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.
No abstract
The ubiquitin (Ub)-26S proteasome pathway is implicated in various cellular processes in higher plants. AtAIRP1, a C3H2C3-type RING (for Really Interesting New Gene) E3 Ub ligase, is a positive regulator in the Arabidopsis (Arabidopsis thaliana) abscisic acid (ABA)-dependent drought response. Here, the AtAIRP2 (for Arabidopsis ABA-insensitive RING protein 2) gene was identified and characterized. AtAIRP2 encodes a cytosolic C3HC4-type RING E3 Ub ligase whose expression was markedly induced by ABA and dehydration stress. Thus, AtAIRP2 belongs to a different RING subclass than AtAIRP1 with a limited sequence identity. AtAIRP2-overexpressing transgenic (35S:AtAIRP2-sGFP) and atairp2 loss-of-function mutant plants exhibited hypersensitive and hyposensitive phenotypes, respectively, to ABA in terms of seed germination, root growth, and stomatal movement. 35S:AtAIRP2-sGFP plants were highly tolerant to severe drought stress, and atairp2 alleles were more susceptible to water stress than were wild-type plants. Higher levels of drought-induced hydrogen peroxide production were detected in 35S:AtAIRP2-sGFP as compared with atairp2 plants. ABA-inducible drought-related genes were up-regulated in 35S:AtAIRP2-sGFP and down-regulated in atairp2 progeny. The positive effects of AtAIRP2 on ABA-induced stress genes were dependent on SNF1-related protein kinases, key components of the ABA signaling pathway. Therefore, AtAIRP2 is involved in positive regulation of ABA-dependent drought stress responses. To address the functional relationship between AtAIRP1 and AtAIRP2, FLAG-AtAIRP1 and AtAIRP2-sGFP genes were ectopically expressed in atairp2-2 and atairp1 plants, respectively. Constitutive expression of FLAG-AtAIRP1 and AtAIRP2-sGFP in atairp2-2 and atairp1 plants, respectively, reciprocally rescued the loss-of-function ABA-insensitive phenotypes during germination. Additionally, atairp1/35S:AtAIRP2-sGFP and atairp2-2/ 35S:FLAG-AtAIRP1 complementation lines were more tolerant to dehydration stress relative to atairp1 and atairp2-2 single knockout plants. Overall, these results suggest that AtAIRP2 plays combinatory roles with AtAIRP1 in Arabidopsis ABAmediated drought stress responses.
Ethylene at low concentrations inhibits the light-induced opening of the bean hypocotyl hook; auxin inhibits the opening by inducing production of ethylene. Light causes a decrease in ethylene production and an increase in the production of carbon dioxide. Hook opening appears to be a response in which ethylene serves as a natural growth regulator and in which carbon dioxide may be involved also as a growth regulator through its antagonism of the action of ethylene.
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