S WOLF and D SHALITIN

Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, Rehovot 761 00, Israel

Background and objectives
It is now well established that most if not all viruses move from cell to cell via plasmodesmata. This process requires an interaction between a specific virally encoded protein, termed movement protein (MP), and proteins within the host plant. Evidence accumulated over recent years has established that these viral MPs have the capacity to alter plasmodesmal function. Transgenic tobacco plants expressing tobacco mosaic virus (TMV) MP have been used to explore the role of plasmodesmata in terms of the transport of photosynthate. Based on the hypothesis that plasmodesmata provide the cell-to-cell pathway for symplasmic movement of carbohydrates, it seemed logical to assume that, if diffusion through plasmodesmata were rate limiting for sucrose transport, major changes in plasmodesmal size exclusion limit (SEL) would result in an increase in the rate of sucrose movement from the mesophyll to the site of loading in the phloem.

Results and conclusions
Analyses of diurnal accumulation of carbohydrates and export rates from source leaves, combined with micro-injection experiments in which plasmodesmal function was determined, established that the TMV-MP has a pleiotropic effect on leaf physiology. These findings suggested that TMV-MP has at least two sites of action: one that modifies plasmodesmal SEL and another that affects carbohydrate metabolism and allocation.

Transgenic potato plants expressing the TMV-MP under the control of several tissue specific promoters were employed to further explore the mode by which this viral protein interacts with cellular metabolism to change carbohydrate allocation. When restricted to photosynthetic tissues, the TMV-MP induced a reduction in the level of sugars and starch in source leaves compared with control potato lines [1]. However, when the TMV-MP was expressed predominantly in phloem parenchyma and companion cells, sucrose and starch accumulated to high levels, whereas the rate of sucrose export from excised petioles was lower than that observed with control plants [2]. Perhaps the most significant result obtained from experiments performed on these potato plants was the discovery that the influence of the TMV-MP on carbohydrate metabolism within source leaves was under developmental control and was exerted only after tuber initiation.

In the light of these results, together with recent findings that plasmodesmata facilitate the cell-to-cell transport of macromolecules including proteins and nucleic acids, we advanced the hypothesis that trafficking of regulatory (information) molecules through plasmodesmata may establish a special supracellular communication network between the companion and mesophyll cells, which operates to regulate carbon partitioning [3]. According to this model, the supracellular communication system facilitates efficient orchestration of the network of biochemical and physiological processes in photosynthate metabolism and transport. The TMV-MP may interfere at several loci within this putative communication network, and therefore can induce an alteration in carbon metabolism within source leaves, either when expressed in the mesophyll or within the phloem. The current study is focused on identifying plant endogenous molecules (proteins) that interact with viral MPs and may be involved in the regulation of photosynthate transport in the phloem.

References
1. Olesinski AA, Almon E, Navot N et al., 1996. Plant Physiology 111, 541-550.
2. Almon E, Horowitz M, Wang H-L et al., 1997. Plant Physiology 115, 1599-1607. 3. Wolf S, Lucas WJ, 1997. In Foyer CH, Quick P, eds, A Molecular Approach to Primary Metabolism in Plants, pp. 219-236.