvest stress appear to be many including cutting or pinching the leaves before harvest, twisting stems, dehydrating the plants, flushing the soil to get rid of nutrients or adding substances like molasses. Again, the citations appear to be addressed to amateur growers with unspecified legal status. We have not seen any treatment of the subject in the scientific literature. One comment we found among the marijuana grower drivel to be humorous was that it is not clear why THC protects the plants against predatory animals who eat them. Does an animal stop eating the leaves of a marijuana plant and walk away because the leaves taste bad? Or is it because the THC causes the animal to forget what it is doing and wander off?
Image source Some xenohormetic phytochemicals
Besides being useful to increase phytochemical and nutritional value, stressing fruits and vegetables can be used as a strategy for preservation and delay of spoilage and prevention of mold.
For example, the 2012 publication Postharvest Stress Treatments in Fruits and Vegetables relates: “Fresh fruits and vegetables are living tissues subject to continuous changes after harvest. While some changes are desirable, most are not. Their commodities are perishable products with active metabolism during the postharvest period. Proper postharvest handling plays an important role in increasing food availability. Postharvest stress treatments have been shown to be generally effective in controlling both insect and fungal pests, reducing physiological disorder or decay, delaying ripening and senescence, and maintaining storage quality in fruits and vegetables. In addition, a moderate stress not only induces the resistance to this kind of severe stress, but also can improve tolerance to other stresses. Postharvest stress treatments can, therefore, be very important to improving shelf life and quality retention during postharvest handling of fruits and vegetables.” It appears from the literature that pretty much the same stresses are good for both food preservation and enhancement of beneficial phytochemical content.
From both theoretical and practical viewpoints, a good stressor to consider is UV-B stress.
Above, we discussed the impact of UV irradiation on fava beans and snow algae. From the theoretical viewpoint, a good research publication related to plant stress and UV-B radiation is When does the stressor cause stress? “In the case of UV-B exposure, key components of the acclimation response are the increased capability of photorepair and the accumulation of UV-B absorbing flavonoids and other phenolics. These pigments have long been thought to accumulate mostly in the vacuoles of epidermal cells and to protect underlying tissues by absorbing UV-B photons. More recently, it has been argued that the main protective role of these phenolics is associated with their antioxidative capabilities (Agati and Tattini, 2010), and this fits the observation that flavonoids can be found in tissues not directly exposed to UV-B and also in sub-cellular domains as far apart as chloroplasts, vacuoles and nuclei, and roots and leaves. The UV-B induced increase in antioxidative defenses is further demonstrated by increases in both the reduction state and pool-size for antioxidants such as ascorbate, glutathione, xanthophylls, and tocopherol (Jansen et al., 2008). — Moreover, numerous studies have reported upregulation of enzymatic antioxidant activities, including Cu or Zn superoxide dismutase (SOD), ascorbate peroxidase (APX), dehydroascorbate reductase (DHR), glutathione peroxidase (GPX), glutathione reductase (GR) and catalase activities (Hideg et al., 2006; Agrawal and Rathore, 2007; Xu et al., 2008). In this issue, Pessoa (2012) further highlights a range of UV-induced biochemical protection responses in algae and aquatic macrophytes. Interestingly, UV-protection appears to be largely dependent on physiological UV acclimation. Few studies have reported evidence for UV-B driven genetic adaptation. In this issue Biswas and Jansen (2012) report that adaptation of local Arabidopsis thaliana accessions comprises the altered regulation of UV acclimation, thus again emphasize the relative importance of induced, physiological processes for UV-B protection. — UV-B as an exploitable regulator in horticulture: UV-B can induce a range of specific plant responses, some of which are particularly desirable from a horticultural perspective. For example, the potential to increase the content of specific phenolic, terpenoid and alkaloid compounds metabolites with nutraceutical or pharmaceutical value, is recognized as a useful tool for commercial plant manipulation (Jansen et al., 2008; Zhang and Bjorn 2009; Schreiner et al., 2012). UV-B can also increase development of colour in, for example, salad leaves (Park et al., 2007) or fruits (Dong et al., 1995), and control plant disease-tolerance and morphology (Wargent et al., 2006). In this issue, Jug and Rusjan (2012) describe several positive effects of UV-B radiation on grapevine biochemistry and physiology, while Ribeiro et al. (2012) review the use of postharvest UV-B applications. Some of the reported UV-B responses are known to be mediated by a dedicated UV-B photoreceptor, UVR8, which operates under low UV-B levels (Jenkins 2009; Heijde and Ulm, 2012). Here, Krasylenko et al. (2012) report on the possible involvement of cytoskeleton components in further downstream signaling. Exploitation of the specific, low UV-B effects requires precision manipulation (wavelength selective cladding materials, UV-reflective mulches and/or supplemental UV-B light systems in pre- or post-harvest settings) whereby general, oxidative stress must be avoided. Clearly, a solid understanding of physiological and environmental conditions that cause UV-B stress is required in order to establish a (stress-free) window-of opportunity for horticultural exploitation.”
From a practical viewpoint, a simple tool for implementing UV stress on fruits and vegetables could be a LED light in a fruit/vegetable corner of a refrigerator.
We have long believed that health and longevity does not necessarily require rocket science or big-pharma interventions. Strawberries provide a case in point, as exemplified in the 2013 Science Daily story Behold the 9-Day Fresh Strawberry: New Approach to Slowing Rot Doubles Berry Shelf Life: “Strawberry lovers rejoice: the days of unpacking your luscious berries from the refrigerator only to find them sprouting wispy goatees of mold may be numbered. A research team from the U.S. Department of Agriculture’s (USDA) Food Components and Health Laboratory in Beltsville, Md., and Sensor Electronic Technology, Inc. (SETi) in Columbia, S.C., has demonstrated that low irradiance ultra-violet (UV) light directed at strawberries over long exposure periods at low temperature and very high humidity — typical home refrigerator conditions — delays spoilage. The team used a novel device incorporating light-emitting diodes (LEDs) that emit UV at wavelengths found in sunlight transmitted through Earth’s atmosphere. — The results, which will be presented next week at the Conference on Lasers and Electro-Optics (CLEO: 2013), are significant because previous attempts using traditional UV light sources for storage of produce resulted in severe drying, and it was unknown if the advantages of long exposure to low-level UV light would be effective against rot. — LEDs are now commonplace thanks to their long life and energy efficiency, as well as their ability to span the wavelength range from near UV to infrared. The full UV spectrum, however, had presented challenges for LED manufacturers — until recently. SETi developed a special technology to fabricate UV LEDs across the entire UV spectrum from UVA to UVC. This flexibility allowed them to tune the emitted light to the wavelengths most effective for this application. — “UV-LEDs presented the opportunity to try low power devices that work well in the cold and can be engineered to work in small spaces such as refrigerator compartments,” says lead USDA researcher Steven Britz, who will present the work at CLEO: 2013. — Using strawberries purchased from a local supermarket, Britz’s team placed one batch in a dark refrigerator and one batch in a refrigerator exposed to UV-LEDs. Results showed the UV-treated berries had their shelf life extended twofold — nine days mold-free — over darkened berries, as judged by weight, moisture content, concentration of select phytochemicals, visible damage, and mold growth. Based on these encouraging results, the team is working to commercialize the technology for home refrigerators.”
The UV LEDs in your refrigerator drawer may be useful for preserving and enhancing the nutritional values of many more fruits and vegetables in addition to strawberries. Most plants respond to UV stress.
For example, Carrots. See the 2012 publication Fresh-cut carrot (cv. Nantes) quality as affected by abiotic stress(heat shock and UV-C irradiation) pre-treatments. Further, we strongly suggest that the UV diodes in your regrigerator be activated by a timer so the light is only on during normal daylight hours. In our first blog entry, we describe how cultivated fruits and vegetables still respond to their circadian rhthyms and make polyphenols better when the light stress is synchronized to the expected light cycle. See the discussion under the heading Light exposure is another stress that can increase phytosubstance content and nutritional value in several fruits and vegetables. It is particularly effective when the light is synchronized to original plant circadian response.
Many other plant stressors that have been studied, including include heat, cold and use of salicylates and jasmonates. Expression of heat shock proteins may play a central role.
The 2013 publication Heat shock proteins as biochemical markers for postharvest chilling stress in fruits and vegetables relates: “Fresh fruits and vegetables have a short postharvest life and are prone to postharvest losses due to mechanical injury, physiological causes and decay. Low temperature storage (LTS) is widely used as a postharvest treatment applied to delay senescence in vegetables and ornamentals and ripening in fruits, so upholding their postharvest quality. But the problem of its application to tropical and subtropical fruits and vegetables is the susceptibility of these to chilling injury (CI) at temperatures below 12 ̊C. Chilling injury is a physiological disorder that greatly reduces fruit quality, frequently rendering the product unsellable. To increase the tolerance of produce to CI and extend storage life, postharvest protocols such as cold storage coupled with heat treatments, temperature preconditioning, intermittent warming, modified and controlled atmosphere storage, ultraviolet (UV) light, and salicylates and jasmonates treatments have been developed. — Membrane damage and reactive oxygen species (ROS) production are multifaceted adverse effects of chilling stress in sensitive fruits and vegetables. They have been attributed to the higher CI tolerance of horticultural products to production and accumulation of heat shock proteins (HSPs), suggesting a central role of HSPs in the acquired tolerance to chilling stress. This beneficial action of HSPs is possible thanks to their chaperone activity. Besides chaperone activity, small HSPs (sHSPs) are able to function as membrane stabilizers and ROS scavengers or to act synergistically with antioxidant system. sHSPs play a key role in maintaining membrane quality attributes such as fluidity and permeability under chilling stress. In fact, the analysis of sHSPs could be envisaged as an ideal method for the assessment of fruits and vegetables’ tolerance to CI and for evaluating the efficiency of postharvest treatments in avoiding CI incidence. This review discusses HSPs and their language of action in mitigation of CI and their potential use as biochemical markers to optimize the use of postharvest treatments. It bridges the division between basic and applied research, and proposes the use of HSPs as biochemical markers of CI.”
The 2010 publication Impact of salicylic acid on post-harvest physiology of horticultural crops reports: “Salicylic acid (SA), an endogenous plant growth regulator, has been found to generate a wide range of metabolic and physiological responses in plants thereby affecting their growth and development. SA as a natural and safe phenolic compound exhibits a high potential in controlling post-harvest losses of horticultural crops. In the present review, we have focused on various intrinsic biosynthetic pathways and effects of exogenous salicylic acid on post-harvest decay and disease resistance, oxidative stress, fruit ripening, ethylene biosynthesis and action, fruit firmness, respiration, antioxidant systems and nutritional quality have also been discussed.”
|Stressing foods as a strategy to increase their nutritional value has been a subject of research for some time.|
An earlier version of the xenobiotic food stress hypothesis was set out in the 2006 online paper The Use of Controlled Postharvest Abiotic Stresses as a Tool for Enhancing the Nutraceutical Content and Adding-Value of Fresh Fruits and Vegetables: “This paper proposes a concept based on applying postharvest abiotic stresses to enhance the nutraceutical content of fresh fruits and vegetables. We hypothesize that selected abiotic stress treatments, such as wounding, phytohormones, temperature, ultraviolet light, altered gas composition, heat shock, and water stress, among others, will affect the secondary metabolism of fresh produce and increase the synthesis of phytochemicals with nutraceutical activity or reduce the synthesis of undesirable compounds. Controlled stresses may be used as tools by the fresh produce industry to enhance the health benefit properties of fresh-cut or whole fresh produce and by the food processing and dietary supplement industries to obtain healthier processed products or enhance extractable nutraceutical yields.”
A number of publications going back to the 1990s, many by Kalidas Shetty and his colleagues, have addressed mechanisms and effects of food stressing including:
A model for the role of the proline-linked pentose-phosphate pathway in phenolic phytochemical bio-synthesis and mechanism of action for human health and bio-synthesis and mechanism of action for human health and environmental applications (2004)
[PDF][PDF] Low microbial load sprouts with enhanced antioxidants for astronaut diet(2003) This is a nice slide presentation that seems to be very advanced for its time. It describes how stress-induced phenolic antioxidants in food-grade plant species can be targeted to enhance therapeutic and health-supporting functional ingredients. Much of the content there is summarized elsewhere in this blog.
An important earlier publication, 2004, Role of proline-linked pentose phosphate pathway in biosynthesis of plant phenolics for functional food and environmental applications: a review by K Shetty relates “… Phenolic compounds such as flavonoids in addition to their inherent antioxidant properties are believed to protect the plants from UV stress . … Phenolic phytochemicals from food-grade plants that are antioxidants are an important part of a healthy diet in a global population that is projected to reach 9 billion in the next 50 years. Such phytochemicals are being targeted for designing conventional foods with added health benefits and are called functional foods. These value-added foods are needed for dietary support to manage major oxidation-linked diseases such as diabetes, cardiovascular diseases, arthritis, cognition diseases and cancer. Plants produce phenolic metabolites during growth and developmental and stress adaptation responses. These phenolic phytochemicals can be targeted for designing functional foods and in order to design consistent food-grade phytochemical profiles for safety and clinical relevancy, — “
The proline-linked pentophosphate pathway (PLPP) has been seen to be critical for the stress-responsive production of plant polyphenols and also to play an important role in human health.
From the previously cited publication: “– in order to design consistent food-grade phytochemical profiles for safety and clinical relevancy, novel tissue culture and bioprocessing technologies have been developed. — These are based on the model that phenolic metabolites in plants are efficiently produced through an alternative mode of metabolism linking proline synthesis with the pentose phosphate pathway. Proline biosynthesis coupled to the pentose phosphate pathway stimulates the synthesis of NADPH2 and sugar phosphates for anabolic pathways, including phenolic and antioxidant response pathways. The reducing equivalents for mitochondrial oxidative phosphorylation are provided by proline replacing NADH, with oxygen being the terminal electron acceptor. Using this system, techniques have been developed to isolate high phenolic clonal lines of food-grade plants from single heterozygous seeds. Applying the same model, elicitation concepts and techniques have been used to over-produce phenolic metabolites in seeds and sprouts. In both clonal and seed sprout systems, exogenous treatment of phenolic phytochemicals from a non-target species elicited endogenous stimulation of phenolic synthesis and potentially an antioxidant response. — From these initial plant antioxidant response investigations, a model has been proposed in which the proline-linked pentose phosphate pathway is critical for modulating protective antioxidant response pathways in diverse biological systems, including humans. The proposed proline-linked pentose phosphate pathway model, when confirmed precisely, provides a mechanism for understanding the mode of action of phenolic phytochemicals in modulating antioxidant pathways in relation to human health. This can provide dietary and nutritional mechanisms as well as new strategies to manage the oxidation-linked diseases through improvement of host physiological response. In other environmental applications, this model can be used to screen and design plants targeted for phytoremediation of aromatic pollutants and adaptation of plants in various stressed environments, including outdoor including outdoor adaptation of tissue culture and transplanted seedlings for better food production.”
Here is an explanation of the biochemistry involved in the synthesis of plant polyphenols and the operation of the PLPP pathway as related to ROS signaling. mainly referring to older literature sources(ref): “Phenolic-enriched fruits and vegetables are important for both postharvest preservation and human health benefits linked to the phenolic-associated antioxidant activity (Adyanthaya et al. 2009). Phenolic phytochemicals are secondary metabolites synthesized by plants, which constitute an important part of the diet in both humans and animals with potential health benefits (Mann 1978; Bravo 1998; Crozier et al. 2000). Phenolic phytochemicals are synthesized by a common biosynthetic pathway which incorporates precursors from both shikimate and/or acetate-malonate pathways (Mann 1978; Strack 1997). The first step in the synthesis of phenolic phytochemicals is the commitment of glucose to the pentose phosphate pathway (PPP), converting glucose-6-phosphate irreversibly to ribulose-5-phosphate. This two-step process also produces reducing equivalents (nicotinamide adenosine dinucleotide phosphate [NADPH]) for cellular anabolic reactions. PPP also generates erythrose-4-phosphate which is channeled to the shikimate pathways to produce phenylalanine, which is directed through the phenylpropanoid pathway to produce phenolic phytochemicals (Mann 1978; Chugh and Sawhney 1999; Shetty et al. 2003; Shetty and Wahlqvist 2004; Vattem et al. 2005). Reactive oxygen species (ROS) like superoxide radicals (O2 -), hydroxyl radicals (OH) and hydrogen peroxide (H2O2) are the products of oxidative dysfunctional biochemical reactions within cells. These ROS, when left unchecked, cause oxidative damage resulting in lipid peroxidation, protein denaturation and mutagenesis. An increase in ROS resulted from different types of stress such as drought, heat stress, metal toxicity, radiation exposure, pathogens and salinity (Bolwell and Wojitaszek 1997; Jimenez et al. 1998; Karpinski et al. 1999; Dat et al. 2000; Hernandez et al. 2001; Quartacci et al.2001; del Rio et al. 2002). ROS is further involved in natural and induced senescence and cell death in plants (Doillard et al. 1987; Thompson et al. 1987; Philosoph-Hadas et al. 1994; Bartoli et al. 1996). For example, studies have indicated an increase in hydroperoxides during pepper, banana, pear and tomato ripening and senescence (Frenkel 1978; Thompson et al. 1987, 1991; Shewfelt and Erickson 1991; Rogiers et al. 1998). In addition, a study has also shown that scald-resistant apples have lower tissue concentration of 536 I. ADYANTHAYA ET AL. H202 when compared with scald-susceptible apples (Rao et al. 1998). Furthermore, the same study showed that H202-metabolizing enzymes like catalase and guaiacol peroxidase (GPX) were more active in scald-resistant varieties of apples; however, superoxide dismutase (SOD) activity was unrelated to resistance or susceptibility to scald (Rao et al. 1998). — Plants quench harmful effects of ROS with antioxidant metabolites and enzymes. Studies have showed many phenolic compounds, especially flavonoids, e.g., quercetin, rutin and catechin have free radical scavenging antioxidant activity to quench ROS (Kalt et al. 1999; Kang et al. 2004; Zhang et al. 2006). In addition, studies in fava beans and peas have shown that exogenous phenolics can stimulate antioxidant enzyme activity to potentially counter ROS (Duval and Shetty 2000; Vattem et al. 2005). Therefore, because ROS is involved in plant development including fruit ripening and senescence, antioxidant response coupling phenolics and antioxidant enzyme response may be recruited to counter ROS and senescence (Hodges et al. 1996; Pastori and Del Rio 1997; Jimenez et al. 1998, 2002, 2003). Previous studies with muskmelon fruits (Lacan and Baccou 1998) and sunflower seeds (Bailly et al. 1996) indicated that delayed senescence in specific tissue types correlated to high antioxidant enzyme response. In order to couple cellular antioxidants like ascorbate, glutathione and phenolic phytochemicals with antioxidant enzymes for effective antioxidant response cellular reducing equivalents such as flavin adenine dinucleotide dihydrogen and NADPH are required. Studies have found that reduced reductant levels increased the rate of senescence in a number of herbaceous species (Philosoph-Hadas et al. 1994; Meir et al. 1995). NADPH is regenerated by the PPP along with sugar phosphates synthesis, supplying precursors for phenolic biosynthesis. Therefore, the enzymes of PPP play an important role in the preservation of plant tissue. — An alternative model for coupling proline synthesis with the PPP has been proposed where proline biosynthesis in response to stress can manage energy and reductant needs of anabolic pathways (Shetty and Wahlqvist 2004). This active metabolic role of proline could have implications for plant senescence where proline can act as an antioxidant or stimulate phenolic-linked antioxidant response (Smirnoff and Cumbes 1989; Reddy andVeeranjaneyulu 1991; Shetty and Wahlqvist 2004). Proline is synthesized via the reduction of glutamate to D1-pyrroline-5-carboxylate (P5C) which is further reduced to proline, with both reactions using NADPH as a reductant. (Hagedorn and Phang 1983; Phang 1985; Shetty andWahlqvist 2004). Because the reduction of P5C in the cytosol requires NADPH, an increase in the proline synthesis would result in a reduction in the NADPH/NADPH+ ratio which has been shown to activate G6PDH (Lendzian 1980; Copeland and Turner 1987). G6PDH catalyzes the first-rate limiting step in the PPP; –“
While this explanation is well-documented, we suspect it can be significantly expanded and updated in terms of current knowledge, for example in terms of plant polyphenols as related to cysteine thiols and activation of gene-activating cell stress responses such as Nrf2 and NF-kappaB(ref).
The 2004 publication A model for the role of the proline-linked pentose-phosphate pathway in phenolic phytochemical bio-synthesis and mechanism of action for human health and bio-synthesis and mechanism of action for human health and environmental applications suggests that the PLPPP pathway may be very important for the health-creating impacts pf plant polyphenols in humans. Frequent readers of the blog will recognize that we have primarily identified such health-creating impacts with activation of the keap1-Nrf2 pathway leading to upregulation of antioxidant response genes(ref), with epigigenetic effects of Histone acetylation and deacetylation(ref)(ref)(ref) and possibly with circulating microvesicles containing plant-based mRNAs(ref). Certain plant polyphenols can also be useful for prevention or treatment of infectuous diseases via inhibition of quorum sensing(ref). Such newer perspectives were missing in the earlier literature.
Some studies have focused on enrichment of plant polyphenols in the course of food processing.
For example the 2005 publication Enhancing health benefits of berries through phenolic antioxidant enrichment: focus on cranberry reports: “Emerging epidemiological evidence is increasingly pointing to the beneficial effects of fruits and vegetables in managing chronic and infectious diseases. These beneficial effects are now suggested to be due to the constituent phenolic phytochemicals having antioxidant activity. Cranberry like other fruits is also rich in phenolic phytochemicals such as phenolic acids, flavonoids and ellagic acid. Consumption of cranberry has been historically been linked to lower incidences of urinary tract infections and has now been shown to have a capacity to inhibit peptic ulcer-associated bacterium, Helicobacter pylori. Isolated compounds from cranberry have also been shown to reduce the risk of cardiovascular diseases. Recent evidence suggests the ability of phytochemical components in whole foods in being more effective in protectively supporting human health than compared to isolated individual phenolic phytochemicals. This implies that the profile of phenolic phytochemicals determines the functionality of the whole food as a result of synergistic interaction of constituent phenolic phytochemicals. Solid state bioprocessing using food grade fungi common in Asian food cultures as well as cranberry phenolic synergies through the addition of functional biphenyls such as ellagic acid and rosmarinic acid along with processed fruit extracts have helped to advance these concepts. These strategies could be further explored to enrich cranberry and cranberry products with functional phytochemicals and further improve their functionality for enhancing health benefits.” The mechanisms of polyphenol enrichment here appears to be not stress-related. It appears to be . simply enrichment of cranberry polyphenols through addition of other critical polyphenols such as ellagic acid and rosmarinic acid.
Post-harvest preservation of apples can lead to increased xenohormetic polyphenol content as well as to their preservation.
Ishan Adyanthaya, Young-In Kwon, Emmanouil Apostolidis, and Kalidas Shetty have generated two papers on this subject, One 2007 publication is APPLE POSTHARVEST PRESERVATION IS LINKED TO PHENOLIC CONTENT AND SUPEROXIDE DISMUTASE ACTIVITY “The postharvest preservation of apples indicated that well-preserved varieties of apples had increased superoxide dismutase (SOD) activity initially, and the activity declined during later storage as apples deteriorated. The increased SOD activity linked to better preservation correlated with higher phenolic content and free-radical scavenging-linked antioxidant activity. Well-preserved varieties were able to maintain a more stable pentose phosphate pathway (measured by the activity of glucose-6-phosphate dehydrogenase) throughout the storage period. Proline content increased with proline dehydrogenase (PDH) activity in the initial storage period, indicatingproline catabolism supporting potential adenosine 5_-triphosphate (ATP) synthesis. During later storage, succinate dehydrogenase activity increased,while PDH activity declined indicating a shift to tricarboxylic acid cycle and likely nicotinamide adenine dinucleotide hydrogen (NADH) generation for ATP synthesis. This shift, coupled with the declining SOD activity, coincides with rapid deterioration. The guaiacol peroxidase activity generally declined in late stages, indicating postharvest deterioration. The postharvest preservation of apples indicated that well-preserved varieties of apples had increased superoxide dismutase (SOD) activity initially,and the activity declined during later storage as apples deteriorated. The increased SOD activity linked to better preservation correlated with higher phenolic content and free-radical scavenging-linked antioxidant activity. Well-preserved varieties were able to maintain a more stable pentose phosphate pathway (measured by the activity of glucose-6-phosphate dehydrogenase) throughout the storage period. Proline content increased with proline dehydrogenase (PDH) activity in the initial storage period, indicating proline catabolism supporting potential adenosine 5_-triphosphate (ATP) synthesis. During later storage, succinate dehydrogenase activity increased, while PDH activity declined indicating a shift to tricarboxylic acid cycle and likely nicotinamide adenine dinucleotide hydrogen (NADH) generation for ATP synthesis. This shift, coupled with the declining SOD activity, coincides with rapid deterioration. The guaiacol peroxidase activity generally declined in late stages, indicating postharvest deterioration.”
Melody Winnig has been pursuing research related to beneficial effects and mechanisms of action of plant polyphenols and factors affecting the longevity of centenarians and super-centenarians. She has contributed significantly to surfacing and interpreting relevant research that has been reported in this blog. She is CEO of Vivace Associates, a consulting company concerned with realization of the practical health benefits achievable through science-informed consumption of plant polyphenols.