Transcription Activator-Like (TAL) Effectors of the Cassava Bacterial Blight Pathogen Xanthomonas axonopodis pv. manihotis
- Author(s): Shybut, Mikel
- Advisor(s): Staskawicz, Brian J
- et al.
Cassava is an essential food crop relied on by hundreds of millions of people worldwide. Xanthomonas axonopodis pv. manihotis (Xam) is the causal agent of cassava bacterial blight (CBB) and the leading destructive bacterial pathogen of cassava. Xam utilizes a unique class of type three effectors known as transcription activator-like (TAL) effectors (TALEs) to activate specific host genes that contribute to virulence and bacterial growth. In Xam, TALEs are often localized to plasmids. TALE-containing plasmids of other Xanthomonas ssp. have been shown to be conjugative, providing a mechanism for the horizontal transfer of TALE virulence components. Here, I characterize the two-TALE containing plasmid pXam46 of Xam isolate CIO151, providing a full draft sequence, TALE virulence assays, and evidence for its mobilizing ability. The potential horizontal transfer of TALEs suggests that TALEs which confer a strong virulence phenotype may be well conserved amongst Xam communities. I screened a subset of 22 global Xam isolates spanning 28 years of evolution from 5 countries over 3 continents for their TALE repertoires. I identified one pair of highly conserved TALEs, including a single repeat variable diresidue (RVD) variant of a pXam46 localized TALE, and 3 additional well-conserved TALEs. Of the two highly conserved TALEs, both contribute to bacterial growth in planta and one is associated with a water soaking disease phenotype. The remaining 3 well-conserved TALEs did not show any measurable contributions to virulence. Some plants contain an evolutionary mechanism to defend against TALEs, carrying executor resistance (R) genes containing TALE binding elements upstream of disease resistance genes. I found that the highly conserved TALE of pXam46 triggers a specific, transcriptionally dependent HR-like phenotype in the non-host Nicotiana benthamiana. Employing RNA-seq, I have identified a list of candidate TALE-upregulated genes that may be involved in the defense response of N. benthamiana. Identifying TALE-triggered R genes as well as conserved TALEs and their susceptibility targets can assist in the design of durable resistance strategies against Xam. Successful strategies may include stacking promoters of multiple conserved TALEs in front of R genes or modifying cassava susceptibility gene promoters to abrogate TALE binding. It is my hope that the basic biology described herein may assist in those efforts.