A valine residue deletion in ZmSig2A, a sigma factor, accounts for a revertible leaf-color mutation in maize

A nuclear-encoded sigma (σ) factor is essential for the transcriptional regulation of plant chloroplast-encoded genes. Five putative maize σ factors have been identified by database searches, but their functions are unknown. We report a maize leaf color mutant etiolated/albino leaf 1 (eal1) that was derived from space mutation breeding. The eal1 mutant displays etiolated or albino leaves that then gradually turn to normal green at the seedling stage. The changes in eal1 leaf color are associated with changes in photosynthetic pigment content and chloroplast development. Map-based cloning revealed that a single amino-acid deletion changing Val₄₈₀-Val₄₈₁-Val₄₈₂ to Val₄₈₀-Val₄₈₁, in the C-terminal domain σ₄ of the putative σ factor ZmSig2A, is responsible for the eal1 mutation. In comparison with the expression level of the wild-type (WT) allele ZmSig2A⁺ in WT plants, much higher expression of the mutant allele ZmSig2A^ΔV in eal1 plants was detected before the eal1 plants turned to normal green. ZmSig2A shows the highest similarity to rice OsSig2A and Arabidopsis SIG2. Ectopic expression of ZmSig2A⁺ or ZmSig2A^ΔV driven by the cauliflower mosaic virus 35S promoter rescued the pale green leaf of the sig2 mutant, but ectopic expression of ZmSig2A^ΔV driven by the SIG2 promoter did not. We propose that the Val deletion generated a new weak allele of ZmSig2A that cannot completely abolish the ZmSig2A function. Some genes involved in chloroplast development and photosynthesis-associated nuclear genes showed significant expression differences between eal1 and WT plants. We conclude that ZmSig2A encoding a σ factor is essential for maize chloroplast development. The eal1 mutant with a weak allele of ZmSig2A represents a valuable genetic resource for investigating the regulation of ZmSig2A-mediated chloroplast development in maize. The eal1 mutation may be useful as a marker for early identification and elimination of false hybrids or transgene transmission in the application of genetic male sterility to commercial hybrid seed production.