The Newly Identified Heat-stress Sensitive Albino 1 Gene Affects Chloroplast Development in Rice

High temperature, a major abiotic stress, significantly affects the yield and quality of crops in many parts of the world. Components of the photosynthetic apparatus are highly susceptible to thermal damage. Although the responses to acute heat stress have been studied intensively, the mechanisms that regulate chloroplast development under heat stress remain obscure, especially in crop plants. Here, we cloned and characterized the gene responsible for the heat-sensitive albino1 (hsa1) mutation in rice (Oryza sativa). The hsa1 mutant harbors a recessive mutation in a gene encoding fructokinase-like protein2 (FLN2); the mutation causes a premature stop codon and results in a severe albino phenotype, with defects in early chloroplast development. The color of hsa1 mutant plants gradually changed from albino to green at later stages of development at various temperatures and chloroplast biogenesis was strongly delayed at high temperature (32 °C). HSA1 expression was strongly reduced in hsa1 plants compared to wild type (WT). HSA1 localizes to the chloroplast and regulates chloroplast development. An HSA1 deletion mutant induced by CRISPR/Cas9 was heat sensitive but had a faster greening phenotype than the original hsa1 allele at all temperatures. RNA and protein levels of plastid-encoded RNA polymerase-dependent plastid genes were markedly reduced in hsa1 plants compared to WT. These results demonstrated that HSA1 plays important roles in chloroplast development at early stages, and functions in protecting chloroplasts under heat stress at later stages in rice.