Differential expression of CmPP16 homologues in pumpkin (Curcurbita maxima), winter squash (C. moschata) and their interspecific hybrid

Grafting is used in vegetable production as an alternative to highly toxic, broad-spectrum soil disinfectants such as methyl bromide and to avoid soil-borne biotic and abiotic stresses. However, deterioration in valuable qualitative characters of the scion occurs in specific graft combinations, such as melon to pumpkin, which makes this environmental-friendly approach less desirable. Understanding the reasons for rootstock-scion incompatibility, and the role of the rootstock, could help to alleviate this problem. We therefore examined the level of expression of CmPP16, a pumpkin (Cucurbita maxima) phloem protein that mediates RNA transport from the rootstock into scion tissue, in different pumpkin and squash (C moschata) varieties and in a pumpkin X squash interspecific hybrid. All are currently in use as rootstocks in grafted melon, watermelon and cucumber production. Our data revealed that two different genetic loci, CmPP16-1 and CmPP16-2, which encode CmPP16 proteins, are expressed in pumpkin and squash, but not in melon. Pumpkin stems accumulated much higher levels of CmPP16 transcripts than squash or interspecific hybrid stems, while in roots CmPP16-expression was higher in squash. The pumpkin cultivar with the highest levels of expression of CmPP16 was the worst rootstock for melon, suggesting a role for CmPP16 in determining scion quality. Our results, if confirmed in additional varieties and species, may provide an effective way to screen for suitable rootstocks in grafted vegetable production.