Cucurbits have been defined as symplastic phloem loaders, translocating mainly raffinose and stachyose. To examine whether the mechanism of phloem loading in cucurbit plants is plastic and if CMV infection activates an apoplastic mode of sugar loading into the sieve tubes, we identified and cloned three sucrose transporter genes in melon plants-CmSUT1, CmSUT2 and CmSUT4. In this book, we demonstrate that CMV infection alters the mechanism of phloem loading in melon plants by activating an apoplastic mode of sugars loading into the sieve tube, probably through the activation of CmSUT1. This alteration was not associated with lower activity of RFOs biosynthesis enzymes. Therefore, there is plasticity in the mechanism of phloem loading in cucurbit plants. We also suggest that CmSUT2 acts as a sugar sensor which is upregulated under stressful conditions, and propose that CmSUT4 is not involved in the process of phloem loading but is probably associated with the compartmentation of sucrose in photosynthetically active cells.