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 degrees C than the wild type, but similar levels of chalkiness to the wild type under mean temperature of 24 degrees 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 degrees 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.