Dynamics of Brassinosteroid Response Modulated by Negative Regulator LIC in Rice

Abstract

Brassinosteroids (BRs) regulate rice plant architecture, including leaf bending, which affects grain yield. Although BR signaling has been investigated in Arabidopsis thaliana, the components negatively regulating this pathway are less well understood. Here, we demonstrate that Oryza sativa LEAF and TILLER ANGLE INCREASED CONTROLLER (LIC) acts as an antagonistic transcription factor of BRASSINAZOLE-RESISTANT 1 (BZR1) to attenuate the BR signaling pathway. The gain-of-function mutant lic-1 and LIC–overexpressing lines showed erect leaves, similar to BZR1–depleted lines, which indicates the opposite roles of LIC and BZR1 in regulating leaf bending. Quantitative PCR revealed LIC transcription rapidly induced by BR treatment. Image analysis and immunoblotting showed that upon BR treatment LIC proteins translocate from the cytoplasm to the nucleus in a phosphorylation-dependent fashion. Phosphorylation assay in vitro revealed LIC phosphorylated by GSK3–like kinases. For negative feedback, LIC bound to the core element CTCGC in the BZR1 promoter on gel-shift and chromatin immunoprecipitation assay and repressed its transcription on transient transformation assay. LIC directly regulated target genes such as INCREASED LEAF INCLINATION 1 ( ILI1) to oppose the action of BZR1. Repression of LIC in ILI1 transcription in protoplasts was partially rescued by BZR1. Phenotypic analysis of the crossed lines depleted in both LIC and BZR1 suggested that BZR1 functionally depends on LIC. Molecular and physiology assays revealed that LIC plays a dominant role at high BR levels, whereas BZR1 is dominant at low levels. Thus, LIC regulates rice leaf bending as an antagonistic transcription factor of BZR1. The phenotypes of lic-1 and LIC–overexpressing lines in erect leaves contribute to ideal plant architecture. Improving this phenotype may be a potential approach to molecular breeding for high yield in rice.

Author Summary

Brassinosteroids (BRs) are phytohormones mediating multiple biological processes, such as development and stress response. They have been used in crops to produce high yield. In rice, the ideal plant architecture for high yield includes effective tillers, as well as height and leaf angle, which is modulated by BRs. Activation of BRI1–mediated BR signaling is well understood, but much less is known about its inactivating mechanism. Here, we found a gain-of-function mutant lic-1 with the phenotype of the ideal rice plant architecture. The C3H-type transcription factor LIC antagonizes BZR1 to repress BR signaling in rice. We used BR to induce the negative regulator LIC and found that it functioned at high BR level, which may restrain plant development. LIC was phosphorylated by GSK3–like kinases. Phosphorylated LIC mainly localized in cytoplasm, whereas dephosphorylated LIC was in nucleus, which was regulated by BR treatment. LIC regulated transcription patterns of the downstream genes in an opposite direction to BZR1. BZR1 activated BR signaling, but the brake module of LIC repressed BR cascade amplification. LIC and BZR1 may balance BR signaling to control growth and development in rice.