Genome sequences of Arabidopsis zund mutants

Plants actively suppress growth and development upon activation of immunity. In turn, when pathogen attack has subsided immune responses are suppressed again. Phytohormones play an important role in regulating this balance and the growth-immunity tradeoff in general. The tradeoff is evident in the Arabidopsis dmr6 dlo1 mutant, which accumulates the immune-activating phytohormone salicylic acid (SA) to high levels resulting in high disease resistance but repression of growth. Little is known about the SA-induced growth tradeoff mechanism. In this study, we performed a genetic suppressor screen on the dmr6 dlo1 double mutant to select mutants with reduced growth reduction and identify suppressors of the SA-mediated tradeoff. We identified 10 zund (giant) mutants, with restored growth but retained resistance to downy mildew. Through bulked segregant analysis and whole-genome sequencing (BSA-seq) we identified three mutant alleles of MED15a, an NPR1 allele, one ICS1/SID2 allele, and a PAD4 splice defect. Genetic complementation of mutants confirmed roles of these genes in the SA-mediated growth-immunity tradeoff. As these genes are known players in SA synthesis, recognition, and signaling, we discuss their application in tweaking SA signaling to optimize the balance between growth and immunity that is important when deploying immunity traits in breeding.