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Characterization of the infection cycle of Phytophthora betacei during disease development on tree tomato (Solanum betaceum).

Natalia Guayazan: Universidad de los Andes

<div><em>Phytophthora betacei </em>is a recently described oomycete plant pathogen closely related to <em>Phytophthora infestans sensu stricto</em>. This plant pathogen infects tree tomato (<em>Solanum betaceum</em>) crops, but is not able to infect potatoes or tomatoes, the main hosts of <em>P.infestans</em>. The aim of this study was to characterize the infection cycle of <em>P.betacei</em> using microscopy and molecular approaches. To this end, susceptible tree tomato plants were inoculated with a sporangial solution, and the progression of the disease was monitored based on six epidemiological parameters. The results indicated <em>P.betacei </em>displays a highly variable disease phenotype. To understand the infection cycle at the cell and molecular level, one aggressive strain was selected and inoculation assays were performed. Samples collected from the inoculated plants 3, 6, 9, 12, 24, 48, 72, and 96 hpi were visualized using Scanning Electron Microscopy (SEM) and light Microscopy (trypan blue staining). Progression of the disease was validated with qPCR by assessing the expression two infection-stage specific markers: <em>Hmp1</em> and <em>Cdc14</em>. Results indicated that <em>P. betacei </em>differs from <em>P. infestans</em> in that the infection cycle of the former is characterized by a longer biotrophic stage, greater lesions and greater sporulation capacity <em> </em>when evaluated on the same host. Varying levels of expression were detected for both genes along the infection cycle, and their profile was consistent with the results observed on the whole plant inoculations. Furthermore, transcriptomic analyses on time course inoculation samples revealed a highly regulated pathogen and host dynamics. De novo assembly of <em>P.betacei</em> transcriptome enabled identification of conserved infection-promoting as well as unique novel effector genes. This study provides insights into the interaction between <em>P.betacei </em>and <em>S.betaceum</em>.</div>