OXI1 and DAD regulate light-induced cell death antagonistically through jasmonate and salicylate levels.

Singlet oxygen (1O2) produced from triplet excited chlorophylls in photosynthesis is a signal molecule that can induce programmed cell death (PCD) through the action of the OXIDATIVE STRESS INDUCIBLE 1 (OXI1) kinase. Here, we identify two negative regulators of light-induced PCD which modulate OXI1 expression: DAD1 and DAD2, homologs of the human anti-apoptotic protein DEFENDER AGAINST CELL DEATH. Overexpressing OXI1 in Arabidopsis (Arabidopsis thaliana) increased plant sensitivity to high light and induced early senescence of mature leaves. Both phenomena rely on a marked accumulation of jasmonate and salicylate. DAD1 or DAD2 overexpression decreased OXI1 expression, jasmonate levels and sensitivity to photooxidative stress. Knock-out mutants of DAD1 or DAD2 exhibited the opposite responses. Exogenous applications of jasmonate upregulated salicylate biosynthesis genes and caused leaf damage in wild-type plants but not in the salicylate biosynthesis mutant sid2, indicating that salicylate plays a crucial role in PCD downstream of jasmonate. Treating plants with salicylate upregulated the DAD genes and downregulated OXI1. We conclude that OXI1 and DAD are antagonistic regulators of cell death through modulating jasmonate and salicylate levels. High light-induced PCD thus results from a tight control of the relative activities of these regulating proteins with DAD exerting a negative feedback control on OXI1 expression.