Figure 10.

Scheme summarizing the influence of increased lutein (Lut) to carotene (Car) ratio on the structure and supramolecular organization of the thylakoid membranes. Carotenoid-dependent retardation in grana folding is related to the decrease in thylakoid fluidity. This is caused by the synergistic effect of overaccumulation of lutein in thylakoid matrix and increased level of nonphosphorylated low-mass PSII supercomplexes due to the decreased level of β-carotene bound with PSII core complex. Decreased thylakoid fluidity results in: (i) reversible binding of Rubisco to the thylakoid surface providing steric hindrance for further membrane folding, (ii) retardation in the PSII core-antennae spectral connectivity, which leads to a limited transfer of excitation energy to the PSII reaction center. Another factor that might further negatively influence membrane folding is an increased light harvesting complex II (LHCII) protein phosphorylation in plants with increased Lut/Car ratio. Highly phosphorylated LHCII pool at least partially forming PSI–LHCI–LHCII supercomplexes, which might compensate a decreased level of light harvesting complex I (LHCI) antennae and influence the PSI efficiency. Interestingly, such plants accumulate additionally δ-Car and γ-Car, which are not typically found in thylakoid membranes of higher plants; however, their presence in chromoplasts was confirmed earlier. We speculate, that δ-Car and γ-Car are mostly present in significantly enlarged plastoglobules of the lutein overaccumulating mutants and do not directly influence thylakoid network structure.