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

The frequent occurrence of chalky rice (Oryza sativa L.) grains becomes a serious problem due to climate change. However, the molecular mechanism underlying chalkiness is largely unknown. In this study, the temperature-sensitive floury endosperm11-2 (flo11-2) mutant was isolated from ion beam-irradiated rice of 1,116 lines. The flo11-2 mutant showed significantly higher chalkiness under mean temperature of 28 °C than the wild type, but similar levels of chalkiness to the wild type under mean temperature of 24 °C. Whole exome sequencing of the flo11-2 mutant showed three causal gene candidates, including Os12g0244100, which encodes 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 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 chalky ratio, indicating that the 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 in the flo11-2 mutant type than in the wild type. The growth of DnaK-defective Escherichia coli cells complemented with DnaK with 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.