Using in vitro plants to study the cassava response to Xanthomonas phaseoli pv. manihotis infection

Cassava Bacterial Blight (CBB), caused by Xanthomonas phaseoli pv. manihotis ( Xpm ), is the most important bacterial disease of cassava ( Manihot esculenta ). Xpm employs Transcription Activator-Like effectors (TALE) to induce the expression of host susceptibility ( S ) genes favoring bacterial growth. The identification of resistant and susceptible cassava varieties as well as the identification of the cassava gene MeSweet10a as a target of the effector TAL20, have been conducted on plants grown from mature stem-cuttings. We explored the possibility of using in vitro plants to study cassava- Xpm interactions. Cassava in vitro plants of varieties 60444 and CM6438–14 were susceptible and resistant, respectively, to infection by strain Xpm668. In addition, the expression of MeSweet10a was induced in the susceptible but not in the resistant variety, which was not associated with polymorphisms in the Effector Binding Element (EBE) reported for TAL20. Three other Xpm strains, Xpm531, Xpm681 and Xpm1061, were also able to induce the TAL20 cognate target gene in the susceptible cassava variety 60444. These results demonstrate the usefulness of in vitro plants to study the phenotypic and molecular responses of cassava during Xpm infection.