Interaction between phloem proteins and viral movement proteins

Recent studies support the concept that long-distance signals are involved in the regulation of resource allocation among the various plant organs. Following the finding that viral movement proteins (MPs) can exert an effect on sugar metabolism and resource allocation at sites distant from their expression, we suggested that the MPs interfere with an element(s) involved in the plant's endogenous long-distance signal network. To provide experi- mental support for this hypothesis, several unique procedures were employed to identify interactions between viral MPs and phloem sap proteins (PSPs) collected from cut petioles of squash (Cucurbita pepo L. subsp. pepo) and melon (Cucumis melo L.) plants. Far-western experiments with blotted PSPs, using both bacteria-overexpressed and in vitro-translated CMV- and TMV-MPs, revealed that the two virally encoded proteins react specifically with more than one PSP. Moreover, isolation of the naturally folded phloem protein in an affinity column containing a TMV-MP-maltose-binding protein indicated, once again, an interaction between the viral protein and similar PSPs. Two melon PSPs with molecular masses of 8 and 23 kDa were found to specifically interact with both the CMV- and TMV-MPs. The possible effects of this interaction in terms of altering the process of phloem transport and resource allocation are discussed.