Sulfite Dynamics Affect Drought Tolerance and Water Status in Arabidopsis and Tomato.

Previously, we showed that altered sulfite homeostasis led to higher water loss in Sulfite oxidase RNA interference (SO Ri) plants than in Arabidopsis wild-type (WT) after sulfite infiltration into rosette leaves. In contrast, SO overexpression (OE) and adenosine-5'-phosphosulfate reductase (apr2) KO resulted in lower water loss than in WT. Accordingly, sulfite homeostasis and drought tolerance under prolonged drought in Arabidopsis thaliana and tomato (Rheinlands Ruhm) were investigated. SO Ri mutants displayed the most severe wilting and water loss, accompanied by sulfite accumulation due to reduced SO expression. In contrast, SO OE, apr2 KO, apr2 KO/SO OE, and gene-edited lines, including SiR OEC2+SO OEC2 and SiR OEC4/SO OE maintained higher relative water content (RWC) by enhancing sulfite oxidation and reduction. Elevated sulfite in SO Ri mutants acted as an antagonist to abscisic acid (ABA), as despite accumulating higher ABA than all other genotypes, SO Ri plants displayed the lowest RWC, consistent with sulfite suppressing ABA signal transduction through impaired ABA perception (PYL5) and enhanced ABA negative feedback (HAI2). SO and SiR overexpression also increased hydrogen sulfide and cysteine levels, contributing to drought resilience. Tomato lines also reflected these patterns in SO OE, SiR OE, and SiR OE/SO OE, showing reduced sulfite and improved RWC compared to WT. Overall, coordinated regulation of SO, SiR, and APR2 maintains sulfite homeostasis, mitigating drought-induced damage and enhancing water retention across species by directly limiting sulfite accumulation and relieving sulfite-driven antagonism of ABA signal transduction.