Inhibition of photosynthesis by viral infection:: Effect on PSII structure and function

The effect of viral infection on photosynthesis was investigated in Nicotiana benthamiana Gray plants infected with different strains of pepper and paprika mild mottle viruses (PMMoV and PaMMoV) and chimeric viral genomes derived from them. In both symptomatic and asymptomatic leaves of virus-infected plants, photosynthetic electron transport in photosystem II (PSII) was reduced. In all cases analyzed, viral infection affected the polypeptide pattern of the oxygen-evolving complex (OEC) in thylakoid membranes. The levels of both the 24 and 16 kDa proteins were reduced to a differing extent when compared with the levels in healthy control. This loss of the OEC extrinsic proteins affected the oxygen evolution rates of thylakoid membranes and leaves from infected plants. Additionally, viral coat protein (CP) was found associated with the chloroplasts and the thylakoid membranes of the infected plants. The CP accumulation level was dependent upon both the post-infection time and the virus analyzed, but independent of the CP itself since hybrid viruses did not behave as their parental viruses with the same CP, with respect to PSII inhibition, CP accumulation rates and OEC protein levels. Modulated chlorophyll (Chl) fluorescence and oxygen evolution measurements carried out in both types of leaves showed that the quantum yield of PSII electron transport was diminished in infected plants with respect to those of control plants. The decrease in electron transport efficiency was mainly caused by a reduction in the fraction of open reaction centers. The infected plants also showed a reduction in the efficiency of excitation capture in PSII by photoprotective thermal dissipation of excess excitation energy.