TaIAA25 negatively regulates wheat alkaline tolerance by inhibiting plasma membrane H⁺-ATPase activity.

Abstract

Soil salinization/alkalization represents a universal challenge that severely constrains crop productivity. Elucidating the molecular mechanisms underpinning saline/alkaline tolerance of wheat (<i>Triticum aestivum</i>) holds profound significance for global food security and sustainable agriculture. In this study, we functionally characterized a wheat alkaline sensitive locus designated <i>Wheat Alkaline Sensitive 1</i> (<i>WAS1</i>), through whole exome-capture sequencing-based bulked segregant analysis and fine mapping of <i>was1</i> mutant populations. This locus encodes TaIAA25, a canonical member of the Auxin/Indole-3-Acetic Acid (Aux/IAA) protein family. A C to T transition in <i>TaIAA25</i> leads to a Pro-to-Ser substitution within its Aux/IAA degron motif. This amino acid substitution enhances the stability of TaIAA25 protein, thereby rendering wheat hypersensitive to alkaline stress. The overexpression of <i>TaIAA25</i> significantly increased sensitivity of wheat to alkaline treatment, whereas <i>iaa25</i> knockout mutants displayed enhanced tolerance to alkaline stress. Consistent with established auxin signaling transduction, TaIAA25 interacts with auxin response factor 16 (TaARF16) to repress the TaARF16-mediated transcriptional activation of small auxin-up RNA gene <i>TaSAUR215</i>. This cascade potentiates the inhibitory effect of D-clade type 2C protein phosphatase (TaPP2C.D) on plasma membrane (PM) H⁺-ATPase activity. Notably, TaIAA25 also directly interacts with the phosphorylation (P) domain in central loop of PM H⁺-ATPase 2 (TaHA2) and represses its binding with the actuator (A) domain, thereby blocking TaHA2-driven proton efflux. Collectively, our findings clarified the crucial role of the classical auxin signaling pathway in plant responses to alkaline stress. Furthermore, we revealed a mechanism wherein an Aux/IAA protein directly modulates PM H⁺-ATPase activity to orchestrate auxin-mediated alkaline stress response.