Reduced Photosystem II Activity and Accumulation of Viral Coat Protein in Chloroplasts of Leaves Infected with Tobacco Mosaic Virus

We previously reported (A Reinero, RN Beachy 1986 Plant Mol Biol 6:291-301) that coat protein (CP) of tobacco mosaic virus (TMV) accumulates in chloroplasts of systemically infected leaves. To determine the significance of such interaction we examined electron transport rates in chloroplasts containing different levels of TMV-CP. Tobacco (Nicotiana tabacum L.) plants were infected with either a TMV strain inducing chlorosis or with a strain inducing mild symptoms, and both the accumulation pattern of TMV-CP inside chloroplasts as well as the rates of photosynthetic electron transport were followed. The CP of the TMV strain inducing chlorosis was detected inside chloroplasts 3 days after infection, and thereafter accumulated at a rapid rate, first in the stroma and then in the thylakoid membranes. On the other hand, the CP of the TMV strain that caused only mild symptoms accumulated in chloroplasts to lower levels and little CP was associated with the thylakoids. In vivo and in vitro measurements of electron transport revealed that photosystem II activity was inhibited in plants infected with the aggressive TMV strain while no reduction was observed in plants infected with the mild strain. The capacity of chloroplasts to synthesize proteins was equivalent in organelles isolated from healthy and virus-infected leaves. The possibility that a large accumulation of TMV-CP inside chloroplasts may affect photosynthesis in virus-infected plants by inhibiting photosystem II activity is discussed.