Light-dependent reversible phosphorylation of the minor photosystem II antenna Lhcb6 (CP24) occurs in lycophytes.

Evolution of vascular plants on Earth had to achieve the most favorable compromise between photosynthesis and photodamage. We analyzed representative species from two divergent lineages of vascular plants, lycophytes and euphyllophytes, with respect to the response of their photosynthesis and light-harvesting properties to increasing light intensity. In the two lycophytes analyzed, Selaginella martensii and Lycopodium squarrosum, as compared to euphyllophytes, the middle-relaxing component of non-photochemical quenching (NPQ) increased at high light. This was deemed to be associated with the occurrence, in both lycophytes, of an additional thylakoid phosphoprotein, beside D2, CP43 and Lhcb1-2. This protein, which showed a light-intensity-dependent reversible phosphorylation, was identified in S. martensii as Lhcb6, a minor antenna subunit of PSII. Lhcb6 is known to have evolved in the context of land colonization. In S. martensii, Lhcb6 was detected as a component of the free LHCII assemblies, but also associated with PSI. Most relevant light-induced changes affected amount and phosphorylation of LHCII assemblies, which possibly mediate PSI-PSII connectivity. We propose that in lycophytes Lhcb6 is involved in light excess energy management, participating in energy balancing between PSI and PSII through a unique reversible phosphorylation, not yet observed in other land plants.