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Although the moss Physcomitrella patens is known to respond to abscisic acid (ABA) by activating gene expression, the transcriptional components involved have not been characterized. Initially, we used the ABA-responsive Em promoter from wheat linked to beta-glucuronidase (GUS) to determine whether ABI3/VP1, transcriptional regulators in the ABA-signaling pathway in angiosperms, were similarly active in the ABA response of P. patens. We show by particle bombardment that ABI3 and VP1 affect Em-GUS expression in P. patens in a manner similar to angiosperms. We also show the involvement of ABI1 in the pathway, utilizing the abi1-1 mutant allele. We isolated three ABI3-like genes from P. patens. Using an Em-like ABA-responsive promoter from P. patens (PpLea1), we demonstrate that PpABI3A, only in the presence of ABA, strongly enhances PpLea1-GUS expression in P. patens. PpABI3A also enhances ABA-induced Em-GUS expression in P. patens. In barley aleurone, PpABI3A transactivates Em-GUS but to a lesser extent than VP1 and ABI3. PpABI3A:GFP is localized to the nucleus of both protonemal cells and barley aleurone, indicating that the nuclear localization signals are conserved. We show that at least a part of the inability of PpABI3A to fully complement the phenotypes of the Arabidopsis abi3-6 mutant is due to a weak interaction between PpABI3A and the bZIP transcription factor ABI5, as assayed functionally in barley aleurone and physically in the yeast-two-hybrid assay. Our data clearly demonstrate that P. patens will be useful for comparative structural and functional studies of components in the ABA-response pathway such as ABI3.
Marella et al. (2006) studied this question.