Chlorophyll b is essential for the growth, photoprotection, and photosystem I assembly in wheat

Chlorophyll (Chl) b deficiency leads to vulnerability to high light and oxidative stress in wheat plants, while the detailed mechanism by which Chl b is involved in photoprotection remains unclear in plants. In this study, the roles of thylakoid protein composition and complexes in photosynthetic electron transport, photoprotective responses, and energy dissipation were investigated in Chl b-deficient mutant lines (ANK-32A) and the wild type (WT) of wheat. Compared to the WT, ANK-32A showed higher non-photochemical quenching (NPQ), slower state transitions, and a significant decline in the amount of Lhca1-4, Lhcb1-3, and PSII-LHCII supercomplexes at the early growth stage. Because of the low Chl b content, ANK-32A also exhibited a low PSI/PSII ratio in the first leaf (the youngest leaf) compared to the WT. In late growth stages, the amounts of Lhcb2, Lhcb3, PSI proteins (Lhca1-4), PSII-LHCII supercomplexes, and PSI and PSII dimers were still lower than in the WT. Immunoblotting analysis and protein mass spectrometry indicated that ANK-32A possessed a high PSI assembly intermediate (PSI*) content relative to the WT. In addition, field experiments further demonstrated that the low Chl content and the PSI efficiency in the flag leaf as well as low yield were observed in ANK-32A compared to the WT. Taken together, this study reveals that chlorophyll b deficiency in wheat alters the organization of thylakoid proteins, which in turn leads to disrupted assembly of PSI complexes, increases PSI photoinhibition, and eventually reduces the photoprotective capacity.