EXPRESSION OF A LUTEOVIRAL MOVEMENT PROTEIN IN TRANSGENIC PLANTS LEADS TO CARBOHYDRATE ACCUMULATION AND REDUCED PHOTOSYNTHETIC CAPACITY IN-SOURCE LEAVES

Elucidating the role of viral genes in transgenic plants revealed that the movement protein (MP) from tobacco mosaic virus is responsible fo r altered carbohydrate allocation in tobacco and potato plants. To stu dy whether this is a general feature of viral MPs, the movement protei n MP17 of potato leafroll virus (PLRV), a phloem-restricted luteovirus , was constitutively expressed in tobacco plants. Transgenic lines wer e strongly reduced in height and developed bleached and sometimes even necrotic areas on their source leaves. Levels of soluble sugars and s tarch were significantly increased in source leaves. Yet, in leaf lami nae the hexose-phosphate content was unaltered and ATP reduced to only a small extent, indicating that these leaves were able to maintain ho meostatic conditions by compartmentalization of soluble sugars, probab ly in the vacuole. On the contrary, midribs contained lower levels of soluble sugars, ATP, hexose-phosphates and UDP-glucose supporting the concept of limited uptake and catabolism of sucrose in the phloem. The accumulation of carbohydrates led to a decreased photosynthetic capac ity and carboxylation efficiency of ribulose-1,5-bisphosphate carboxyl ase/oxygenase (rubisco) probably owing to decreased expression of phot osynthetic proteins. In parallel, levels of pathogenesis-related prote ins were elevated which may be the reason for the obtained limited res istance against the unrelated potato virus Y (PVY)(N) in the transgeni c tobacco plants. Ultrathin sections of affected leaves harvested from 2-week-old plants revealed plasmodesmal alterations in the phloem tis sue while plasmodesmata between mesophyll cells were indistinguishable from wild-type. These data favour the phloem tissue to be the primary site of PLRV MP17 action in altering carbohydrate metabolism.