Differential colonization and defence responses of resistant and susceptible melon lines infected by Fusarium oxysporum race 1·2

A green fluorescent protein (GFP)-expressing strain of Fusarium oxysporum f.sp. melonis race 1·2 (FOM 1·2-GFP) was used to visualize infection of a susceptible melon cultivar, Ein Dor (ED). At 1-2 days post-inoculation (d.p.i.), the fungus grew on the root epidermis and adhered to epidermal cell borders. By day 4, the mycelium crossed the cortex and endodermis through narrow pores in cell walls and reached xylem vessels, where it sporulated and produced secondary hyphae that grew upwards. Colonization dynamics of ED seedlings were compared with those of a genetically resistant line, BIZ. FOM 1·2-GFP colonized the resistant plant's vascular system, but the incidence of seedling infection was lower than in ED, suggesting stronger defence responses in BIZ expressed at the pre-xylem stage of infection. Infection of the vascular system of BIZ was slower: at 11 d.p.i., FOM 1·2-GFP only colonized the lower hypocotyl sections of BIZ, whilst the upper hypocotyls of ED were already infected, indicating that BIZ also restricted FOM 1·2 movement in the xylem. The expression patterns of three defence genes were compared between the resistant and susceptible genotypes using real-time PCR. Transcript levels of phenylalanine ammonia lyase (PAL), chitinase (CHI) and hydroperoxide lyase (HPL) were induced to a greater extent in BIZ than in ED. A constitutive two- to fourfold difference between BIZ and ED in the basal levels of all three transcripts was also apparent. Both the constitutive and inducible defence responses could contribute to reduced vascular colonization of the resistant genotype.