Glutathione regulates transcriptional activation of iron transporters via S-nitrosylation of bHLH factors to modulate subcellular iron homoeostasis.

Glutathione (GSH) is known to regulate iron (Fe) deficiency response in plants but its involvement in modulating subcellular Fe homoeostasis remains elusive. In this study, we report that the GSH-deficient mutants, cad2-1 and pad2-1 displayed increased sensitivity to Fe deficiency with significant downregulation of the vacuolar Fe exporters, AtNRAMP3 and AtNRAMP4, and the chloroplast Fe importer, AtPIC1. Moreover, the pad2-1 mutant accumulated higher Fe levels in vacuoles but lower Fe levels in chloroplasts compared to wild type (Columbia ecotype [Col-0]) under Fe limited conditions. Exogenous GSH treatment enhanced chloroplast Fe contents in Col-0 but failed to do so in the nramp3nramp4 double mutants demonstrating that GSH plays a role in modulating subcellular Fe homoeostasis. Pharmacological experiments, mutant analysis, and promoter assays revealed that this regulation involves the transcriptional activation of Fe transporter genes by a GSH-S-nitrosoglutathione (GSNO) module. The Fe responsive bHLH transcription factors (TFs), AtbHLH29, AtbHLH38, and AtbHLH101 were found to interact with the promoters of these genes, which were, in turn, activated via S-nitrosylation (SNO). Taken together, the present study highlights the role of the GSH-GSNO module in regulating subcellular Fe homoeostasis by transcriptional activation of the Fe transporters AtNRAMP3, AtNRAMP4, and AtPIC1 via SNO of bHLH TFs during Fe deficiency.

Rice lectin protein r40c1 imparts drought tolerance by modulating S-adenosylmethionine synthase 2, stress-associated protein 8 and chromatin-associated proteins.

Lectin proteins play an important role in biotic and abiotic stress responses in plants. Although the rice lectin protein Osr40c1 has been reported to be regulated by drought stress, the mechanism of its drought tolerance activity has not been studied so far. In this study, it is shown that expression of the Osr40c1 gene correlates with the drought tolerance potential of various rice cultivars. Transgenic rice plants overexpressing Osr40c1 were significantly more tolerant to drought stress than the wild-type plants. Furthermore, ectopic expression of the Osr40c1 gene in tobacco yielded a similar result. Interestingly, the protein displayed a nucleo-cytoplasmic localization and was found to interact with a number of drought-responsive proteins such as S-adenosylmethionine synthase 2 (OsSAM2), stress-associated protein 8 (OsSAP8), DNA-binding protein MNB1B (OsMNB1B), and histone 4 (OsH4). Silencing of each of these protein partners led to drought sensitivity in otherwise tolerant Osr40c1-expressing transgenic tobacco lines indicating that these partners were crucial for the Osr40c1-mediated drought tolerance in planta. Moreover, the association of Osr40c1 with these partners occurred specifically under drought stress forming a multi-protein complex. Together, our findings delineate a novel role of Osr40c1 in imparting drought tolerance by regulating OsMNB1B, OsSAM2, and OsH4 proteins, which presumably enables OsSAP8 to induce downstream gene expression.