The frequent occurrence of chalky rice (Oryza sativa L.) grains becomes a serious problem as a result of climate change. The molecular mechanism underlying chalkiness is largely unknown, however. In this study, the temperature‐sensitive floury endosperm11‐2 (flo11‐2 ) mutant was isolated from ion beam‐irradiated rice of 1116 lines. The flo11‐2 mutant showed significantly higher chalkiness than the wild type grown under a mean temperature of 28°C, but similar levels of chalkiness to the wild type grown under a mean temperature of 24°C. Whole‐exome sequencing of the flo11‐2 mutant showed three causal gene candidates, including Os12g0244100 , which encodes the plastid‐localized 70‐kDa heat shock protein 2 (cpHSP70‐2). The cpHSP70‐2 of the flo11‐2 mutant has an amino acid substitution on the 259th aspartic acid with valine (D259V) in the conserved Motif 5 of the ATPase domain. Transgenic flo11‐2 mutants that express the wild‐type cpHSP70‐2 showed significantly lower chalkiness than the flo11‐2 mutant. Moreover, the accumulation level of cpHSP70‐2 was negatively correlated with the chalky ratio, indicating that cpHSP70‐2 is a causal gene for the chalkiness of the flo11‐2 mutant. The intrinsic ATPase activity of recombinant cpHSP70‐2 was lower by 23% at V _max for the flo11‐2 mutant than for the wild type. The growth of DnaK‐defective Escherichia coli cells complemented with DnaK with the D201V mutation (equivalent to the D259V mutation) was severely reduced at 37°C, but not in the wild‐type DnaK. The results indicate that the lowered cpHSP70‐2 function is involved with the chalkiness of the flo11‐2 mutant.

中文翻译：

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面粉ENDOSPERM11-2编码质体HSP70-2，涉及水稻（Oryza sativa L.）谷物的温度依赖性白垩度。

由于气候变化，白垩米（Oryza sativa L.）谷物的频繁发生成为一个严重的问题。但是，很大程度上白垩化的分子机制尚不清楚。在本研究中，从1116系离子束辐照水稻中分离了对温度敏感的粉状胚乳11-2（flo11-2）突变体。flo11-2突变体显示出的白垩度明显高于在28°C的平均温度下生长的野生型，但白垩化水平与在24°C的平均温度下生长的野生型相似。flo11-2突变体的全外显子测序显示了三个因果候选基因，包括Os12g0244100，它编码质体定位的70kDa热激蛋白2（cpHSP70-2）。flo11-2突变体的cpHSP70-2在第259位天冬氨酸上具有一个氨基酸取代，该氨基酸在ATPase域的保守基序5中为缬氨酸（D259V）。表达野生型cpHSP7070-2的转基因flo11-2突变体的白垩化度明显低于flo11-2突变体。此外，cpHSP70-2的积累水平与白垩比率负相关，表明cpHSP70-2是flo11-2突变体白垩化的因果基因。重组cpHSP70-2的固有ATP酶活性的较低23％V_最大为flo11‐2突变体比野生型突变体。带有D201V突变（相当于D259V突变）的DnaK互补的DnaK缺陷型大肠杆菌细胞的生长在37°C时严重降低，而野生型DnaK则没有。结果表明，降低的cpHSP70-2功能与flo11-2突变体的白垩度有关。