Guard cells : transcription factors regulate stomatal movements and water loss

The regulation of the opening of stomata allows the plant to cope with the conflicting needs of ensuring a sufficient uptake of CO2 for photosynthesis and of minimizing water loss. Guard cells integrate internal signals and environmental stimuli to modulate stomatal aperture for plant survival under diverse conditions. Evidence suggests that modulation of transcription plays an important role in controlling guard cell activity, even though the details of this level of regulation remain mostly unknown. Recent work from different groups shows that two members of the R2-R3MYB transcription factors family of Arabidopis thaliana, AtMYB60 and AtMYB61, are necessary for stomatal movements. Here we report the characterization of AtMYB60 gene and of its promoter sequence. AtMYB60 is specifically expressed in guard cell and its expression is negatively modulated during drought. A null mutation in AtMYB60 results in the constitutive reduction of stomatal opening and in decreased wilting under water stress conditions. Transcript levels of a limited number of genes are altered in the mutant, many of which involved in the plant response to stress. We also report the deletion analysis of AtMYB60 gene promoter through transgenic lines harbouring different portions fused to the GUS reporter gene. We identified the minimal promoter fragment necessary and sufficient for AtMYB60 gene expression in guard cells and some putative regulators proteins binding to this portion. Our data indicate that AtMYB60 is a transcriptional modulator of physiological responses in guard cells and open new possibilities to engineering stomatal responses to improve plant survival during drought.