Bioengineering and Molecular Manipulation of Ethylene Signaling System for Crop Disease Management

Abstract

Ethylene signaling system is one of the most important signaling systems which can activate plant innate immunity. This system can be manipulated for inducing systemic resistance (ISR) against fungal, oomycete, bacterial and viral pathogens. Under natural conditions endogenous ethylene content is very low and its level can be increased using several molecular techniques. Increase in ethylene biosynthesis may induce enhanced defense responses. Hence several attempts were made to induce ethylene biosynthesis in plants for disease management. Transgenic rice lines overexpressing ACC synthase gene, OsACS2, have been generated and these transgenic plants show increased levels of endogenous ethylene. The transgenic lines overexpressing OsACS2 show increased resistance to the rice blast pathogen Magnaporthe oryzae and the rice sheath blight pathogen Rhizoctonia solani. ERF belonging to the APETELA2 (AP2)/ETHYLENE RESPONSIVE ELEMENT BINDING PROTEIN (EREBP) transcription factor family is the important group of transcription factors functioning downstream in ethylene signaling system. Several attempts have been made to develop transgenic plants using ERF genes from different plants. An ERF transcription factor gene, GbERF2, was cloned from cotton and used to develop transgenic tobacco plants for management of tobacco diseases. The transgenic tobacco plants constitutively accumulate high levels of transcripts of PR-1b, PR2, and PR4 genes. These plants show enhanced resistance to the necrotrophic fungal pathogen Alternaria longipes. Transgenic tobacco plants overexpressing the rice OsBIERF3 gene have been developed. The transgenic tobacco plants show enhanced resistance against both viral and bacterial pathogens. Transgenic tobacco plants expressing tobacco Tsi1 gene encoding an EREBP-AP2-type transcription factor show enhanced resistance against Pseudomonas syringae pv. tabaci. The transgenic hot pepper plants expressing the tobacco Tsi1 exhibit resistance to Pepper mild mottle virus, Cucumber mosaic virus, Xanthomonas campestris pv. vesicatoria and Phytophthora capsici. ERF1-V, an ethylene-responsive element-factor gene of the AP2/ERF transcription factor gene family was cloned from Haynaldia villosa, a wild species of wheat. Transgenic wheat plants overexpressing EFR1-V show enhanced resistance to the powdery mildew pathogen Blumeria graminis f. sp. tritici. EIN2 is a membrane protein that acts as the central regulator of ethylene signaling pathways. The rice plants overexpressing OsEIN2 show enhanced resistance against the rice blast pathogen Magnaporthe oryzae. Ethylene signaling system can be activated by using different rhizobacteria. Pseudomonas fluorescens WCS417r induces systemic resistance (ISR) in plants. It activates the transcription factor MYB72 and the activated MYB72 gene interacts with EIL3 transcription factor, resulting in activation of ET signaling system. Trichoderma asperellum triggers ISR against several pathogens. Ethylene signal transduction pathway has been shown to be involved in the ISR induced by T. asperellum. The major action of Trichoderma appears to be suppression of the action of both ETR1 and CTR1, the negative regulators of ethylene signaling. Pythium oligandrum treatment induces disease resistance against several pathogens. It induces biosynthesis of ethylene in plants. It also activates ethylene signaling pathway. Activation of the ethylene-dependent signaling pathway was accompanied by increased expression of genes encoding ethylene receptors and ethylene-responsive transcription factors.