Late Blight of Tomato - Translational Genomics for Crop Breeding: Biotic Stress - page 246

(a) (b)

(c) (d)

Fig. 13.1. Phytophthora infestans infection cycle on susceptible tomato cv. Rumba, as visualized by dual stain and
confocal laser microscopy (Nowicki et al. 2012). Red channel: pathogen-specific stain and plant autofluorescence; green
channel: callose-specific stain; black and white: DIC. (a) Germinating spore [s] develops infection tube and attempts leaf
tissue penetration by formation of appressorium [a]. Host plant's primary line of defense is rapid formation of callose-rich
papilla [p] directly underneath the attempted penetration area. (b) Same as A, but visible in DIC. (c) Upon successful infection,
the pathogen grows in the leaf mesophyll, as denoted by the numerous hyphae. (d) At the end of the infection cycle, the
hyphae emerge through the abaxial leaf-side stomata, followed by sporangial development. Rapid field infestation is achieved
through successful colonization; descendant sporangia will drive the epidemics through the season. For a color version of
this figure, please refer to the color plate.

The sexual life cycle of P. infestans requires
the mating of individuals with opposite mat-
ing types, known as A1 and A2 (Gallegly and
Galindo 1958; Gisi et al. 2011). The mating
types are not dimorphic forms of P. infestans ,

but are compatibility types differentiated by mat-
ing hormones (Judelson 1997). When mycelia of
the two mating types interact, mating hormones
induce gametangial formation in the opposing
mating types, resulting in sexual reproduction. In