Sulfate availability affects <scp>ABA</scp> levels and germination response to <scp>ABA</scp> and salt stress in <i><scp>A</scp>rabidopsis thaliana</i>

Sulfur-containing compounds play a critical role in the response of plants to abiotic stress factors including drought. The phytohormone abscisic acid (ABA) is the key regulator of responses to drought and high-salt stress. However, our knowledge about interaction of S-metabolism and ABA biosynthesis is scarce. Here we report that sulfate supply affects synthesis and steady-state levels of ABA in Arabidopsis wild-type seedlings. By using different mutants of the sulfate uptake and reduction pathway, we confirmed the impact of sulfate supply on steady-state ABA content in Arabidopsis and demonstrated that this impact was due to cysteine availability. Loss of the chloroplast sulfate transporter3;1 function (sultr3;1) resulted in significantly decreased aldehyde oxidase (AO) activity and ABA levels in seedlings and seeds. These mutant phenotypes could be reverted by exogenous application of cysteine or ectopic expression of SULTR3;1. In addition the sultr3;1 mutant showed a decrease of xanthine dehydrogenase activity, but not of nitrate reductase, strongly indicating that in seedlings cysteine availability limits activity of the molybdenum co-factor sulfurase, ABA3, which requires cysteine as the S-donor for sulfuration. Transcription of ABA3 and NCED3, encoding another key enzyme of the ABA biosynthesis pathway, was regulated by S-supply in wild-type seedlings. In contrast, ABA up-regulated the transcript level of SULTR3;1 and other S-metabolism-related genes. Our results provide evidence for a significant co-regulation of S-metabolism and ABA biosynthesis that operates to ensure sufficient cysteine for AO maturation and highlights the importance of sulfur for stress tolerance of plants.