Microscopical and molecular characterization of the infection cycle of Phytophthora betacei during disease development on tree tomato (Solanum betaceum).

Phytophthora betacei is a recently described oomycete plant pathogen closely related to Phytophthora infestans sensu stricto. This plant pathogen naturally infects tree tomato (Solanum betaceum) but has not been reported on tomatoes and potatoes, the primary hosts of P. infestans. The aim of this study was to characterize the infection cycle of P. betacei using microscopy and molecular approaches. Several strains were inoculated in susceptible tree tomato plants and disease progression was monitored via six epidemiological parameters. Although different P. betacei strains displayed a highly variable disease phenotype, the most aggressive one was chosen for further plant inoculations. Samples at different time points of the infection cycle were analyzed at the cellular level via light and scanning electron microscopy (SEM) and at the molecular level via qRT-PCR of infection-stage-specific markers. The infection cycle of P. betacei differed from that of P. infestans in having a longer biotrophic stage, larger lesions, and higher sporulation capacity. Additionally, P. betacei transcriptomic profiles were monitored along the infection cycle via RNAseq and evidenced a changing expression landscape that supports an elongated hemibiotrophic transition and a clear distinction from what is being expressed in the mycelium or the sporangia. This study provides novel insights into the interaction between P. betacei and S. betaceum.