Ultrastructure, autofluorescence, callose deposition and lignification in susceptible and resistant muskmelon leaves infected with the powdery mildew fungus Sphaerotheca fuliginea

Abstract Leaves of the susceptible cv. Ananas-Yokneam (AY) and of resistant cvs PI 124111F, PI 124112, PMR-45, PMR-6 of muskmelon ( Cucumis melo ) were inoculated with either race 1 or race 2 of Sphaerotheca fuliginea and examined microscopically after staining with Calcofluor, basic aniline blue or phloroglucinol. At 20–24 h post inoculation of the susceptible AY at 23 °C, the fungal spores (both race 1 or race 2) developed one or two germtubes which penetrated into one or two epidermal cells. The penetration zones were surrounded with callose-like material but no autofluorescence nor lignin-like materials were observed in the penetrated epidermal cells. A similar response was observed in PMR-45 inoculated with race 2 (compatible). In contrast, the fungus developed a single germtube on the resistant PI 124111F, PI 124112, and PMR-6 inoculated with either race, as well as in PMR-45 inoculated with race 1, which induced autofluorescence, callose accumulation and lignification in the penetrated epidermal cells. Electron microscopical studies revealed that the rapid collapse of epidermal cells in the resistant cultivars was accompanied by the accumulation of callose-like deposits in cell walls and around haustoria, electron-opaque deposits in the plasma membrane and electron-opaque deposits between the cell wall and the plasma membrane. Occasionally, callose also appeared in epidermal and mesophyll cells adjacent to the penetrated cells. By 96–120 h post inoculation, abundant sporulation was observed in the compatible interactions whereas only 1–3 germtubes with no sporulation were seen in the incompatible interactions. Heat shock or chemical inhibitors (cycloheximide, blasticidin-S, cordycepin, 2-deoxy-D-glucose, α-aminooxyacetic acid, 2,4-dinitrophenol and sodium azide) failed to induce susceptibility in the resistant cultivars. The results suggest a similar structural response to powdery mildew in C. melo cultivars carrying the resistance genes Pm-1 (PMR-45), Pm-2 (PMR-6), Pm-3 (PI 124111F), Pm-4 (PI 124112), Pm-5 (PI 124112) and Pm-6 (PI 124111F).