FLOURY ENDOSPERM11‐2 encodes plastid HSP70‐2 involved with the temperature‐dependent chalkiness of rice (Oryza sativa L.) grains

Abstract

Summary 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 Vmax 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.

The conditional chalky grain mutant ’flo11-2ʹ of rice (Oryza sativa L.) is sensitive to high temperature and useful for studies on chalkiness

“…Therefore, it was considered that these irradiation conditions would likely also be effective for rice. Although the mutated gene flo11-2 was not homozygous yet at M 3 generation (Tabassum et al, 2020), the number of chalky grains observed in the flo11-2 mutant seemed to fluctuate depending on temperature (data not shown). The averaged T mean during 30 DAH at nearby local meteorological observatories considerably varied in the two locations and the 3 years (2006)(2007)(2008), to 25.8°C, 27.7°C, and 26.3°C, respectively.…”

“…Lee et al, 2017). The flo11-2 mutant has three homozygous mutations which cause nonsynonymous substitutions or frameshift in the coding sequence, including Bet v I allergen family protein, hypothetical protein, and cpHSP70-2 (Tabassum et al, 2020). Because the three proteins are seemingly not related with ABA metabolism and signaling, the high preharvest sprouting coupled with high chalky ratio could be not the direct effect of the mutations but the indirect effect through the poor starch accumulation.…”

“…In this study to comprehend the effect of HT on chalky grains in the wide range of grain filling stages, we isolated and used conditional chalky grain mutant 'flo11-2ʹ for the comparative quantitative analysis. The mutant has a single amino acid substitution at the conserved ATPase domain of cpHSP70-2, which is important for translocation of amyloplast-localized proteins, such as granule bound starch synthase I, resulting in lower ATPase and chaperone activities than that of wild type (Tabassum et al, 2020). The mutant showed extreme chalkiness under averaged T mean of 28°C during a grain filling in natural environment, whereas the agronomical traits were similar to wild type and chalkiness becomes less severe under cool temperature (averaged T mean of 24°C).…”

Genetic and Molecular Factors Determining Grain Weight in Rice

“…Previous research reported that Heat Shock Proteins (HSPs) also contribute to starch granule development. FLO11-2 encodes a plastid-localized cpHSP70-2 that contributes to chalkiness in a temperature-dependent manner ( Tabassum et al, 2020 ). FLO10 encodes a pentatricopeptide repeat protein that acts as a chaperone in the trans -splicing of mitochondrial nad1 intron 1 during endosperm development ( Wu et al, 2019 ).…”

Effects of high temperature on rice grain development and quality formation based on proteomics comparative analysis under field warming

“…The sHsps itself cannot refold unnatural proteins, but it has a significant effect in binding unnatural proteins, and forming a stable complex. The function of sHSP is similar to other ATP-dependent members such as Hsp70, thereby assisting the correct folding and configuration of the protein for further processing (Tabassum et al, 2020). Grain storage proteins such as glutelin precursors are synthesized in the endoplasmic reticulum, and must be folded and modified with the help of a series of molecular chaperones to form trimers, which are then transported out of the endoplasmic reticulum through vesicles, and eventually transported to protein storage vacuoles to form protein bodies (PB) II (Ren et al, 2020).…”