Redox-mediated regulation of ABSCISIC ACID-INSENSITIVE 5 affects seed germination and seedling development in Arabidopsis

Seed germination and seedling establishment are complex traits regulated by post-translational modifications, which converge on key molecular devices such as the bZIP transcription factor ABSCISIC ACID-INSENSITIVE 5 (ABI5). This represents a molecular hub in the abscisic acid signaling pathway, which represses seed germination and seedling establishment. ABI5 is post-translationally modified by nitric oxide through Cys153-specific S-nitrosylation (SNO), leading to its degradation. Despite the physiological effects of redox-sensitive proteins, the specificity and molecular mechanisms underlying this type of regulation during seed germination and post-germination developmental checkpoints remain unknown. Here, we show the effect of the redox environment on the formation of ABI5 complexes, highlighting the relevance of Cys153. Mutation of this key residue and the phosphorylation status influence the subcellular localization of ABI5. We establish an enzymatic system underlying the recently determined reversibility of SNO mediated by thioredoxin h5 (TRXh5). Furthermore, seeds overexpressing the redoxins ROXY10 and ROXY21 show a misregulation in germination and in the accumulation of the ABI5 protein. These results provide a physiological link between redox regulation and the abscisic acid signaling pathway through the control of ABI5 that is crucial for a successful seedling establishment.