On your grocery store shelves, recently harvested fruit and vegetables are not just fresh, they are alive. The modular nature of plants enables leaves and other organs to function autonomously even after they are severed from the parent plant. But can plants still respond to environmental cues after they are harvested? Janet Braam of Rice University in Houston, Texas, and colleagues decided to investigate.
The researchers had previously determined that in Arabidopsis, a wild relative of the common cabbage, fluctuating levels of the defense hormone jasmonate, controlled by a circadian biological clock, enable the plants to prepare for potential insect attacks. They also knew that jasmonates control the accumulation of glucosinolates, which repel herbivorous insects, and they wondered whether those chemicals also underwent daily fluctuations.
To find out, they grew Arabidopsis plants for three weeks under a twelve-hour light-dark cycle, then switched them to constant light. Glucosinolate levels indeed changed cyclically, rising at dawn (when herbivorous insects tend to become active) and declining at dusk. The team then exposed plants to cabbage looper caterpillars (Trichoplusia ni). The plants had been grown in the lab under a light-dark cycle that was either in phase or out of phase with that of the insects. Sure enough, the plants suffered greater damage and the caterpillars fattened more quickly when plant and caterpillar light-dark cycles were mismatched.
The team then turned to the common cabbage, Brassica oleracea, wondering whether harvested food crops could still be entrained to light-dark cycles. To their surprise, experiments similar to those conducted on Arabidopsis showed that even in snippets of cabbage leaves from a grocer’s, glucosinolate levels were still controlled by a biological clock, although the antiherbivore levels were lower in plants that had been refrigerated. Restarting a cabbage clock (by subjecting the vegetable to a light-dark cycle) increased the plant’s resistance to caterpillar munching, provided cabbage and caterpillars were in the same “time zone.” But when cabbage entraining was delayed more than three days after purchase, the circadian rhythm could not be reinstated. Other harvested crops such as lettuce, spinach, zucchini, sweet potato, carrot, and blueberry also had significantly enhanced resistance to caterpillars after being reentrained.
The new findings hint that crops may benefit from storage that maintains light-dark cycles, not only for pest resistance, but also because many food crops have beneficial phytochemicals such as glucosinolates, which have anti-cancer and antimicrobial properties. The health benefits conferred by those phytochemicals may be lost or reduced when vegetables and fruit are stored in constant light or darkness. (Current Biology)