OsPK2encodes a plastidic pyruvate kinase involved in rice endosperm starch synthesis, compound granule formation and grain filling

First author: Yicong Cai; Affiliations: China National Rice Research Institute (中国水稻研究所): Hangzhou, China

Corresponding author: Peisong Hu (胡培松)

Starch is the main form of energy storage in higher plants. Although several enzymes and regulators of starch biosynthesis have been defined, the complete molecular machinery remains largely unknown. Screening for irregularities (不规则) in endosperm formation in rice represents valuable prospect for studying starch synthesis pathway. Here, we identified a novel rice white‐core endosperm and defective grain filling (籽粒灌浆) mutant, ospk2, which displays significantly lower grain weight, decreased starch content and alteration of starch physicochemical properties (理化性质) when compared to wild‐ ype grains. The normal starch compound granules (颗粒) were drastically reduced and more single granules filled the endosperm cells of ospk2. Meanwhile, the germination rate of ospk2 seeds after 1‐year storage was observably reduced compared with wild‐ ype. Map‐ased cloning of OsPK2 indicated that it encodes a pyruvate kinase 丙酮酸激酶 (PK, ATP: pyruvate 2‐O‐phosphotransferase, EC 2.7.1.40), which catalyses an irreversible (不可逆的) step of glycolysis (糖酵解). OsPK2 has a constitutive expression in rice and its protein localizes in chloroplasts. Enzyme assay showed that the protein product from expressed OsPK2 and the crude protein (粗蛋白) extracted from tissues of wild‐ ype exhibits strong PK activity; however, the mutant presented reduced protein activity. OsPK2 (PKpα1) and three other putative rice plastidic isozymes (同功酶), PKpα2, PKpβ1 and PKpβ2, can interact to form heteromer (异聚体). Moreover, the mutation leads to multiple metabolic disorders. Altogether, these results denote new insights into the role of OsPK2 in plant seed development, especially in starch synthesis, compound granules formation and grain filling, which would be useful for genetic improvement of high yield and rice grain quality.

淀粉是高等植物保存能量的主要形式。尽管目前基因鉴定了一些参与淀粉生物合成的酶与调控子，然而整个分子机制还缺少研究。对于水稻中异常胚乳组织形成的鉴定是一种研究淀粉合成通路的有效手段。本文，作者鉴定了一个新的胚乳中心为白色，并且籽粒灌浆会出现缺陷的水稻突变体ospk2，该突变体与野生型相比种子重量会减轻，淀粉含量会减少，另外淀粉的理化性质也会发生改变。突变体中淀粉颗粒会急剧减少，大多数胚乳细胞只含有单个的淀粉颗粒。同时，在经历过一年的储藏后，突变体种子的萌发率显著要比野生型低。对于OsPK2的图位克隆显示这是一个编码丙酮酸激酶的基因，其催化糖酵解途径中某一不可逆反应。在水稻中，OsPK2具有组成型表达的特性，其蛋白定位于叶绿体中。酶试验显示野生型中OsPK2基因翻译的蛋白产物和植株组织粗提的蛋白产物具有很强的丙酮酸激酶活性，而突变体的酶活性要低得多。另外，OsPK2属于PKpα1，其另外的三个旁系同源PKpα2，PKpβ1和PKpβ2可以相互作用，形成异聚体。此外，突变体会导致多个代谢途径紊乱。综上，本文的试验结果揭示了OsPK2基因在水稻种子发育中的新作用，尤其是在淀粉合成、淀粉颗粒的形成以及籽粒灌浆等过程中发挥重要作用，这些可以作为未来对水稻籽粒质量和种子产量进行遗传改良的参考。

通讯：胡培松 (http://www.chinariceinfo.com/leader/cn/hupeisong/)

个人简介：1986年，浙江农业大学，学士；1991年，中国农业科学院，硕士；2002年，南京农业大学，博士。

研究方向：水稻遗传与品质改良。

doi: https://doi.org/10.1111/pbi.12923

Journal: Plant Biotechnology Journal

First Published date: 17 April, 2018

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