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

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.The phytohormone abscisic acid (ABA) 1 modulates plant developmental processes such as seed formation, dormancy, and germination, as well as plant responses to environmental stresses such as drought, cold, high salinity, pathogen attack, and UV radiation (1-6). Plant responses to ABA are mediated at several molecular levels including transcription, RNA processing, post-translational modification, and metabolism of the secondary messengers (reviewed in Refs. 5-7). Recent data indicate that the ABA signaling pathways appear to be conserved among higher plant species and even bryophytes (8, 9).Both ABA-resistant and ABA-hypersensitive mutants have been extremely valuable in helping define ABA signaling pathways. Studies of mutants in several plant species suggest that the ABA signaling is mediated by a membrane-bound metal sensor (10, 11), type 2C serine/threonine protein phosphatases (12-14), a Ser/Thr protein kinase (15, 16), a protein farnesyl transferase (17), a steroid reductase (16), an inositol polyphosphate 1-phosphatase (18), and several transcription factors (19 -26). In addition, the mutant studies also 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 guanosi...

The Mg-Chelatase H Subunit ofArabidopsisAntagonizes a Group of WRKY Transcription Repressors to Relieve ABA-Responsive Genes of Inhibition

“…The WRKY domain proteins are a superfamily of transcription factors with up to 100 representatives in Arabidopsis, and the family members appear to be involved in the regulation of plant development and pathogen defense (Eulgem et al, 2000;Ulker and Somssich, 2004;Pandey and Somssich, 2009). Several WRKY proteins were suggested to be involved in ABA signaling in creosote bush (Larrea tridentata; Zou et al, 2004), rice (Oryza sativa; Xie et al, 2005Xie et al, , 2006, and barley (Xie et al, 2007). Most recently, genetic evidence was provided for the involvement of the Arabidopsis WRKY2 transcription factor in regulation of ABA signaling (Jiang and Yu, 2009).…”

Isolation and characterization of a Vitis vinifera transcription factor, VvWRKY1, and its effect on responses to fungal pathogens in transgenic tobacco plants

“…In plants, many WRKY proteins are involved in the defence against attack by phytopathogens such as bacteria (Dellagi et al, 2000;Asai et al, 2002;Deslandes et al, 2002;Dong et al, 2003), fungi (Beyer et al, 2001;Asai et al, 2002;Kalde et al, 2003), and viruses (Wang et al, 1998;Yang et al, 1999;Yoda et al, 2002;Liu et al, 2004). Furthermore, a role in various physiological processes has also been suggested, including embryogenesis, seed coat and trichome development, senescence, regulation of biosynthetic pathways, and hormonal signalling (Ulker and Somssich, 2004;Lagace and Matton, 2004;Xu et al, 2004;Zhang et al, 2004;Zou et al, 2004;Xie et al, 2005).…”

Quantitative Statistical Analysis of cis-Regulatory Sequences in ABA/VP1- and CBF/DREB1-Regulated Genes of Arabidopsis

“…These include a variety of classes of transcription factors, including bZIP, Zn-finger, AP2/EREBP, MYB, bHLH, NAC, and WRKY. Opaque2-like bZIP factors and a WRKY transcription factor were recently shown to interact with ABI3 and activate expression of ABA-regulated genes (Lara et al, 2003;Zou et al, 2004), thereby suggesting that additional transcription factors are yet to be identified in ABA signaling during seed development. Moreover, several transcription factors in these classes are known to be activated by exogenous application of ABA as well as abiotic stress conditions, such as cold and drought (Urao et al, 1993;Soderman et al, 1996Soderman et al, , 1999Lee and Chun, 1998;Choi et al, 2000;Uno et al, 2000;Kim et al, 2001;Denekamp and Smeekens, 2003;Shen et al, 2003;Sakamoto et al, 2004;Tran et al, 2004;Villalobos et al, 2004;Xue and Loveridge, 2004).…”