SiTCD1, encoding a P-type PPR protein, affects early chloroplast development under low temperature in foxtail millet

Chloroplast gene expression relies on nucleus-encoded factors for RNA metabolism processing, but the mechanisms under cold stress remain poorly understood. In this study, we isolated and characterized a foxtail millet ( Setaria italica ) mutant, temperature-sensitive chlorophyll-deficient ( sitcd1 ), which exhibited reduced chlorophyll content and abnormal chloroplasts, resulting in an albino phenotype during early leaf development at low temperatures (20°C during the day and 18°C at night). Map-based cloning revealed that SiTCD1 encoded a P-type PPR protein localized in chloroplasts. In sitcd1 background, transgenic lines of SiTCD1 overexpression appeared nearly normal green leaves under L20/D18 condition. SiTCD1 was especially expressed in earlier development of leaves under low temperature. Additionally, SiTCD1 directly bound to the plastid gene atpF in vitro, which might participate in the splicing of plastid gene atpF under low temperature. RNA-seq indicated that the expression of genes related to metabolism (such as porphyrin, chlorophyll and glutathione metabolism), which required ATP for energy, was down-regulated in sitcd1 , resulting in decreased chlorophyll content, GSH, and its redox couple (GSH/GSSG) at low temperature. As sitcd1 exhibited more sensitive at the bud bursting stage than germination and seedling stage under cold stress, we identified two haplotypes of SiTCD1 ( SiTCD1 Hap1 and SiTCD1 Hap2 ) in 195 accessions, and found that accessions carrying the SiTCD1 Hap2 allele were more tolerant to cold stress than those with the SiTCD1 Hap1 allele at the bud bursting stage. In summary, our results suggest that SiTCD1 is essential for early chloroplast development under low temperature in foxtail millet.