Abstract
HT1 (HIGH LEAF TEMPERATURE 1) is the first component associated with changes in stomatal aperture in response to CO2 to be isolated by forward genetic screening. The HT1 gene encodes a protein kinase expressed mainly in guard cells. The loss-of-function ht1-1 and ht1-2 mutants in Arabidopsis thaliana have CO2-hypersensitive stomatal closure with concomitant reductions in their kinase activities in vitro In addition to these mutants, in this study we isolate or obtaine five new ht1 alleles (ht1-3, ht1-4, ht1-5, ht1-6, and ht1-7). Among the mutants, only ht1-3 has a dominant mutant phenotype and has widely opened stomata due to CO2 insensitivity. The ht1-3 mutant has a missense mutation affecting a non-conserved residue (R102K), whereas the other six recessive mutants have mutations in highly conserved residues in the catalytic domains required for kinase activity. We found that the dominant mutation does not affect the expression of HT1 or the ability to phosphorylate casein, a universal kinase substrate, but it does affect autophosphorylation activity in vitro A 3D structural model of HT1 also shows that the R102 residue protrudes from the surface of the kinase, implying a role for the formation of oligomers and/or interaction with its targets. We demonstrate that both the loss-of-function and gain-of-function ht1 mutants have completely disrupted CO2 responses, although they have normal responses to ABA. Furthermore, light-induced stomatal opening is smaller in ht1-3 and much smaller in ht1-2 Taken together, these results indicate that HT1 is a critical regulator for CO2 signaling and is partially involved in the light-induced stomatal opening pathway.
Highlights
Plants control CO2 uptake for photosynthesis and regulate signals and endogenous stimuli, integrate this information transpirational water loss through stomatal pores
We performed phosphorylation assays with several kinase inhibitors and confirmed that HT1 is a Group C Raf-like. These findings demonstrate that the features of this protein kinase and others vary among the subgroups of Group C MAPKKKs
These results indicate that the kinase activity of Raf-like MAPKKK HT1 is important for stomatal CO2 responses
Summary
Plants control CO2 uptake for photosynthesis and regulate signals and endogenous stimuli, integrate this information transpirational water loss through stomatal pores. The global annual mean concentration of atmospheric CO2 has steadily increased from a pre-industrial value of about 280 ppm to 395 ppm as of 2013 (National Oceanic and Atmospheric Administration data; http://www.esrl.noaa.gov/gmd/ccgg/ insitu.html). This rise in [CO2] has caused a significant decrease in stomatal conductance in many species on a global scale (Medlyn et al, 2001) and affects plant ecosystems (Ainsworth and Rogers, 2007; Keenan et al, 2013). Understanding the molecular basis for stomatal sensitivity to atmospheric CO2 will improve our ability to predict future ecosystem responses; the detailed mechanisms by which CO2 effects changes in stomatal aperture remain largely unknown
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