Effect of the Absence of α Carbonic Anhydrase 2 on the PSII Light-Harvesting Complex Size in Arabidopsis thaliana

The absence of α-carbonic anhydrase 2 (α-CA2) in <i>Arabidopsis thaliana</i> leads to higher contents of chlorophylls <i>a</i> and <i>b</i>, and to a reduced chlorophyll <i>a</i>/<i>b</i> ratio, suggesting an increased PSII antenna compared to the wild type (WT). The evaluation of the OJIP kinetics of chlorophyll fluorescence in leaves of WT and α-carbonic anhydrase 2 knockout (α-CA2-KO) plants revealed higher apparent photosystem II (PSII) light-harvesting antenna size in the mutants. The higher levels of both Lhcb1 and Lhcb2 proteins in α-CA2-KO plants compared to WT plants were demonstrated using immunoblotting. Gene expression analysis showed increased <i>lhcb1</i> expression levels in mutants, whereas the <i>lhcb2</i> and <i>lhcb3</i> genes were downregulated. The content of hydrogen peroxide (H₂O₂) in leaves, as well as the production of H₂O₂ within the thylakoid membranes ("membrane" H₂O₂) was lower in α-CA2-KO plants as compared with WT plants. The expression levels of the genes encoding regulating proteins, which are involved in retrograde chloroplast-nucleus signaling, were lower in the α-CA2-KO than in the WT. The changes in the PSII light-harvesting complex size in the absence of α-CA2 correlates with the decreased accumulation of H₂O₂ in the leaves of mutants. It is suggested that this led to lower expression levels of the genes related to retrograde signal transduction from the chloroplast to the nucleus. The results of this study support previous conclusions regarding the involvement of α-CA2 in photosynthetic processes and its location within the chloroplasts of Arabidopsis.