Effect of tobacco mosaic virus movement protein on photosynthesis in transgenic tobacco plants

The tobacco mosaic virus movement protein (TMV-MP) exerts a pleiotropic effect in tobacco plants. In addition to its effect on plasmodesmal size exclusion limit, this protein causes the accumulation of carbohydrates in source leaves and alterations in biomass distribution, resulting in a significantly lower root-to-shoot ratio. In view of the close interaction between the photosynthetic apparatus and carbohydrate metabolism, transgenic tobacco plants expressing TMV-MP under the control of the CaMV 35S promoter were used to study this protein's effect on the photosynthetic system. Gas-exchange analyses indicated that the TMV-MP reduces photosynthetic rate via its influence on mesophyllic factors. The photochemical components of the photosynthetic apparatus, examined by means of chlorophyll fluorescence, revealed no effect on chlorophyll antenna size, susceptibility to photoinhibition, or the variable-to-maximal fluorescence ratio (Fv/Fm). Non-photochemical quenching was higher in TMV-MP-expressing as compared with control plants, whereas the former's photosynthetic yield was lower. These results indicated that the TMV-MP does not exert any effect on the photochemical components of the photosynthetic system. Further study indicated 30% lower phosphate content in TMV-MP-expressing leaves as compared with controls. External application of phosphate solution to detached leaves caused a significant increase in leaves' phosphate content. Moreover, this treatment increased the electron transfer rate (ETR) of TMV-MP-expressing plants to control values. The presented findings indicated that the orthophosphate level constitutes a limiting factor to photosynthesis in TMV-MP-expressing tobacco plants.