Arabidopsis Basic Leucine-Zipper Transcription Factors TGA9 and TGA10 Interact with Floral Glutaredoxins ROXY1 and ROXY2 and Are Redundantly Required for Anther Development

Abstract

ROXY1 and ROXY2 are CC-type floral glutaredoxins with redundant functions in Arabidopsis (Arabidopsis thaliana) anther development. We show here that plants lacking the basic leucine-zipper transcription factors TGA9 and TGA10 have defects in male gametogenesis that are strikingly similar to those in roxy1 roxy2 mutants. In tga9 tga10 mutants, adaxial and abaxial anther lobe development is differentially affected, with early steps in anther development blocked in adaxial lobes and later steps affected in abaxial lobes. Distinct from roxy1 roxy2, microspore development in abaxial anther lobes proceeds to a later stage with the production of inviable pollen grains contained within nondehiscent anthers. Histological analysis shows multiple defects in the anther dehiscence program, including abnormal stability and lignification of the middle layer and defects in septum and stomium function. Compatible with these defects, TGA9 and TGA10 are expressed throughout early anther primordia but resolve to the middle and tapetum layers during meiosis of pollen mother cells. Several lines of evidence suggest that ROXY promotion of anther development is mediated in part by TGA9 and TGA10. First, TGA9 and TGA10 expression overlaps with ROXY1/2 during anther development. Second, TGA9/10 and ROXY1/2 operate downstream of SPOROCYTELESS/NOZZLE, where they positively regulate a common set of genes that contribute to tapetal development. Third, TGA9 and TGA10 directly interact with ROXY proteins in yeast and in plant cell nuclei. These findings suggest that activation of TGA9/10 transcription factors by ROXY-mediated modification of cysteine residues promotes anther development, thus broadening our understanding of how redox-regulated TGA factors function in plants.

The ROXY1 C-Terminal L**LL Motif Is Essential for the Interaction with TGA Transcription Factors

“…TGA transcription factors belong to a group of bZIP proteins that are categorized into 10 groups (A-I and S) in Arabidopsis. TGA transcription factors belong to group D, and all members of this group have been investigated (Running and Meyerowitz, 1996;Chuang et al, 1999;Jakoby et al, 2002;Zhang et al, 2003;Kesarwani et al, 2007;Murmu et al, 2010). Nuclear transcriptional coactivators of animals can contain one or more copies of the L**LL motif.…”

“…TGA transcription factors belong to the basic leucine zipper (bZIP) protein superfamily and comprise 10 members in Arabidopsis: TGA1 to TGA7, PERIANTHIA (PAN), TGA9, and TGA10 (Jakoby et al, 2002;Murmu et al, 2010). TGA1 to TGA7 are implicated as regulators of PATHOGENESIS-RELATED (PR) genes and hence disease resistance in Arabidopsis (Zhang et al, 2003;Kesarwani et al, 2007).…”

“…Expression of a dominant-negative form of PAN (PAN-SRDX) uncovered additional redundant functions of TGA proteins during later petal morphogenesis . Recently, TGA9 and TGA10 have also been shown to participate in flower development and to be required for microspore formation (Murmu et al, 2010).…”

“…Genetic analysis and protein interaction experiments indicate that the CC-type GRXs might modify TGA transcription factors by catalyzing thiol-disulfide exchange or deglutathionylation reactions through conserved Cys residues, thereby regulating their activity and related biological processes Ndamukong et al, 2007;Murmu et al, 2010). Upon pathogen infection and salicylic acid accumulation, NONEXPRESSOR OF PR GENES1 (NPR1) is translocated from the cytoplasm into the nucleus and interacts with TGA transcription factors.…”

“…Similar to the a-helical L**LL motifs of transcriptional coactivators that interact with liganded nuclear receptors as well as many other transcription factors in animals (Le Douarin et al, 1996;Heery et al, 1997;Voegel et al, 1998), a hydrophobic face formed by the conserved Leu residues in the L**LL motif of ROXY1 likely mediates the interaction with TGA transcription factors. Although crucial Cys residues of TGA proteins were identified as potential targets to be posttranslationally modified by ROXYs (Després et al, 2003;Murmu et al, 2010), little is known about the TGA domains mediating the interaction with ROXYs. Domain truncation experiments demonstrate that the second Glnrich domain (Q2) and an intervening region between bZIP and the first Gln-rich region (Q1) may form a ROXY1-interacting interface.…”