Dual located WHIRLY1 affects salicylic acid homeostasis via coordination of ICS1, 1 PAL1 and BSMT1 during Arabidopsis plant aging

Salicylic acid (SA) homeostasis determines also developmental senescence and is spatiotemporally controlled by various mechanisms, including biosynthesis, transport and conjugate formation. The alteration of WHIRLY1 (WHY1), a repressor of leaf natural senescence, with respect to allocation in the nucleus or chloroplast causes a perturbation in SA homeostasis, resulting in adverse plant senescence phenotypes. Loss of WHY1 resulted in a 5 days earlier SA peak compared to wild type plants which accumulated SA at 42 days after germination. SA accumulation coincided with an early leaf senescence phenotype, which could be prevented by ectopic expression of the nuclear WHY1 isoform (nWHY1). However, expressing the plastid WHY1 isoform (pWHY1) greatly enhanced cellular SA levels. A global transcriptional analysis in WHY1 loss-of-function background by expressing either pWHY1 or nWHY1 indicated that hormone metabolism related genes were most significantly altered. The pWHY1 isoform predominantly affected stress related gene expression, while the nWHY1 controlled rather developmental gene expression. Chromatin immunoprecipitation-qPCR (ChIP-qPCR) assays indicated that nWHY1 directly binds to the promoter region of isochorismate synthase (ICS1) to activate its expression at later stage, but indirectly activated S-adenosyl-L-methionine-dependent methyltransferase (BSMT1) gene expression via ethylene response factor 109 (ERF109), while repressing phenylalanine ammonia lyase (PAL1) expression via R2R3-MYB member 15 (MYB15) at the early stage of development. Interestingly, rising SA levels exerted a feedback effect by inducing nWHY1 modification and pWHY1 accumulation. Thus, the alteration of WHY1 organelle isoforms and the feedback of SA intervened in a circularly integrated regulatory network during developmental or stress-induced senescence in Arabidopsis.