水稻剑叶的倾角直接影响籽粒灌浆并影响产量。植物甾醇类激素-油菜素内酯（BR）能调控水稻叶夹角，且能促进作物生长，广泛应用于生产。植物细胞中BR信号转导的“激活”组分与通路业已在模式植物中明晰，但其信号系统中的“制动”元件与机制知之甚少。

中国科院院植物所种康研究组及其合作者发现并证实了一个水稻C3H锌指蛋白（LIC）作为负调控转录因子，介导一条新的BR“信号制动”通路。实验证明LIC及其互作基因网络通过控制叶枕近轴面细胞伸长而介导叶片倾角的改变。这一成果于2012年4月26日在线出版在《PLoS Genetics》上，这对于水稻高产分子设计育种具有重要意义。

研究人员在LIC基因的功能获得性突变体和超表达株系中观察到具有叶片直立、密穗等高产农艺形状，而反义转基因植株叶夹角变大。LIC基因编码C3H锌指类转录因子，负调控其靶基因，这些基因包含BR信号系统的核心“激活因子”BZR1及其下游基因。另一方面，LIC基因又被BZR1结合在转录水平上直接受到抑制。LIC蛋白作为GSK3/BIN2类激酶底物直接被磷酸化。而LIC蛋白的磷酸化修饰控制其在细胞质和细胞核间的穿梭，从而阻碍对下游基因的转录调控。LIC与BZR1对下游基因的调控作用方向相反。在高浓度BR条件下，LIC抑制BR“信号激活”因子的转录，从而阻遏BR信号，显示其信号“制动”功能。而在低浓度BR条件下，BZR1抑制LIC的转录表达，表现出“信号激活”状态。LIC与BZR1功能拮抗，在不同生理浓度区间协同调控BR信号平衡。

这一发现揭示了植物BR信号途径调控的崭新机制。该机制与叶片倾角调节和密穗等高产性状遗传材料将为水稻分子设计育种提供分子功能模块元件。

论文摘要：

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.

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