Thylakoid membrane oxidoreductase LTO1/AtVKOR is involved in ABA-mediated response to osmotic stress in Arabidopsis.

Arabidopsis lumen thiol oxidoreductase 1 (LTO1) - the At4g35760 gene product - was previously found to be related to reactive oxygen species (ROS) accumulation. Here, we show that ROS accumulated in a mutant Arabidopsis line (lto1-2, mutant of LTO1/AtVKOR) under osmotic stress at a higher level than that observed in wild-type and transgenic complemented plants of the lto1-2 mutant (lto1-2C, transgenic complemented plants of lto1-2). Because ROS accumulation in osmotic stress is triggered by abscisic acid (ABA), an ABA-responsive gene, Annexin 1 (AnnAt1), was selected to study the response. Osmotic stress or exogenous ABA can significantly upregulate the transcription of AnnAt1 in wild-type and lto1-2C plants. Only a slight change in the transcriptional abundance of AnnAt1 was observed under osmotic stress in the lto1-2 mutant, but exogenous ABA application could increase the expression of AnnAt1, which suggested that exogenous ABA had a partial complementation role. Because the transcription of AnnAt1 is regulated by ABRE (ABA-responsive elements) binding proteins (AREBs)/ABRE binding factors (ABFs), the expression of AREBs/ABFs was also analyzed. The transcription of AREBs/ABFs in the lto1-2 mutant was not induced by osmotic stress but was significantly upregulated by exogenous ABA, which significantly differs from the wild-type and lto1-2C plant responses. Similarly, the expression of another ABA-responsive gene, RD29B (responsive to desiccation stress gene 29B), in the lto1-2 mutant was also upregulated by exogenous ABA. The partial complementation of mutants by ABA indicated that the ABA signal transduction pathway was not significantly affected in the lto1-2 mutant. Taken together, these results suggest that LTO1 is involved in ABA-mediated response to osmotic stress, possibly by affecting the biosynthesis of endogenous ABA.