The Ca²⁺/calmodulin2‐binding transcription factor <scp>TGA</scp>3 elevates <i><scp>LCD</scp></i> expression and H₂S production to bolster Cr⁶⁺ tolerance in Arabidopsis

Abstract

Heavy metal (HM) contamination on agricultural land not only reduces crop yield but also causes human health concerns. As a plant gasotransmitter, hydrogen sulfide (H₂ S) can trigger various defense responses and help reduce accumulation of HMs in plants; however, little is known about the regulatory mechanisms of H₂ S signaling. Here, we provide evidence to answer the long-standing question about how H₂ S production is elevated in the defense of plants against HM stress. During the response of Arabidopsis to chromium (Cr⁶⁺ ) stress, the transcription of L-cysteine desulfhydrase (LCD), the key enzyme for H₂ S production, was enhanced through a calcium (Ca²⁺ )/calmodulin2 (CaM2)-mediated pathway. Biochemistry and molecular biology studies demonstrated that Ca²⁺ /CaM2 physically interacts with the bZIP transcription factor TGA3, a member of the 'TGACG'-binding factor family, to enhance binding of TGA3 to the LCD promoter and increase LCD transcription, which then promotes the generation of H₂ S. Consistent with the roles of TGA3 and CaM2 in activating LCD expression, both cam2 and tga3 loss-of-function mutants have reduced LCD abundance and exhibit increased sensitivity to Cr⁶⁺ stress. Accordingly, this study proposes a regulatory pathway for endogenous H₂ S generation, indicating that plants respond to Cr⁶⁺ stress by adjusting the binding affinity of TGA3 to the LCD promoter, which increases LCD expression and promotes H₂ S production. This suggests that manipulation of the endogenous H₂ S level through genetic engineering could improve the tolerance of grains to HM stress and increase agricultural production on soil contaminated with HMs.