Functional Analysis Of SULTR3;1and MiR395-Directed Regulation Of Sulfur Assimilation In Arabidopsis Thaliana

Plants play a prominent role as sulfur reducers in the global sulfur cycle. Sulfate, the major form of inorganic sulfur utilized by plants, is absorbed and transported by specific sulfate transporters into plastids, especially chloroplasts where it is reduced and assimilated into cysteine before entering other metabolic processes. However, how sulfate is transported into chloroplast remains unresolved. No plastid-localized sulfate transporters have been reported in higher plants so far. Here we report that SULTR3;1is chloroplast envelope-localized and affects the sulfate level in chloroplasts. Moreover, the loss-of-SULTR3;1significantly reduces endogenous ABA level to about half of the wild type and affects the response of seed germination and root growth to exogenous ABA, indicating sulfur levels have a significant impact on ABA synthesis. Our results also show that low sulfate levels transcriptionally activate NCED3and ABA3, two key genes in ABA biosynthesis. ABA in turn activates several sulfate transporters and genes of sulfur assimilation. Taken together, our results has resolved a longstanding issue how sulfate is transported into chloroplasts and suggest that sulfur assimilation and ABA biosynthesis pathway can crosstalk at the transcriptional level, which provides an efficient mechanism for plants to combat environmental stresses. Arabidopsis miR395induced by sulfur starvation is known to regulate sulfur metabolism by targeting the transcript for three ATP sulfurylases and one sulfate transporter. However, little is known about the influence of miR395on sulfur assimilation. Here we created miR395overexpression lines to investigate the relation between ATP sulfurylase activity and miR395expression by up-regulating miR395expression without changing other components. The result showed that miR395over-expression led to decreased ATP sulfurylase activity when compared with wildtype under sulfur deficiency rather than sulfur sufficiency. Consequently, the reduced ATPS activity led to lower cysteine and glutathione levels under low sulfur conditions but not sulfur sufficient conditions. Meanwhile, the ATP sulfurylase activity in all lines increased when transferred from sulfur sufficiency to sulfur deficiency. Under sulfur starvation, plants are in a dilemma to deal with the decreased sulfate uptake and the increased sulfate assimilation to meet the increasing demands of sulfur-containing metabolites which are important for plants to overcome the stress conditions. Because many physiological processes including photosynthesis are sensitive to sulfate concentration in cells, plants need an effective mechanism to avoiding the rapid declining of sulfate content. Our results confirm that miR395worked as a protective mechanism to maintain the sulfate concentration in cells, protecting plants from unnecessary excessive ATP sulfurylase activity under sulfur starvation.