Abstract
Members of the Class III homeodomain leucine zipper (Class III HD-Zip) gene family are central regulators of crucial aspects of plant development. To better understand the roles of five Class III HD-Zip genes in rice (Oryza sativa) development, we investigated their expression patterns, ectopic expression phenotypes, and auxin responsiveness. Four genes, OSHB1 to OSHB4, were expressed in a localized domain of the shoot apical meristem (SAM), the adaxial cells of leaf primordia, the leaf margins, and the xylem tissue of vascular bundles. In contrast, expression of OSHB5 was observed only in phloem tissue. Plants ectopically expressing microRNA166-resistant versions of the OSHB3 gene exhibited severe defects, including the ectopic production of leaf margins, shoots, and radialized leaves. The treatment of seedlings with auxin quickly induced ectopic OSHB3 expression in the entire region of the SAM, but not in other tissues. Furthermore, this ectopic expression of OSHB3 was correlated with leaf initiation defects. Our findings suggest that rice Class III HD-Zip genes have conserved functions with their homologs in Arabidopsis (Arabidopsis thaliana), but have also acquired specific developmental roles in grasses or monocots. In addition, some Class III HD-Zip genes may regulate the leaf initiation process in the SAM in an auxin-dependent manner.
Highlights
Members of the Class III homeodomain leucine zipper (Class III HD-Zip) gene family are central regulators of crucial aspects of plant development
We searched for full-length cDNAs of each of the five OSHB genes in a gene database, and found four fulllength cDNAs corresponding to OSHB1 to OSHB4
To better understand how leaf primordia initiate in the 2,4-D-treated shoot apical meristem (SAM), we examined the expression pattern of the Class I KNOX gene OSH1, whose downregulation is a marker of P0 leaf primordium (Sentoku et al, 1999)
Summary
Members of the Class III homeodomain leucine zipper (Class III HD-Zip) gene family are central regulators of crucial aspects of plant development. Dominant gain-of-function mutants of Class III HD-Zip genes in Arabidopsis show meristem and leaf polarity defects (McConnell and Barton, 1998; McConnell et al, 2001; Zhong and Ye, 2004; Ochando et al, 2006). Consistent with their role in the SAM and the adaxial identity of the lateral organs, all three of these genes are expressed in the SAM and in the adaxial domain of the lateral organs (Zhong and Ye, 1999; McConnell et al, 2001; Otsuga et al, 2001; Emery et al, 2003; Prigge et al, 2005). Dominant gainof-function mutants of PHB, PHV, and REV show radialized vascular bundles with xylem surrounding phloem (McConnell and Barton, 1998; Emery et al, 2003; Zhong and Ye, 2004)
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