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Summary: The timing and extent of cell division are crucial for the correct patterning of multicellular organism. In Arabidopsis, root ground tissue maturation involves the periclinal cell division of the endodermis to generate two cell layers: endodermis and middle cortex. However, the molecular mechanism underlying this pattern formation remains unclear.Here, we report that phytohormone brassinosteroid (BR) and redox signal hydrogen peroxide (H2O2) interdependently promote periclinal division during root ground tissue maturation by regulating the activity of SHORT‐ROOT (SHR), a master regulator of root growth and development.BR‐activated transcription factor BRASSINAZOLE RESISTANT1 (BZR1) directly binds to the promoter of SHR to induce its expression, and physically interacts with SHR to increase the transcripts of RESPIRATORY BURST OXIDASE HOMOLOGs (RBOHs) and elevate the levels of H2O2, which feedback enhances the interaction between BZR1 and SHR. Additionally, genetic analysis shows that SHR is required for BZR1‐promoted periclinal division, and BZR1 enhances the promoting effects of SHR on periclinal division.Together, our finding reveals that the transcriptional module of BZR1‐SHR fine‐tunes periclinal division during root ground tissue maturation in response to hormone and redox signals. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]