Mutants for photosystem I subunit D of Arabidopsis thaliana: effects on photosynthesis, photosystem I stability and expression of nuclear genes for chloroplast functions

Summary In Arabidopsis thaliana, the D‐subunit of photosystem I (PSI‐D) is encoded by two functional genes, PsaD1 and PsaD2, which are highly homologous. Knock‐out alleles for each of the loci have been identified by a combination of forward and reverse genetics. The double mutant psad1‐1 psad2‐1 is seedling‐lethal, high‐chlorophyll‐fluorescent and deficient for all tested PSI subunits, indicating that PSI‐D is essential for photosynthesis. In addition, psad1‐1 psad2‐1 plants show a defect in the accumulation of thylakoid multiprotein complexes other than PSI. Of the single‐gene mutations, psad2 plants behave like wild‐type (WT) plants, whereas psad1‐1 markedly affects the accumulation of PsaD mRNA and protein, and photosynthetic electron flow. Additional effects of the psad1‐1 mutation include a decrease in growth rate under greenhouse conditions and downregulation of the mRNA expression of most genes involved in the light phase of photosynthesis. In the same mutant, a marked decrease in the levels of PSI and PSII polypeptides is evident, as well as a light‐green leaf coloration and increased photosensitivity. Increased dosage of PsaD2 in the psad1‐1 background restores the WT phenotype, indicating that PSI‐D1 and PSI‐D2 have redundant functions.