Whole and natural food advocates have long contended that vegetable produce remains alive until it is prepared for the table and eaten. The fact that root vegetables like potatoes, turnips, carrots and onions, et al. will continue to sprout in storage, and that dried beans and grains can be induced to do so with introduction of moisture and warm temperature, is tangible evidence that they are on to something.
Now a team of researchers from the biochemistry departments of Rice University at Houston, and the University of California at Davis have determined that produce in distribution channels, on supermarket shelves, and in storage after purchase by consumers continues to respond to the entrainment of circadian rhythms in daily light–dark cycles similarly to the way whole plants respond to cycles of day and night. The biologists determined that continued to be exposed to diurnal light-dark cycles were more resistant to insects in storage and produced natural insect repellant chemicals made for plant defense, called glucosinolates, that also may contribute positively to maintenance of human health, during the light hours.
Research at UC Davis some years ago on the Biochemical Genomics Of Glucosinolate Biosynthesis In Arabidopsis Thaliana identifies glucosinolates as a diverse class of plant-specific secondary metabolites synthesized by species of the Brassicaceae family (Cabbage, Broccoli, Brussels Sprouts, et al.).
A 2001 report published by the USDA’s Research, Education & Economics Information System (REEIS) notes that while the biological functions of glucosinolates are unknown, their breakdown products have a variety of biological activities in plants, animals, and humans, ranging from plant defense to prevention of cancer. The authors proposed molecular genetics and genomics-based strategies to identifying such genes, and reverse genetics and biochemical approaches to testing their activities in glucosinolate biosynthesis.
Current research at Rice and UC Davis is profiled in a paper published this week in the journal Current Biology. The paper, entitled “Postharvest Circadian Entrainment Enhances Crop Pest Resistance and Phytochemical Cycling,” is co-authored by Professor & Chair, Biochemistry and Cell Biology Janet Braam (Corresponding Author), graduate students Danielle Goodspeed and John D. Liu, Faculty Fellow E. Wassim Chehab, and undergraduate student Zhengji Sheng of the Braam Lab at Rice University’s Biochemistry and Cell Biology department, and Marta Francisco and Daniel J. Kliebenstein of the Department of Plant Biology, College of Biological Sciences, University of California, Davis at Davis, California.
The Rice U. Braam Lab’s focus of research is on understanding how plants perceive environmental conditions and respond in ways that may make them better able to withstand abiotic and biotic environmental stress. The roles of the jasmonate phytohormone, the circadian clock, autophagy, chlorophyll biosynthetic pathways, and calmodulin and calmodulin-like proteins in plant biology are specific areas of ongoing research. The lab uses use Arabidopsis thaliana as its model system for studying plant biology.
In the Current Biology report’s abstract, the co-authors note that the circadian clock of diverse crops can be re-entrained postharvest, with restored diurnal light-dark rhythms enhancing pest resistance and phytochemical cycling. Additionally, defense glucosinolates accumulate with circadian rhythmicity in Arabidopsis, and an anticancer phytochemical, 4MSO, accumulates cyclically in re-entrained cabbage.
The scientists summarize that the modular design of plants enables individual plant organs to manifest autonomous functions and continue aspects of metabolism, such as respiration, even after separation from the parent plant. Consequently they hypothesize that harvested vegetables and fruits may retain a capacity to perceive and respond to external stimuli, and demonstrate that the circadian clock of postharvest cabbage (Brassica oleracea) is entrainable by restoration of light-dark cycles and results in enhanced herbivore resistance.
In addition, the researchers found that entrainment of Arabidopsis plants and postharvest cabbage causes cyclical accumulation of metabolites that function in plant defense, and that in edible crops, these metabolites also have potent anticancer properties. Finally, they show that the phenomena of postharvest entrainment and enhanced herbivore resistance are widespread among diverse crops, concluding that sustained clock entrainment of postharvest crops may be a simple mechanism that can be employed to promote pest resistance and the nutritional value of plant-derived food.
Watch the video below from Rice University:
In a Popular Science report on the new research, Francie Diep notes that a similar effect was noted with other foods tested, including lettuce, spinach, zucchini, sweet potatoes, carrots and blueberries, none of which make glucosinolates, but it is deduced that they must make some natural protection agent.
Ms. Diep is not optimistic that supermarkets would be persuaded to make major changes for the sake of enhancing some fairly unstudied nutrients, but she thinks it’s cool to know that carrots lying quietly on their shelves are actually doing live plant biochemistry as you push your cart by. Consumers also need persuading to eat more of these foods, replacing less health-promoting dietary choices. All the glucosinolates and other such agents in the world won’t help if people don’t eat them. For example, a study by Professor Scott Leatherdale, a Cancer Care Ontario Research Chair in Population Studies with the School of Public Health and Health Systems at the University of Waterloo, Ontario funded by Health Canada and published in the latest issue of the journal BMC Public Health finds that barely seven percent of Canadian high school seniors reported adequate fruit and vegetable consumption.
A 1992 paper published by the National Academy of Science entitled “A major inducer of anticarcinogenic protective enzymes from broccoli: isolation and elucidation of structure” by Zhang Y, Talalay P, Cho CG, and Posner GH of the Johns Hopkins University School of Medicine Department of Pharmacology and Molecular Sciences at Baltimore, found that consumption of vegetables, especially crucifers, reduces the risk of developing cancer. The authors noted that although the mechanisms of this protection were unclear, consumption of vegetables induces enzymes of xenobiotic metabolism and thereby accelerates the metabolic disposal of xenobiotics (cancer inducing foreign agents). They also determined that sulforaphane is the most potent inducer, and the presence of oxygen on sulfur enhances its potency, and that the induction of detoxication enzymes by sulforaphane may be a significant component of the anticarcinogenic action of broccoli.