Solar UVB-inducible ethylene alone induced isoflavonoids in pods of field-grown soybean, an important defense against stink bugs

Solar UV-B radiation enhances constitutive and inducible plant defenses against herbivore insects. Although inducible defenses in leaves damaged by lepidotperan larvae and other chewing insects are well known, almost nothing is noted for the response of reproductive organs in response to stink bug attack. Here we measured the induction of phytohormones and flavonoids in undamaged and damaged pods by stink bug (Piezodorus guildinii) adults. We compared two different soybean cultivars grown under two levels of solar UV-B radiation. Stink bug and mechanical damage increased isoflavonoid content in pods of both cultivars and solar UV-B tended to enhance induction, without affecting isoflavonoids in undamaged pods. While pod flavonols levels did not change by stink bug attack, mechanical damage or to solar UV-B, herbivory and mechanical damage increased seed isoflavonoids, and solar UV-B did not affect seed isoflavonoid content. Although stink bug damage induced ethylene (ET) from pods of cv. Charata, herbivory did not affect ET emission in cv. Williams. Solar UV-B radiation increased ET emission in mechanically damaged pods of cv. Charata, but did not affect other phytohormones. Both mechanical and stink bug damage induced jasmonates in seed of both cultivars. Application of the ET precursor ACC alone, but not MeJA or SA increased isoflavonoid content in pods of cv. Charata. Our field experiments suggest that herbivory- and solar UV-B radiation-induced ET by itself is sufficient to increase the level of defensive isoflavonoids in pods against stink bugs.