AUTHOR=Tsuzuki Tomo , Takahashi Koji , Tomiyama Masakazu , Inoue Shin-ichiro , Kinoshita Toshinori 

TITLE=Overexpression of the Mg-chelatase H subunit in guard cells confers drought tolerance via promotion of stomatal closure in Arabidopsis thaliana

JOURNAL=Frontiers in Plant Science

VOLUME=4

YEAR=2013

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2013.00440

DOI=10.3389/fpls.2013.00440

ISSN=1664-462X

ABSTRACT=<p>The Mg-chelatase H subunit (CHLH) has been shown to mediate chlorophyll biosynthesis, as well as plastid-to-nucleus and abscisic acid (ABA)-mediated signaling. A recent study using a novel <italic>CHLH</italic> mutant, <italic>rtl1</italic>, indicated that CHLH specifically affects ABA-induced stomatal closure, but also that CHLH did not serve as an ABA receptor in <italic>Arabidopsis thaliana</italic>. However, the molecular mechanism by which CHLH engages in ABA-mediated signaling in guard cells remains largely unknown. In the present study, we examined CHLH function in guard cells and explored whether <italic>CHLH</italic> expression might influence stomatal aperture. Incubation of <italic>rtl1</italic> guard cell protoplasts with ABA induced expression of the ABA-responsive genes <italic>RAB18</italic> and <italic>RD29B</italic>, as also observed in wild-type (WT) cells, indicating that <italic>CHLH</italic> did not affect the expression of ABA-responsive genes. Earlier, ABA was reported to inhibit blue light (BL)-mediated stomatal opening, at least in part through dephosphorylating/inhibiting guard cell H<sup>+</sup>-ATPase (which drives opening). Therefore, we immunohistochemically examined the phosphorylation status of guard cell H<sup>+</sup>-ATPase. Notably, ABA inhibition of BL-induced phosphorylation of H<sup>+</sup>-ATPase was impaired in <italic>rtl1</italic> cells, suggesting that CHLH influences not only ABA-induced stomatal closure but also inhibition of BL-mediated stomatal opening by ABA. Next, we generated <italic>CHLH-GFP-</italic>overexpressing plants using <italic>CER6</italic> promoter, which induces gene expression in the epidermis including guard cells. <italic>CHLH</italic>-transgenic plants exhibited a closed stomata phenotype even when brightly illuminated. Moreover, plant growth experiments conducted under water-deficient conditions showed that <italic>CHLH</italic> transgenic plants were more tolerant of drought than WT plants. In summary, we show that <italic>CHLH</italic> is involved in the regulation of stomatal aperture in response to ABA, but not in ABA-induced gene expression, and that manipulation of stomatal aperture via overexpression of <italic>CHLH</italic> in guard cells improves plant drought tolerance.</p>