Antagonistic HLH/bHLH Transcription Factors Mediate Brassinosteroid Regulation of Cell Elongation and Plant Development in Rice andArabidopsis     

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.

Dynamic Cytology and Transcriptional Regulation of Rice Lamina Joint Development

“…Genes GRAS19 (Chen et al, 2013), LAZY1 (Li et al, 2007), CYP51G3 (Xia et al, 2015), and GSR1 ) present high expression levels at the lamina joint, validating their roles in lamina joint development. In addition, genes involved in the biosynthesis and signaling of BR and auxin, which have been implicated in important roles in the leaf angle, present diverse expression patterns, including decreased expression (CYP51G3 and CYCU4:1), increased expression (DLT [Tong et al, 2009] and GRAS19 and GH3-2 [Tong et al, 2012]), constant high expression levels (LIC and LAZY1 [Zhang et al, 2012]), and constant low expression levels (ILI1 [Zhang et al, 2009a] and REL1 [Chen et al, 2015a]), suggesting the complex regulation of leaf angles via different pathways.…”

“…Deficiency of lamina joint structure (Lee et al, 2007), suppressed longitudinal elongation of adaxial parenchyma cells, and increased division of abaxial sclerenchyma cells of the lamina joint (Zhang et al, 2009a;Sun et al, 2015) result in leaf erectness. Conversely, increased expansion or proliferation of adaxial parenchyma cells leads to increased leaf inclination (Zhao et al, 2010(Zhao et al, , 2013Zhang et al, 2015).…”

A Triple Helix-Loop-Helix/Basic Helix-Loop-Helix Cascade Controls Cell Elongation Downstream of Multiple Hormonal and Environmental Signaling Pathways inArabidopsis

“…PREs are HLH proteins similar to the human Inhibitor of DNA binding (Id) protein, which lacks the basic domain required for DNA binding but dimerizes with DNA binding bHLH factors to inhibit their DNA binding (Ruzinova and Benezra, 2003;Zhang et al, 2009). Similarly, PREs also antagonize their interacting HLH or bHLH proteins.…”

“…PRE1 was initially identified as a positive regulator of GA responses , but other studies have elucidated roles of PRE1 and its homologs in growth regulation by a wide range of signals, including BR, auxin, and light (Zhang et al, 2009;Bai et al, 2012;Oh et al, 2012;Chapman et al, 2012). Overexpression of PRE1 and its rice (Oryza sativa) homolog INCREASED LAMINA INCLINATION1 (ILI1) increased BR-induced cell elongation in both Arabidopsis and rice (Zhang et al, 2009), and overexpression of PRE3/ATBS1/TMO7 suppressed the bri1 mutant ). PRE6/KIDARI has been implicated in light responses (Hyun and Lee, 2006), whereas PRE3/ATBS1/TMO7 is a target of an auxin response factor and required for root development (Schlereth et al, 2010).…”

Rice leaf inclination2, a VIN3-like protein, regulates leaf angle through modulating cell division of the collar

“…Downregulation of OsLIC, which encodes a CCCH-type zinc-finger protein with transcription activation activity, results in the increased angles of rice leaf and tiller through regulating the BR signaling [24]. The pair of antagonistic HLH/bHLH transcription factors, ILI1 and IBH1, mediates the BR regulation of lamina inclination in rice [25]. In addition, increased expression of the OsAGO7 gene, the ortholog of the Arabidopsis ZIPPY (ZIP/AGO7, belonging to the Argonaute family), resulted in the upward curling of the leaf blade and enhanced erectness of the leaf [26].…”