Confirmation report of Bacterial Leaf Streak disease of rice caused by Xanthomonas oryzae pv. oryzicola in Senegal.

Xanthomonas oryzae pv. X. oryzicola (Xoc), the causal agent of Bacterial Leaf Streak (BLS), is considered as one of the most important emerging pathogens of rice in Africa. This disease is estimated as responsible of 20 to 30% yield loss (Sileshi et Gebeyehu 2021) and has been characterized in several west African countries including Mali and Burkina Faso since 2003 and more recently in Ivory Coast (Wonni et al. 2014, Diallo et al. 2021). Presence of BLS symptoms in Senegal were reported by Trinh in 1980 but, to our knowledge, BLS occurrence has never been validated further and no strain of Xoc have ever been isolated from Senegalese rice fields. Xoc is transmitted by seeds which contribute to its spread through the rice trade (Sileshi et Gebeyehu 2021). To confirm Trinh's observations, we surveyed rice fields between 2014 and 2016 in eight different regions where rice is produced in Senegal. Typical disease symptoms characterized by yellow-brown to black translucent leaf streaks sometimes along with exudates, were detected in fields of several regions and collected. Leaf pieces were successively sanitized, rinsed in sterile water, and symptomatic fragments were ground using the Qiagen Tissue Lyser System (QIAGEN, Courtaboeuf, France). The leaf powder was diluted in 1.5 ml of sterile water and incubated for 30 minutes at room temperature. Ten µl of the suspension was streaked on semi-selective PSA medium and incubated at 28°C for 3 to 7 days. Characteristic round, convex, mucous, straw-yellow Xoc candidate colonies were purified from six individual leaf samples from three distinct sites in the northern Senegal River Valley. To confirm their identity, isolated strains were tested for pathogenicity and molecular characterization. All isolates were subjected to the multiplex PCR developed for the identification of X. oryzae pathovars (Lang et al., 2010) and revealed the same PCR profile (two amplicons of 324 and 691 base pairs) similar to that of the Xoc reference strain BLS256. Leaves of 5-week-old plants of O. sativa cv. Kitaake were infiltrated with a needleless syringe containing a bacterial suspension set at an optical density of 0.5. Upon seven days of incubation under greenhouse conditions (27 ± 1°C with a 12-hour photoperiod), all infiltrated spots (2 spots on 3 plants per isolate) developed water-soaked lesions similar to those caused by control strain BLS256, except when leaves were infiltrated with water. Symptomatic leaf tissues were ground and plated on PSA medium, resulting in colonies with typical Xanthomonas morphology that were diagnosed as Xoc by multiplex PCR typing, thus fulfilling Koch's postulate. At last, four of the isolates were subjected to gyrB sequencing upon PCR amplification using the universal primers XgyrB1F and XgyrB1R (Young et al., 2008). Analysis of 780bp partial gyrB sequences of strains S18-3-4, S23-1-12, S52-1-4 and S52-1-10 highlighted 100% identity with the gyrB sequence of strain BLS256 (Acc. No. CP003057). To our knowledge, this is the first report of BLS in Senegal which is supported by molecular characterization methods. This study validates the presence of BLS in Senegal and will serve as a basis for future efforts of rice breeding for locally adapted resistance. More studies are needed to clarify the spatial distribution and prevalence of BLS in Senegal as rice cultivation is expanding rapidly in the country.

TAL Effectors with Avirulence Activity in African Strains of Xanthomonas oryzae pv. oryzae.

BACKGROUND: Xanthomonas 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. RESULTS: With 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. CONCLUSION: We 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.