Effector-induced susceptibility as a quantitative trait: Transcription activator-like (TAL) effectors are important avirulence (disease causing) proteins produced by many Xanthomonas species. They are able to regulate transcriptional activity of host genes that contain a matching sequence in the promoter of the target gene, termed effector binding element (EBE). A subset of TAL effectors target host genes to enrich bacterial infection causing disease, and thus are termed susceptibility (S) genes. In response, the host has evolved mechanisms of resistance to circumvent bacterial infection. In rice, these include: 1) dominant non-transcriptional, including NLR resistance proteins which fall under the typical ligand-receptor mechanism; 2) dominant TAL effector-dependent resistance, which leads to the transcriptional activation of major resistance (R) genes; and 3) loss of susceptibility, which results from polymorphisms in the EBE of rice S-genes.
The first two mechanism describe above depend on the binary interaction between a single dominant bacterial avirulence gene and one major R-gene in the host. While some effectors are in the dominant avirulence category, most are auxiliary avirulence effectors and incrementally contribute to pathogen virulence. Their conservation in many bacterial strains of Xoo raises questions we are studying in the Huerta Lab:
What are these auxiliary avirulence effectors targeting in the host genome?
How are these contributing to disease?
Does host allelic variation impact disease severity to these auxiliary avirulence factors?
Figure 1. Predicted Tal7b gene targets overlap with QTL conferring resistance to PXO99A pHMI::tal7b. (a) Integrated physical map of rice chromosomes 1, 3, 5, 8, 10, 11 and 12 showing QTL identified in PXO99A pHMI::EV (cyan), PXO99A pHMI::tal7b (magenta), predicted Tal7b targets (horizontal lines) and previously described resistance loci for bacterial blight (vertical lines). (b) Sequence logo showing predicted EBE of Tal7b using predicted targets in PXO99A pHMI::tal7b‐specific QTL only. (c) Sequence logo showing predicted EBE of Tal7b using predicted targets from a genome‐wide scan. (d) Predicted Tal7b gene targets per QTL unique for PXO99A pHMI::tal7b. (e) Top ten predicted Tal7b gene targets that overlap with PXO99A pHMI::tal7b‐specific QTL.
Huerta, Alejandra I., Emily E. Delorean, Ana M. Bossa‐Castro, Bradley W. Tonnessen, Chitra Raghavan, Rene Corral, Álvaro L. Pérez‐Quintero, Hei Leung, Valérie Verdier, and Jan E. Leach. (2020) Resistance and susceptibility QTL identified in a rice MAGIC population by screening with a minor‐effect virulence factor from Xanthomonas oryzae pv. oryzae.” Plant Biotechnology Journal (2020). https://doi.org/10.1111/pbi.13438
Huerta, A.I., Cohen, S.P., Verdier, V., Leach, J.E. (2018). Molecular genetics of bacterial blight and bacterial leaf streak and their impact on future control strategies in: Rice diseases: Biology and selected management practices. Mew TW, Hibino H, Savary S, Vera Cruz CM, Opulencia R, Hettel GP, eds. 2017. Los Baños (Phillipines) International Rice Research Institute. PDF e- book.
Triplett, L. R., Cohen, S. P., Heffelfinger, C., Schmidt, C. L., Huerta, A. I., Tekete, C., Verdier, V., Bogdanove A.J. & Leach, J. E. (2016). A resistance locus in the American heirloom rice variety Carolina Gold Select is triggered by TAL effectors with diverse predicted targets and is effective against African strains of Xanthomonas oryzae pv. oryzicola. The Plant Journal, 87(5), 472-483. https://doi.org/10.1111/tpj.13212