Mitochondrial NADPH dehydrogenase B1 negatively regulates Arabidopsis tolerance to drought stress through affecting ABA-triggered stomatal closure.

Water use efficiency (WUE) is a decisive factor for plant growth and yield under drought conditions. Mitochondrial NADPH dehydrogenase B1 (NDB1) plays an essential role in plant development and stress adaptation by balancing the cytosolic NADPH/NADP ratio. Here, the function and the regulatory mechanism of NDB1 in Arabidopsis tolerance to drought stress were investigated. AtNDB1 transcription was up-regulated in roots and markedly induced in guard cells, but down-regulated in leaves under drought. Natural drought experiments confirmed that Atndb1 mutant had a higher survival rate and lower water loss rate compared with wild type (WT), complementary line (Atndb1) and overexpression line (AtNDB1), indicating increased drought tolerance when AtNDB1 is disrupted. Moreover, Atndb1 mutant had increased abscisic acid (ABA) sensitivity at germination and seedling stages. Compared with WT, the higher ABA level in Atndb1 was responsible for decreased stomatal aperture under drought. Hydrogen peroxide (HO) and Ca were involved in the ABA-induced stomatal regulation in Atndb1 under drought. ABA-induced HO accumulation in guard cells was abolished by the plasma membrane NADPH oxidase (NOX) inhibitor. The high NADPH/NADP  ratio in Atndb1 was involved in the HO production and transport through NOX and aquaporins, respectively. In Atndb1, reduced cuticle permeability and stomatal transpiration helped maintain high WUE. RNA-seq analysis revealed that differentially expressed genes involved in the ABA signaling pathway, cuticle biosynthesis and photosynthesis play vital roles in Atndb1 tolerance to drought stress. Taken together, the high NADPH/NADP  ratio in Atndb1 facilitates maintaining the ABA-dependent stomatal regulation, and the high photosynthetic capacity and WUE increase its seed yield.