Homeodomain protein ATHB6 is a target of the protein phosphatase ABI1 and regulates hormone responses in Arabidopsis

ABA (phytohormone avscisic acid) : phtocormone growth inhibitor C 15 H 20 O 4 (M.W= ) 1964, P.F Wareing isolated dormancy-inducible factor from Betula Pubescence ⇒ dormin

Function of ABA : role in regulating plant responses to adverse environmental cues – water shortage high osmolarity ⇒ elevation of plant low temperature ABA level leads to physiological adaptations ⇒ stomatal closure growth inhibition differential gene regulation for metabolic developmental adjustment

PP2C : protein serine/threonin phosphatase of PPM family present in both eukaryotes and prokaryotes no similarity seqencing structurally similar with PPP family consists of two domains ⇒ N-terminal catalytic domain - 6α-helices, 11β –sheets - two Mn + coordnated by four aspartate, Glu residue ⇒ 90 residue C-terminal domainof 3α-helices

ABI1 : ABA-INSENSITIVE encodes a member of the 2C class of protein serine/threonine phosphatase negative regulator of abscisic acid signaling ATHB6 : isolated in low stringency screen for homeobox contain genes by use ATHB6 cDNA as a probe encodes a class 1 homeodomain-leucine zipper protein

Homepdomain-leucine zipper protein ⇓ approximately 60 amino acid residues producing either three or four α-helices and N-terminal arms : HD-Zip protein large family of transcription factors apparently unique to plant contain a leucine zipper domain C-terminal to the domain actives as protein dimers in the binding of DNA

Arabidopsis thaliana genome -Contanis 42 HD-Zip encoding gene -Designated Arabidopsis thaliana homeobox gene ⇒ ATHBz four classes HD-Zip Ⅰ ~ Ⅳ ATHB6 - Induced by water deficit conditions - by ABA indicating a role in the ABA mediated drought stress response

ABI1 : protein phoosphatase 2C key componen of signal transduction in arabidopsis refulates diverse responses to phytohormone abscisic acid (ABA) ATHB6 : analysing proteins capable of interacting with ABI1 identified homeodomain protein ATHB6 regulater of the ABA signal pathway

interaction between ATHB6 and ABI1 -Intact protein phosphatase domain -N-terminal domain of ATHB6 containing DNA-binding site recognizes a cis-element present in ATHB6 promoter core motif (CAATTATTA) mediated ATHB6, ABA-dependent gene expression in protoplasts controll transgenic plants containing a luciferase gene negative regulator of the ABA signal to act downstream of ABI1

Fig. 1. Interaction of ATHB6 with ABI1 in the yeast two-hybrid system and in vitro.

Fig. 2. Dependence of ABI1 and ATHB6 interaction on the functional catalytic domain of PP2C.

Fig. 3. Specific binding of ATHB6 to a cis-element present in the ATHB6 promoter.

Fig. 4. Requirement for a functional cis-element and ATHB6 for ABA-mediated promoter activation in Arabidopsis protoplasts.

Fig. 5. ABA- and ABI1-dependent activation of the ATHB6 promoter.

Fig. 6. Analysis of ABA responses in Arabidopsis plants with ectopic expression of ATHB6.