Sigma factors are initiation factors that promote the attachment of plastid-encoded RNA polymerase (PEP) to specific initiation sites and are then released. Regulates transcription in chloroplast in a DG1-dependent manner. Involved in light-dependent chloroplast development. Required during early plant development and primary leaf formation. Interacts (via N-terminus) with DG1 (via C-terminus). Expressed in seedling, accumulating progressively. Present in leaves but not in roots. Induced by red light. Slightly induced by blue light. Phosphorylated to acquire sigma activity; site-specific phosphorylation regulates promoter affinity. Phosphorylation at Ser-174 by chloroplastic CK2 requires prior phosphorylation at Ser-177. Phosphorylation at either Ser-94, Ser-95 or Ser-174 is required for sigma activation. Chlorophyll-defective seedlings with reduced levels of chloroplast-encoded transcripts (e.g. atpB, atpE and rbcL). Delayed light-dependent chloroplast development. Constitutive acclimation to light stress. Suppressor of FLU disruption phenotype; seedlings missing both FLU and SIGF do not bleach when grown under non-permissive dark/light conditions. Belongs to the sigma-70 factor family.