Arabidopsis HT1 kinase controls stomatal movements in response to CO2

Guard cells, which form stomata in leaf epidermes, sense a multitude of environmental signals and integrate this information to regulate stomatal movements 1 , 2 . Compared with the advanced understanding of light and water stress responses in guard cells 2 , 3 , 4 , the molecular mechanisms that underlie stomatal CO 2 signalling have remained relatively obscure. With a high-throughput leaf thermal imaging CO 2 screen, we report the isolation of two allelic Arabidopsis mutants ( high leaf temperature 1 ; ht1-1 and ht1-2 ) that are altered in their ability to control stomatal movements in response to CO 2 . The strong allele, ht1-2 , exhibits a markedly impaired CO 2 response but shows functional responses to blue light, fusicoccin and abscisic acid (ABA), indicating a role for HT1 in stomatal CO 2 signalling. HT1 encodes a protein kinase that is expressed mainly in guard cells. Phosphorylation assays demonstrate that the activity of the HT1 protein carrying the ht1-1 or ht1-2 mutation is greatly impaired or abolished, respectively. Furthermore, dominant-negative HT1(K113W) transgenic plants, which lack HT1 kinase activity, show a disrupted CO 2 response. These findings indicate that the HT1 kinase is important for regulation of stomatal movements and its function is more pronounced in response to CO 2 than it is to ABA or light.