Abstract
BackgroundXanthomonas oryzae pv. oryzae (Xoo) causes bacterial leaf blight, a devastating disease of rice. Among the type-3 effectors secreted by Xoo to support pathogen virulence, the Transcription Activator-Like Effector (TALE) family plays a critical role. Some TALEs are major virulence factors that activate susceptibility (S) genes, overexpression of which contributes to disease development. Host incompatibility can result from TALE-induced expression of so-called executor (E) genes leading to a strong and rapid resistance response that blocks disease development. In that context, the TALE functions as an avirulence (Avr) factor. To date no such avirulence factors have been identified in African strains of Xoo.ResultsWith respect to the importance of TALEs in the Rice-Xoo pathosystem, we aimed at identifying those that may act as Avr factor within African Xoo. We screened 86 rice accessions, and identified 12 that were resistant to two African strains while being susceptible to a well-studied Asian strain. In a gain of function approach based on the introduction of each of the nine tal genes of the avirulent African strain MAI1 into the virulent Asian strain PXO99A, four were found to trigger resistance on specific rice accessions. Loss-of-function mutational analysis further demonstrated the avr activity of two of them, talD and talI, on the rice varieties IR64 and CT13432 respectively. Further analysis of TalI demonstrated the requirement of its activation domain for triggering resistance in CT13432. Resistance in 9 of the 12 rice accessions that were resistant against African Xoo specifically, including CT13432, could be suppressed or largely suppressed by trans-expression of the truncTALE tal2h, similarly to resistance conferred by the Xa1 gene which recognizes TALEs generally independently of their activation domain.ConclusionWe identified and characterized TalD and TalI as two African Xoo TALEs with avirulence activity on IR64 and CT13432 respectively. Resistance of CT13432 against African Xoo results from the combination of two mechanisms, one relying on the TalI-mediated induction of an unknown executor gene and the other on an Xa1-like gene or allele.
Highlights
Cultivated plants constantly face multiple abiotic and biotic stresses, the latter of which are estimated to cause from 17 to 30% of global yield losses on five of the most important crops including rice (Savary et al 2019)
Germplasm Screening for Transcription Activator-Like Effector (TALE)‐Dependent Resistance Against African Xanthomonas oryzae pv. oryzae (Xoo) Uncovers Three Resistant Rice Varieties To search for African Xoo tales with avr activity, we established a gain-of-function approach consisting in the trans-expression of these tal genes in a virulent recipient strain of Xoo
We first screened a germplasm of 86 accessions of rice and selected those that were susceptible to the Asian Xoo strain PXO99A and resistant to the reference African Xoo strains MAI1 and BAI3
Summary
Cultivated plants constantly face multiple abiotic and biotic stresses, the latter of which are estimated to cause from 17 to 30% of global yield losses on five of the most important crops including rice (Savary et al 2019). Lachaux et al Rice (2022) 15:9 blight (BLB) caused by the bacterial phytopathogen Xanthomonas oryzae pv. BLB may cause up to 50% of yield loss depending on rice variety, growth stage of infection, geographic location and environmental conditions (Liu et al 2014). Oryzae (Xoo) causes bacterial leaf blight, a devastating disease of rice. Some TALEs are major virulence factors that activate susceptibility (S) genes, overexpression of which contributes to disease development. The TALE functions as an avirulence (Avr) factor To date no such avirulence factors have been identified in African strains of Xoo
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