CYSTM1, a drought-responsive cysteine-rich transmembrane module protein, negatively affects drought tolerance in Arabidopsis thaliana
Drought stress represents a major challenge for plant growth and survival, requiring coordinated regulation of physiological and molecular responses to maintain water balance. The cysteine-rich transmembrane module (CYSTM) family comprises a highly conserved group of small proteins whose functions in plant stress adaptation remain largely unknown. Here, we identified CYSTM1 (AT1G05340) as a drought-responsive gene in Arabidopsis thaliana whose expression increased in response to prolonged water deficit, osmotic stress, high salinity, and abscisic acid (ABA). Analysis of a loss-of-function T-DNA insertion mutant (cystm1) showed no growth phenotype compared to the wild-type under well-watered conditions. However, the mutant plants exhibited enhanced drought tolerance, whereas independent complementation lines restored the wild-type phenotype. The improved drought performance of the cystm1 plants was associated with reduced stomatal aperture and decreased transpiration. Mutant plants also displayed an enhanced seedling establishment under osmotic stress conditions. Loss of CYSTM1 altered ABA-associated responses at physiological and transcriptional level, resulting in reduced sensitivity to exogenous ABA and attenuated expression of ABA- and drought-responsive genes during water deficit. Co-expression analysis showed that genes with expression pattern similar to CYSTM1 include many genes related to drought response as well as other members of the CYSTM family. Further expression analyses revealed selective transcriptional reprogramming within the CYSTM family following the loss of CYSTM1, suggesting crosstalk or functional interdependence within the CYSTM gene family. Collectively, these findings identify CYSTM1 as a novel negative effector of drought tolerance.