Melon phloem-sap proteome: developmental control and response to viral infection

In addition to small molecules such as sugars and amino acids, phloem sap contains macromolecules, including mRNA and proteins. It is generally assumed that all molecules in the phloem sap are on the move from source to sink, but recent evidence suggests that the macromolecules' direction of movement can be controlled by endogenous plant mechanisms. To test the hypothesis that the phloem-sap protein profile is affected by local metabolic activities, we analyzed the phloem-sap proteome in young and mature tissues of melon plants. We also examined the effect of cucumber mosaic virus (CMV) infection and expression of CMV movement protein in transgenic melon plants on the phloem protein profile. Sap collected from cut sections of young stems or petioles contained specific proteins that were absent from sap collected from mature stems or petioles. Most of these proteins were involved in defense response and protection from oxidative stress, suggesting that they play a role in maintaining safe activity of the sieve tubes in young tissues. Phloem sap collected from CMV-infected plants and transgenic plants expressing the CMV movement protein contained only a few additional proteins with molecular masses of 18 to 75 kDa. Here again, most of the additional proteins were associated with stress responses. Our study indicated that the proteome of phloem sap is dynamic and under developmental control. Entry and exit of proteins from the sieve tube can be regulated at the tissue level. Moreover, the plant can maintain regulation of protein trafficking from companion cells to sieve elements under viral infection or other perturbations in plasmodesmal function.