Sigma factors are initiation factors that promote the attachment of plastid-encoded RNA polymerase (PEP) to specific initiation sites and are then released (Probable). Controls the transcription of the psaA and psaB genes in chloroplast, and thus maintains the abundance of the core protein complex PsaA-PsaB of photosystem I (PSI) in the thylakoid membrane (PubMed:17651366). Maintains PSI activity, sufficient rate of electron transfer from PSII to PSI, and photochemical efficiency (PubMed:17651366). Expressed in shoots (PubMed:9421493). Expressed in the tips of fully elongated leaves (PubMed:15118338). Expressed in leaf blades (PubMed:17651366). Induced by light in dark-grown seedllings. Pale-green leaf phenotype due to reduced chlorophyll content. Belongs to the sigma-70 factor family.