Another ABA-binding protein was isolated from Vicia faba leaves (Zhang et al. 2002). It was identified as magnesium protoporphyrin IX chelatase large subunit H (Mg-chelatase H subunit, CHLH). ABA receptor CHLH/ABAR was identified and characterized in chloroplasts in A. thaliana (Shen et al. 2006). CHLH/ABAR forms with other sub-units (CHLD and CHLI) a complex, which is involved in chlorophyll biosynthesis, catalyzing insertion of Mg2+ into protoporphyrin-IX. ABAR
seems to be involved in retrograde signaling (Mochizuki et al. 2001; Nott et al. 2006; Wang and Zhang 2008), i.e., signaling between the plas-tid and nucleus. As many plastid genes are coded by the nucleus, tight regulation of their expression, e.g., under stress conditions, is crucial for coordinated expression in both organelles.
C-terminal part of ABAR molecule was found to play a central role in ABA binding and signaling (Wu et al. 2009). Localization of ABAR was reported to be dependent on the concentration of Mg2+. Receptor was localized in the envelope fraction at high Mg2+ (5 mM) or in the stroma fraction, when Mg2 + level was low (1 mM). At physiological conditions, ABAR was localized to both inner and outer envelope membranes. Recently, Shang et al. (2010) specified ABAR structure as a transmembrane protein which spans the chloroplast envelope, with N- and C-terminal sequences exposed to cytosol. ABAR C-terminal sequence interacts with ABA in contrast to the N-terminal part, which does not bind ABA, but is functionally required for ABA signaling. C-terminal deletion results in the localization of ABAR predominantly to the stroma. It might be caused by an endocytosis-like mechanism, which involves chloroplast membrane trafficking in response to low Mg(+ stress. C-terminal domain is interacting with transcription factor WRKY40, and to lesser extent with WRKY18 and WRKY60. WRKY-mediated signaling is rather complex; WRKY60 seems to be a regulator which may balance the WRKY40/WRKY18 mediated ABA signaling. The three WRKYs as well as ABAR are expressed ubiquitously in different organs/ tissues, which may indicate their role at the whole plant level (Wu et al. 2009) .
ABAR-WRKY40 interaction is highly stimulated at the presence of ABA. ABA is required for the migration of WRKY40 molecule from the nucleus to the cytosol. This was demonstrated by application of exogenous ABA, which restored WRKY40 cytosolic distribution in ABA-deficient mutant. ABA also downregulates transcript as well as protein level of WRKY40. WRKY40 represses expression of several ABA-responsive genes via binding to their promoter region, namely, to TGAC W-box sequence. These genes involve bZIP transcription factors, ABI5 and ABF4, and Apetala-2 (AP2) domain transcription factor, ABI4, DREB1A, DREB2A, MYB2, and RAB18.
One of the genetically best-characterized and physiologically most important transcription factors is ABI5, which controls seed germination and postgermination growth. In response to high levels of ABA that recruit WRKY40 from the nucleus to cytosol and promote ABAR-WRKY40 interaction, AB15 transcription inhibition is relieved. Binding of WRKY40 to AB15 promoter region indicates that ABI5 may function directly downstream of WRKY40 in the ABAR-WRKY40-mediated ABA signaling.
Another interesting role of ABAR was recently suggested by Legnaioli et al. (2009), who showed that ABAR could mediate connection between drought ABA signaling and circadian clock.
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