Even a point mutation in the psaA gene mediates chlorophyll deficiency. The role of the plastid signal may perform the redox state of the compounds on the acceptor-side of PSI. Two extranuclear variegated mutants of sunflower, Var1 and Var33, were investigated. The yellow sectors of both mutants were characterized by an extremely low chlorophyll and carotenoid content, as well as poorly developed, unstacked thylakoid membranes. A full-genome sequencing of the cpDNA revealed mutations in the psaA gene in both Var1 and Var33. The cpDNA from the yellow sectors of Var1 differs from those in the wild type by only a single, non-synonymous substitution (Gly734Glu) in the psaA gene, which encodes a subunit of photosystem (PS) I. In the cpDNA from the yellow sectors of Var33, the single-nucleotide insertion in the psaA gene was revealed, leading to frameshift at the 580 amino acid position. Analysis of the photosynthetic electron transport demonstrated an inhibition of the PSI and PSII activities in the yellow tissues of the mutant plants. It has been suggested that mutations in the psaA gene of both Var1 and Var33 led to the disruption of PSI. Due to the non-functional PSI, photosynthetic electron transport is blocked, which, in turn, leads to photodamage of PSII. These data are confirmed by immunoblotting analysis, which showed a significant reduction in PsbA in the yellow leaf sectors, but not PsaA. The expression of chloroplast and nuclear genes encoding the PSI subunits (psaA, psaB, and PSAN), the PSII subunits (psbA, psbB, and PSBW), the antenna proteins (LHCA1, LHCB1, and LHCB4), the ribulose 1.5-bisphosphate carboxylase subunits (rbcL and RbcS), and enzymes of chlorophyll biosynthesis were down-regulated in the yellow leaf tissue. The extremely reduced transcriptional activity of the two protochlorophyllide oxidoreductase (POR) genes involved in chlorophyll biosynthesis is noteworthy. The disruption of NADPH synthesis, due to the non-functional PSI, probably led to a significant reduction in NADPH-protochlorophyllide oxidoreductase in the yellow sectors of Var1 and Var33. A dramatic decrease in chlorophyllide was shown in the yellow sectors. A reduction in NADPH-protochlorophyllide oxidoreductase, along with photodegradation, has been suggested as a result of chlorophyll deficiency.
Keywords: Helianthus annuus; Photosystem I; Plastom mutations; Variegation.