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

## TL;DR

Removing a specific carbonic anhydrase in Arabidopsis increases the size of the photosystem II antenna and changes chlorophyll levels.

## Contribution

This study reveals a novel role of α-carbonic anhydrase 2 in regulating the PSII antenna size and chlorophyll composition in Arabidopsis.

## Key findings

- α-CA2 knockout plants show increased chlorophyll a and b contents and a larger PSII antenna.
- Lhcb1 and Lhcb2 protein levels are higher in α-CA2-KO plants compared to wild type.
- Hydrogen peroxide levels and retrograde signaling gene expression are reduced in α-CA2-KO plants.

## Abstract

The absence of α-carbonic anhydrase 2 (α-CA2) in Arabidopsis thaliana leads to higher contents of chlorophylls a and b, and to a reduced chlorophyll a/b 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 lhcb1 expression levels in mutants, whereas the lhcb2 and lhcb3 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.