Effect of CMV infection and high temperatures on the enzymes involved in raffinose family oligosaccharide biosynthesis in melon plants

Ultrastructural and molecular studies have provided experimental evidence for the classification of cucurbits as symplastic loaders, mainly translocating the raffinose family oligosaccharides (RFOs) raffinose and stachyose. Earlier studies established that cucumber mosaic virus (CMV) infection causes a significant increase in the sucrose-to-RFO ratio in the phloem sap of melon plants. The alteration in phloem sap sugar composition was associated with upregulation of CmSUT1 transcript within the vascular bundles. The current research aimed to explore the effect of CMV infection on the enzymes involved in symplastic phloem loading and RFO biosynthesis. Viral infection did not affect the activity of either raffinose or stachyose synthases in source leaves, but caused upregulation of the respective transcripts. Interestingly, activity of galactinol synthase was higher in CMV-infected leaves, associated with upregulation of CmGAS2. A significant increase in CmGAS2 expression in source leaves of melon plants exposed to high temperatures indicated that this response is common for both biotic and abiotic stresses. However, the effect of CMV or heat stress on phloem sap sugar composition is not due to alteration in RFO biosynthesis.