Title : Ethylene alone may play a key role in modulating induction of isoflavonoids, an important defense against herbivore insects in field grown soybean
Solar radiation can enhance protection against insect pests and increase yield of crops, studies of plant secondary metabolites induced by solar UV-B radiation get more and more attention in the context of potential utilization in agriculture for increasing crop resistance to herbivore insects. However, the mechanism and traits involved in the UV-B mediated increment of plant resistance are largely unknown in crops species, such as soybean. Here we determined in undamaged and damaged leaves by Anticarsia gemmatalis larvae and in pods attached by stink bugs of two soybean cultivars (cv.) grown under attenuated or full solar UV-B radiation changes in jasmonates, ethylene, salicylic acid, trypsin protease inhibitor activity, flavonoids and mRNA expression of genes related with defenses. Ethylene emission induced by herbivory was synergistically increased in plants grown under solar UV-B radiation and was positively correlated with malonylgenistin concentration, TPI activity and expression of IFS2 and the defensive protein PR2, while was negatively correlated with leaf consumption and stink bug damage. The precursor of ethylene ACC applied exogenously to soybean was enough to strongly induce leaf isoflavonoids. Our results showed that in field-grown soybean isoflavonoids were regulated by both herbivory and solar UV-B inducible ET, while flavonols were regulated by solar UV-B radiation and not by herbivory or ET. Traditionally ET has been considered an important modulator of plant responses against herbivores with a secondary participation in induction of defenses and capable of trigger a synergistic effect on defense induction in combination with jasmonic acid. However, our study suggests that although ET can modulate UV-B-mediated priming of inducible plant defenses, some plant defenses, such as isoflavonoids are regulated by ET alone.