No time for candy: Plants down-regulate herbivory-induced extrafloral nectar production when challenged by competitors

Background/Question/Methods

Plant fitness is often defined by the activation of plastic responses to herbivory and competition with neighbors. These responses involve expression of chemical defenses and adaptive changes in plant morphology. Plants detect the proximity of competitors using the phytochromes, which are sensitive to changes in the red to far-red (R:FR) ratio of sunlight. It is now well established that upon perception of low R:FR ratios, which signal the proximity of competitors, phytochromes often reduce resource allocation to direct chemical defenses against herbivores and pathogens (such as defensive proteins and secondary compounds). In addition to direct defenses, many plants use indirect defenses, such as volatile compounds and extrafloral nectaries, to attract beneficial canopy arthropods that are natural enemies of herbivorous organisms. The effects of informational light signals on the expression of indirect defenses has not been investigated. We studied extrafloral nectaries of passion fruit (Passiflora edulis) leaves as a model of a defense system mediated by beneficial organisms, and characterized the effects of various combinations of light quality and herbivory treatments on the activity of these nectaries.

Results/Conclusions

Petioles of P. edulis leaves carried extrafloral nectaries that were physiologically active and routinely visited by predatory ants and other canopy arthropods. We found that the activity of these nectaries was strongly increased in response to wounding (simulated herbivory), which increased both nectar production and total sugar secretion. As with the induction of many plant defenses, both responses could also be elicited by spraying the plants with methyl jasmonate (MeJA). When P. edulis plants were exposed to low R:FR treatments that mimicked the proximity of other plants, they responded with increased internode and petiole elongation, which are typical morphological components of the so-called shade-avoidance syndrome. Low R:FR ratios reduced nectar production, and strongly suppressed the response triggered by simulated herbivory or MeJA application. We conclude that light quality is a strong regulator of the expression of indirect defenses, which is consistent with the emerging paradigm that phytochromes are central modulators of adaptive phenotypic plasticity. In the case of extrafloral nectar secretion in passion fruit, phytochrome appears to regulate the expression of this mutualistic defense system by controlling plant sensitivity to the defense hormone jasmonate.