Dihydroxyacid dehydratase is important for gametophyte development and disruption causes increased susceptibility to salinity stress in Arabidopsis

Dihydroxyacid dehydratase (DHAD) catalyses a key step in the branched-chain amino acid (BCAA) biosynthetic pathway that exists in numerous organisms, including bacteria, fungi, and plants, but not humans. In Arabidopsis thaliana, DHAD is encoded by a single gene (AT3G23940), but its biological function in controlling plant development remains uncharacterized. In this study, we showed that DHAD is highly expressed in most vegetative and reproductive tissues. It is an essential gene, and complete disruption caused partial sterility in both male and female gametophyte phases. In addition, reduced expression of DHAD in knockdown mutants resulted in a reduction in the accumulation of all three BCAAs in roots and, as a consequence, led to a shorter root phenotype, which could be restored by an exogenous supplement of free BCAAs. Interestingly, the knockdown mutants became hypersensitive to salt stress, not to heavy metal stress, implying that BCAAs may act as osmolytes in salt tolerance. This would be the second amino acid shown to confer such a function in addition to the well-documented proline. Our results provide evidence that BCAA biosynthesis plays important roles in gametophyte and root development, and BCAA homeostasis contributes to the adaptation of Arabidopsis to salinity stress.