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Role and Interrelationship of G α Protein, Hydrogen Peroxide and Nitric Oxide in UV-B-Induced Stomatal Closure in Arabidopsis Leaves

Heterotrimeric G proteins have been shown to transmit UV-B signals in mammalian cells, but whether they also transmit UV-B signals in plant cells is not clear. In this paper, we report that 0.5 W m -2 UV-B induces stomatal closure in Arabidopsis thaliana by eliciting a cascade of intracellular signaling events including G α protein, hydrogen peroxide (H 2 O 2 ) and nitric oxide (NO). UV-B triggered a significant increase in H 2 O 2 or NO levels associated with stomatal closure in wild type, but these effects were abolished in the single and double mutants of AtrbohD and AtrbohF or in the Nia1 mutants respectively. Furthermore, we found that UV-B-mediated H 2 O 2 and NO generation are regulated by GPA1, the G α -subunit of heterotrimeric G proteins. UV-B-dependent H 2 O 2 and NO accumulation were nullified in gpa1 knockout mutants but enhanced by overexpression of a constitutively active form of GPA1 (cG α ). In addition, exogenously applied H 2 O 2 or NO rescued the defect in UV-B-mediated stomatal closure in gpa1 mutants, whereas cG α AtrbohD/F and cG α nia1 constructs exhibited a similar response as AtrbohD/F and Nia1 did respectively. Finally, we demonstrated that G α activation of NO production depends on H 2 O 2 . The mutants of AtrbohD and AtrbohF had impaired NO generation in response to UV-B, but UV-B-induced H 2 O 2 accumulation was not impaired in Nia1 . Moreover, exogenously applied NO rescued the defect in UV-B-mediated stomatal closure in the mutants of AtrbohD and AtrbohF . These findings establish a signaling pathway leading to UV-B-induced stomatal closure, which involves GPA1-dependent activation of H 2 O 2 production and subsequent Nia1-dependent NO accumulation.