A MYB transcription factor participates in Ca signaling in Arabidopsis thaliana.

Plant MYB transcription factors play key roles in an extremely wide variety of processes, from cellular growth and plant development to secondary metabolism and the response to stresses. A differential screening analysis performed to determine genes modulated by cadmium treatment in Brassica juncea resulted in the identification of a gene orthologous to an Arabidopsis thaliana MYB transcription factor, that was induced after two hours of cadmium exposure. The corresponding A. thaliana MYB transcription factor, MYB59, has previously been reported as involved in the regulation of cell cycle and root growth. The MYB59 locus produces three splicing isoforms, that are also up-regulated in response to cadmium treatment, as well as modulated by other abiotic stresses. myb59 knock-out mutant shows a mild phenotype of altered growth in comparison to wild-type, associated with reduced dimensions of leaf cells. In addition, myb59 mutant is more tolerant to cadmium during seed germination compared to wild-type. The transcriptomic comparison on leaves of wild-type and myb59 plants highlights the modulation of several genes involved in calcium homeostasis and signaling: in particular, 15% of all up-regulated genes in myb59 consisted of transcripts encoding calmodulin-like proteins (CMLs), with calcium-binding properties and involved in calcium signaling. The analysis of stomatal opening indicates that myb59 responds differently than wild-type to exogenous calcium supplementation; moreover, the moderate, even though appreciable, alteration in the patterns of calcium fluxes in guard cells supports the hypothesis that MYB59 transcription factor is involved in the regulation of calcium signaling rather than transport and homeostasis. These results suggest that MYB59 participates in the control of both cell growth and response to abiotic stress by regulating the processes of calcium signaling.