A WRKY Gene from Creosote Bush Encodes an Activator of the Abscisic Acid Signaling Pathway

Xiaolu Zou,Jeffrey R Seemann,Dawn Neuman,Qingxi J Shen

doi:10.1074/jbc.m408536200

Abstract

The creosote bush (Larrea tridentata) is a xerophytic evergreen C3 shrub thriving in vast arid areas of North America. As the first step toward understanding the molecular mechanisms controlling the drought tolerance of this desert plant, we have isolated a dozen genes encoding transcription factors, including LtWRKY21 that encodes a protein of 314 amino acid residues. Transient expression studies with the GFP-LtWRKY21 fusion construct indicate that the LtWRKY21 protein is localized in the nucleus and is able to activate the promoter of an abscisic acid (ABA)-inducible gene, HVA22, in a dosage-dependent manner. The transactivating activity of LtWRKY21 relies on the C-terminal sequence containing the WRKY domain and a N-terminal motif that is essential for the repression activity of some regulators in ethylene signaling. LtWRKY21 interacts synergistically with ABA and transcriptional activators VP1 and ABI5 to control the expression of the HVA22 promoter. Co-expression of VP1, ABI5, and LtWRKY21 leads to a much higher expression of the HVA22 promoter than does the ABA treatment alone. In contrast, the Lt-WRKY21-mediated transactivation is inhibited by two known negative regulators of ABA signaling: 1-butanol, an inhibitor of phospholipase D, and abi1-1, a dominant negative mutant protein phosphatase. Interestingly, abi1-1 does not block the synergistic effect of LtWRKY21, VP1, and ABI5 co-expression, indicating that LtWRKY21, VP1, and ABI5 may form a complex that functions downstream of ABI1 to control ABA-regulated expression of genes.

Highlights

The phytohormone abscisic acid (ABA)1 modulates plant developmental processes such as seed formation, dormancy, and germination, as well as plant responses to environmental
ABREs and CE3 are bound by bZIP proteins (24, 34 – 43); CE1 is bound by ABI4 [44]; RY/Sph elements are bound by those containing B3 domains [19, 20, 45,46,47,48,49,50,51,52,53,54,55]; AT-rich elements are bound by homeodomain leucine zipper proteins [56, 57]; MYC sites are bound by AtMYC [58]; and MYB sites are bound by AtMYB [58, 59]
We focused on WRKY proteins because they regulate plant response to various stresses (60 –70), likely mediating ABA responses

Summary

Introduction

The phytohormone abscisic acid (ABA) modulates plant developmental processes such as seed formation, dormancy, and germination, as well as plant responses to environmental. The mutant studies suggest that RNA processing plays an important role in the regulation of ABA signaling [27] because several ABA response mutants are impaired in a double-stranded RNA-binding protein [28], a mRNA CAP-binding protein [29], or a U6-related Sm-like small ribonucleoprotein [18]. In line with these reports, an ABA-induced maize glycine-rich protein can bind to uridine- and guanosinerich RNA fragments [30]. ABREs and CE3 are bound by bZIP proteins (24, 34 – 43); CE1 is bound by ABI4 [44]; RY/Sph elements are bound by those containing B3 domains [19, 20, 45,46,47,48,49,50,51,52,53,54,55]; AT-rich elements are bound by homeodomain leucine zipper proteins [56, 57]; MYC sites are bound by AtMYC [58]; and MYB sites are bound by AtMYB [58, 59]

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Results

Conclusion