AtPUB18 and AtPUB19 are homologous U-box E3 ubiquitin ligases in Arabidopsis ( Arabidopsis thaliana ). AtPUB19 is a negative regulator of abscisic acid ( ABA )-mediated drought responses, whereas the role of AtPUB18 in drought responses is unknown. Here, loss-of-function and overexpression tests identified AtPUB18 as a negative regulator in ABA -mediated stomatal closure and water stress responses. The atpub18-2atpub19-3 double mutant line displayed more sensitivity to ABA and enhanced drought tolerance than each single mutant plant; therefore, AtPUB18 and AtPUB19 are agonistic. Stomatal closure of the atpub18-2atpub19-3 mutant was hypersensitive to hydrogen peroxide ( H 2 O 2 ) but not to calcium, suggesting that AtPUB18 and AtPUB19 exert negative effects on the ABA signaling pathway downstream of H 2 O 2 and upstream of calcium. AtPUB22 and AtPUB23 are other U-box E3 negative regulators of drought responses. Although atpub22atpub23 was more tolerant to drought stress relative to wild-type plants, its ABA -mediated stomatal movements were highly similar to those of wild-type plants. The atpub18-2atpub19-3atpub22atpub23 quadruple mutant exhibited enhanced tolerance to drought stress as compared with each atpub18-2atpub19-3 and atpub22atpub23 double mutant progeny; however, its stomatal behavior was almost identical to the atpub18-2atpub19-3 double mutant in the presence of ABA , H 2 O 2 , and calcium. Overexpression of AtPUB18 and AtPUB19 in atpub22atpub23 effectively hindered ABA -dependent stomatal closure, but overexpression of AtPUB22 and AtPUB23 in atpub18-2atpub19-3 did not inhibit ABA -enhanced stomatal closure, highlighting their ABA -independent roles. Overall, these results suggest that AtPUB18 has a linked function with AtPUB19, but is independent from AtPUB22 and AtPUB23, in negative regulation of ABA -mediated drought stress responses. Glossary ABA abscisic acid H 2 O 2 hydrogen peroxide Ub ubiquitin qRT quantitative reverse transcription RT reverse transcription DAB 3,3′-diaminobenzidine bZIP basic leucine zipper cDNA complementary DNA
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