Change in fast Chl a fluorescence transients, 2 dimensional protein profile and pigment protein interactions during state transitions in Arabidopsis thaliana

• The OJIP transients curves were quenched in state II.
• Lhcb2 is phosphorylated in state II resulting in formation of 4 Lhcb2 isoforms.
• Pigment–pigment interactions in state II leads to macro-aggregation.
• All the above mentioned changes were absent in the stn7 mutant.

Photosynthetic organisms during acclimation to light, differences in the amount of energy absorbed by photosystems leads to an imbalance in the energy distribution between photosystem (PS) II and PSI. Here, we describe the changes in fast chlorophyll (Chl) a fluorescence transients (OJIP) in wild type and stn7 under state I and state II light conditions. Fluorescence quenching in the OJIP transients recorded from state II exposed wt leaves is due to mobilization of LHCII to PSI. Similar kind of quenching was not observed in stn7 plants exposed to state II light. OJIP transients can be used to study the changes in Chl a fluorescence upon state transitions in A. thaliana. Immunoblotting and 2 dimensional gel electrophoresis studies have shown that phosphorylated Lhcb2 under state II condition exhibited 4 isoforms, whereas dephosphorylated Lhcb2 exhibited 3 isoforms in state I. Phosphorylation and migration of LHCII to PSI resulted in changes in the pigment protein profile of the thylakoid membranes in state II from wt. The increase in circular dichroism (CD) signals at 663 nm and 679 nm was due to association of chirally active trimeric LHCII to PSI–LHCI supercomplex leading to macro-aggregation of pigment–pigment complexes in state II pre-illuminated conditions in wt A. thaliana.