The effect of viral infection on photosynthesis was investigated infected plants. The CP accumulation level was dependent upon both the post-infection time and the virus analyzed, but in Nicotiana benthamiana Gray plants infected with different strains of pepper and paprika mild mottle viruses (PMMoV independent of the CP itself since hybrid viruses did not behave as their parental viruses with the same CP, with and PaMMoV) and chimeric viral genomes derived from them. In both symptomatic and asymptomatic leaves of virus-respect to PSII inhibition, CP accumulation rates and OEC infected plants, photosynthetic electron transport in photosys-protein levels. Modulated chlorophyll (Chl) fluorescence and tem II (PSII) was reduced. In all cases analyzed, viraloxygen evolution measurements carried out in both types of infection affected the polypeptide pattern of the oxygen-evolv-leaves showed that the quantum yield of PSII electron transport was diminished in infected plants with respect to those of ing complex (OEC) in thylakoid membranes. The levels of control plants. The decrease in electron transport efficiency both the 24 and 16 kDa proteins were reduced to a differing extent when compared with the levels in healthy control. This was mainly caused by a reduction in the fraction of open loss of the OEC extrinsic proteins affected the oxygen evolu-reaction centers. The infected plants also showed a reduction tion rates of thylakoid membranes and leaves from infected in the efficiency of excitation capture in PSII by photoprotecplants. Additionally, viral coat protein (CP) was found associ-tive thermal dissipation of excess excitation energy. ated with the chloroplasts and the thylakoid membranes of the phenomenon under biotic stress, as is also observed under abiotic stress conditions. However, the mechanism of action of the viral infection upon PSII remains unclear (see Balachandran et al. 1997).Chlorophyll (Chl) fluorescence measurements show that the PSII photochemical efficiency was decreased by the viral infection (Hodgson et al. 1989, van Kooten et al. 1990, Balachandran and Osmond 1994, Balachandran et al. 1994a). Studies of imaging PSII fluorescence quenching (Balachandran et al. 1994b) also demonstrated that in the youngest, most susceptible leaves of tobacco plants systemically infected with tobbaco mosaic virus (TMV), distur-

We have previously shown that tobamovirus infection induces an inhibition of photosystem II electron transport, disturbing the oxygen-evolving complex (OEC). In the infected plants, the OEC polypeptide pattern was modified when compared to healthy plants, the levels of the PsbP and PsbQ extrinsic proteins being lowered to different extents. In this work we have further investigated by two-dimensional polyacrylamide gel electrophoresis (2-DE) the changes on the OEC protein pattern of thylakoid membranes isolated from Nicotiana benthamiana Domin plants infected with the Spanish strain of pepper mild mottle virus. When the thylakoid membranes from healthy plants were analyzed for the presence of PsbO and PsbP proteins by 2-DE (pI range 4-7) and further immunoassayed by using specific-antisera against these two proteins, it was observed that four polypeptides cross-reacted with each antiserum. These data, along with the N-terminal amino acid sequence determined for the eight polypeptides, indicate that the N. benthamiana PsbO and PsbP proteins correspond to protein families. In the silver-stained 2-DE gels of thylakoid membranes isolated at different days postinoculation from virus-infected plants, it was observed that the content of PsbP polypeptides decreased dramatically with respect to those of PsbO, during the progress of the infection. Interestingly, there was a differential decrease of the different PsbP proteins, indicative of a distinct regulation of their expression.

Changes of thermoluminescence characteristics as well as the O2-evolving capacity was analysed in chloroplasts isolated from Nicotiana benthamiana infected with pepper and paprika mild mottle viruses and their chimeric hybrids. The electron transport activity in thylakoids of virus-infected plants was inhibited and could be restored by adding DPC or Ca2+ which indicated that the virus infection altered the oxygen-evolving complex. In thermoluminescence characteristics of plants infected with either viruses, the first well defined response was a shift in the peak position of the B band from 20 °C to 35 °C corresponding to S3(S2)QB- and S2QB- charge recombinations, respectively, which showed an inhibition in the formation of higher S states in the water splitting system. Simultaneously, a new band appeared around 70 °C due to chemiluminescence of lipid peroxidation. Further progress of the viral infection dramatically decreased the intensity of bands originated from charge recombinations with a concomitant increase of the band at 70 °C indicating the general oxidative breakdown of injured thylakoids

No abstract

No abstract