Abstract
In rice (Oryza sativa), brassinosteroids (BRs) induce cell elongation at the adaxial side of the lamina joint to promote leaf bending. We identified a rice mutant (ili1-D) showing an increased lamina inclination phenotype similar to that caused by BR treatment. The ili1-D mutant overexpresses an HLH protein homologous to Arabidopsis thaliana Paclobutrazol Resistance1 (PRE1) and the human Inhibitor of DNA binding proteins. Overexpression and RNA interference suppression of ILI1 increase and reduce, respectively, rice laminar inclination, confirming a positive role of ILI1 in leaf bending. ILI1 and PRE1 interact with basic helix-loop-helix (bHLH) protein IBH1 (ILI1 binding bHLH), whose overexpression causes erect leaf in rice and dwarfism in Arabidopsis. Overexpression of ILI1 or PRE1 increases cell elongation and suppresses dwarf phenotypes caused by overexpression of IBH1 in Arabidopsis. Thus, ILI1 and PRE1 may inactivate inhibitory bHLH transcription factors through heterodimerization. BR increases the RNA levels of ILI1 and PRE1 but represses IBH1 through the transcription factor BZR1. The spatial and temporal expression patterns support roles of ILI1 in laminar joint bending and PRE1/At IBH1 in the transition from growth of young organs to growth arrest. These results demonstrate a conserved mechanism of BR regulation of plant development through a pair of antagonizing HLH/bHLH transcription factors that act downstream of BZR1 in Arabidopsis and rice.
Highlights
Plant development and morphogenesis is driven by cell elongation and expansion as well as cell division
Our results demonstrate that ILI1-Os IBH1 and Paclobutrazol Resistance1 (PRE1)-At IBH1 are two conserved pairs of antagonistic HLH/basic helix-loop-helix (bHLH) transcription factors that function downstream of BZR1 to mediate BR regulation of cell elongation in rice and Arabidopsis
These results suggest that rice ILI1 and IBH1 can promote and inhibit BR responses in Arabidopsis as they do in rice. Consistent with such conserved roles of ILI1 and IBH1, the PRE1-OX plants displayed similar phenotypes as the ILI1-OX plants (Figure 5E), whereas At IBH1-OX plants showed similar dwarf phenotypes (Figure 5F) as the Os IBH1-OX plants. These results indicated that the function of ILI1/PRE1 and IBH1 is conserved in monocots and dicots, and ILI1/PRE1 promotes cell elongation, whereas IBH1 inhibits cell elongation
Summary
Plant development and morphogenesis is driven by cell elongation and expansion as well as cell division. Extensive molecular genetic and biochemical studies in Arabidopsis thaliana have illustrated a BR signal transduction pathway from the BRI1 receptor kinase at the cell surface to the BZR transcription factors, which regulate BR-responsive gene expression (Vert et al, 2005; Wang et al, 2006b). In the absence of BR, the BZR transcription factors BZR1 and BZR2 ( named BES1) are phosphorylated by the GSK3-like kinase BIN2 (He et al, 2002; Li and Nam, 2002; Yin et al, 2002) and inactivated due to retention in the cytoplasm through interaction with 14-3-3, degradation by the proteasome, and loss of DNA binding activity (He et al, 2002; Vert and Chory, 2006; Gampala et al, 2007). BSU1 dephosphorylates BIN2 at a conserved phosphotyrosine residue and inactivates BIN2, leading to nuclear accumulation of unphosphorylated BZR1 and BZR2 (Kim et al, 2009) and BRresponsive genes expression and plant growth
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