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Review
. 2014 May;33(5):683-96.
doi: 10.1007/s00299-014-1578-7. Epub 2014 Mar 26.

Brassinosteroid-mediated regulation of agronomic traits in rice

Affiliations
Review

Brassinosteroid-mediated regulation of agronomic traits in rice

Cui Zhang et al. Plant Cell Rep. 2014 May.

Abstract

Brassinosteroids (BRs) are a group of steroid phytohormones with wide-ranging biological activity. Genetic, genomic and proteomic studies have greatly advanced our understanding of BR signaling in Arabidopsis and revealed a connected signal transduction pathway from the cell surface receptor kinase BRASSINOSTEROID-INSENSITIVE1 (BRI1) and BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) to the BRASSINAZOLE-RESISTANT1 (BZR1) family of transcription factors and their targets mediating physiological functions. However, compared with the dicot model plant Arabidopsis, much less is known about BR signaling in rice, which is a monocot. In this review, we provide an update on the progress made by BR studies in rice and discuss how BR regulates various important agronomic traits to determine rice grain yield. Specifically, we discuss the function of novel components including LEAF AND TILLER ANGLE INCREASED CONTROLLER (LIC), DWARF and LOW-TILLERING (DLT), DWARF1 (D1) and TAIHU DWARF1 (TUD1) in rice BR signaling, and provide a rice BR-signaling pathway model that involves a BRI1-dependent pathway as well as a G-protein α subunit-mediated signaling pathway. The recent significant advances in our understanding of BR-mediated molecular mechanisms underlying agronomic traits will be of great help for rice molecular breeding.

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Figures

Fig. 1
Fig. 1
BR-signaling pathways in rice and Arabidopsis. The left panel shows the BR signal transduction pathway in Arabidopsis. BR directly interacts with BRI1 and BAK1 to form the BRI1–BR–BAK1 complex. Sequential transphosphorylation between BRI1 and BAK1 activates BRI1, which then phosphorylates BSKs/CDG1. The active BSKs/CDG1 phosphorylates BSU1, which dephosphorylates and inactivates BIN2. BZR1 and BES1/BZR2 are dephosphorylated by PP2A and move into the nucleus to induce expression of PRE1, but repress IBH1 expression. PRE1, IBH1 and HBI1 form an antagonistic cascade to regulate plant growth. SBI1 (SUPPRESSOR OF BRI1) is involved in the deactivation of BRI1 through methylation of PP2A. The right panel shows the BR-signaling pathway in rice. BR binding to OsBRI1 promotes association with OsBAK1, and inactivates OsBIN2 via an unknown mechanism. OsBIN2 phosphorylates OsBZR1, LIC and DLT, and inhibits their activity. OsBZR1 induces the expression of ILI1, but represses the expression of IBH1, LIC and DLT; whereas LIC represses the expression of OsBZR1. The heterotrimeric G-protein α subunit known as D1/RGA1 in rice is involved in another BR-signaling pathway in which D1/RGA1 and TUD1 act together to mediate BR signaling. Red indicates the components play positive roles in BR signaling, whereas green denotes the components that play negative roles
Fig. 2
Fig. 2
A diagram of LIC antagonism of BZR1 to balance rice BR signaling. The images in the black circles show the leaf bending of the gain-of function LIC mutant and the antisense line for LIC. The red sketches show the pulvinus (triangle-like, linker between the sheath and leaf) mediating leaf bending. Interior to those images are schematic diagrams of cells, including the nucleus, showing LIC regulation. The LIC overexpression line 1 (OX1) and the gain-of-function mutant (lic-1) display erect leaves similar to the BR-biosynthesis mutant d2; the LIC antisense line 2 (AS2) shows inclined leaves. LIC is a negative regulator of BR signaling while BZR1 is a positive regulator. LIC represses its targets, such as BZR1 and ILI1, on a transcriptional level and inhibits adaxial cell elongation of the pulvinus in the lic-1 mutant and the overexpression lines. This causes the plants to show erect leaves. In LIC antisense lines, BZR1 function is dominant. BZR1 represses its targets, such as LIC and IBH1, to promote cell elongation. The plants therefore show increased leaf angle. LIC and BZR1 antagonize each other to balance BR dynamics during development
Fig. 3
Fig. 3
BR biosynthetic pathway and the enzymes involved in each reaction. Most of the enzymes shown are from Arabidopsis; those in red are the enzymes that have been identified only in rice (Choi et al. ; Fujioka et al. ; Hong et al. , ; Kwon M ; Noguchi et al. ; Tanabe et al. ; Vriet et al. 2013). CR campesterol, CN campestanol, CT cathasterone, TE teasterone, DT dehydroteasterone, TY typhasterol, CS castasteronel, BL brassinolide

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