Chen Yujuan, Hui Suozhen, Li Huijuan, Jiao Guiai, Cao Ruijie, Zhou Liang, Wang Jingxin, Mawia Amos Musyoki, Yang Lingwei, Wu Yu, Zhang Yuanyaun, Sheng Zhonghua, Shao Gaoneng, Zhao Fengli, Wang Ling, Lyu Yusong, Tang Shaoqing, Hu Shikai, Hu Peisong
State Key Laboratory of Rice Biology and Breeding, China National Rice Research Institute, Hangzhou, China.
Plant Biotechnol J. 2025 Jul;23(7):2793-2808. doi: 10.1111/pbi.70103. Epub 2025 Apr 29.
Grain chalkiness leads to the deterioration of grain appearance quality, which affects grain processing quality and the market value of rice. Gibberellin plays a crucial role in seed germination and plant growth, but its mechanism on endosperm starch synthesis and rice grain chalkiness formation remains largely elusive. Here, we identified a grain white belly (chalkiness in the belly area of grain) gene, SWB9, which encodes a kinesin-4 protein with a conserved ATPase domain and a coiled-coil domain. The mutation of SWB9 affects the starch structure, resulting in a grain white belly. SWB9 regulates endogenous gibberellin synthesis and accumulation in endosperm by directly binding to the promoter of ent-kaurene oxidase genes (KO1, KO2 and KOL5) encoding gibberellin-biosynthetic enzymes, and negatively regulates their expression. The loss of SWB9 function resulted in higher gibberellin content in the endosperm of swb9 than that of the wild type. Besides, a MYB transcription factor, MYB61 binds to the promoter of SWB9 and activates its expression. The grain of myb61 showed the same white belly phenotype as swb9, while overexpression of SWB9 in myb61 inhibited the grain white belly phenotype. Furthermore, the exogenous GA treatment showed increased grain chalkiness, and high gibberellin treatment can induce the reduced expression of MYB61, and then weaken the inhibitory effect of SWB9 on the expression of KO1, KO2 and KOL5, so as to break the homeostasis of endogenous gibberellin in the endosperm. Meanwhile, MYB61 directly binds to the promoter of amylopectin synthesis-related genes, SSIIa, BEIIb, ISA1 and PUL, at the GAMYB element and activates their expression, further affecting the distribution of amylopectin chain length. Our findings uncover a new insight into the gibberellin dose-dependent feedback regulation loop in rice endosperm that determines grain chalkiness formation.
籽粒垩白会导致籽粒外观品质下降，影响稻米加工品质和市场价值。赤霉素在种子萌发和植物生长中起关键作用，但其在胚乳淀粉合成和稻米籽粒垩白形成方面的机制仍不清楚。在此，我们鉴定出一个籽粒腹白（籽粒腹部区域的垩白）基因SWB9，它编码一种具有保守ATP酶结构域和卷曲螺旋结构域的驱动蛋白-4蛋白。SWB9的突变影响淀粉结构，导致籽粒腹白。SWB9通过直接结合编码赤霉素生物合成酶的内根-贝壳杉烯氧化酶基因（KO1、KO2和KOL5）的启动子，调节胚乳中内源赤霉素的合成和积累，并负向调节它们的表达。SWB9功能缺失导致swb9胚乳中的赤霉素含量高于野生型。此外，一个MYB转录因子MYB61与SWB9的启动子结合并激活其表达。myb61的籽粒表现出与swb9相同的腹白表型，而在myb61中过表达SWB9则抑制了籽粒腹白表型。此外，外源GA处理显示籽粒垩白增加，高赤霉素处理可诱导MYB61表达降低，进而削弱SWB9对KO1、KO2和KOL5表达的抑制作用，从而打破胚乳中内源赤霉素的稳态。同时，MYB61在GAMYB元件处直接与支链淀粉合成相关基因SSIIa、BEIIb、ISA1和PUL的启动子结合并激活它们的表达，进一步影响支链淀粉链长的分布。我们的研究结果揭示了水稻胚乳中决定籽粒垩白形成的赤霉素剂量依赖性反馈调节环的新见解。
