Adenosine 5' phosphosulfate reductase and sulfite oxidase regulate sulfite-induced water loss in Arabidopsis.

Chloroplast-localized adenosine-5'-phosphosulphate reductase (APR) generates sulfite and plays a pivotal role in sulfate reduction to cysteine. The peroxisome-localized sulfite oxidase (SO), oxidizes excess sulfite to sulfate. Wild-type (WT), SO RNA-interference (SO Ri) and SO overexpression (SO OE) Arabidopsis mutants were infiltrated with sulfite. In SO Ri plants, water loss was increased due to enhancement of stomatal aperture compared to WT leaves, whereas in SO OE plants, stomatal aperture was smaller than that of the WT plants, and hence water loss was lower. Sulfite application also limited sulfate and ABA-induced stomatal closure in WT and SO Ri. The increases in APR activity in response to sulfite infiltration into WT and SO Ri leaves resulted in an increase in sulfite beyond the level of the applied sulfite, indicating that APR has an important role in sulfite-induced increases in stomatal aperture. Notably, sulfite-induced H2O2 generation by NADPH oxidase, led to enhanced APR expression and sulfite production. Suppression of APR by inhibiting NADPH oxidase and glutathione reductase2 (GR2) by diphenyleneiodonium, or mutation in APR2 or GR2, resulted in decrease in sulfite production and stomatal aperture size, further supporting the role of APR in stomatal aperture size. The importance of APR and SO in the set-up of sulfite level in leaves, and the significance of sulfite level in water loss were further demonstrated during fast and harsh drought stress in root-detached WT, gr2 and SO modified plants. The role of SO in sulfite homeostasis in relation to water consumption was shown in well-watered plants.