ABA-induced SA accumulation causes higher susceptibility in B. juncea as compared to tolerant genotypes against A. brassicae

Alternaria brassicae, a necrotrophic pathogen causes Alternaria blight in members of the Brassicaceae family. An extensive yield-oriented breeding strategy has rendered Indian mustard (Brassica juncea) susceptible to many fungal pathogens however, Alternaria blight is one of the most pressing challenges of all because it causes almost 40-60% yield loss. Variable degree of tolerance is reported in wild relatives of B. juncea, Sinapis alba (White mustard) and camelina sativa (False flax) have been reported to exhibit moderate and high tolerance respectively against A. brassicae. Phytohormones are the essential regulator of the intricate mechanism of plant defence response. The hormones salicylic acid (SA) and jasmonic acid (JA) have been widely studied and recognized as important regulators of plant immune response. In the last decade, research has pointed out that other hormones like abscisic acid (ABA) also participate equally in plant defence. However, the role of ABA in defence responses and its cross-talk with SA and JA has not been fully understood in terms of Brassica-A. brassicae system. In this investigation, three genotypes-B. juncea, S. alba and C. sativa were selected and their response to exogenous application of SA, JA and ABA and their combination with A. brassicae were studied. Disease assessment, gene expression analysis and quantitative estimation of phytohormones showed that the B. juncea exhibited a weak JA-mediated defence response against A. brassicae and synergy between SA-ABA shifted the signalling mechanism to SA-mediated response leading to susceptibility in B. juncea. Tolerant genotypes, S. alba and C. sativa exhibited a robust JA-mediated response against A. brassicae and JA-ABA was found antagonistic in Brassica-A. brassicae phyto-pathosystem.