Identification of polymorphisms in the Cassava RXAM1 Gene and the association with Cassava Bacterial Blight Resistance

Cassava (Manihot esculenta Crantz) is an essential crop in tropical regions providing food for about one billion people around the world. The most important bacterial disease is cassava bacterial blight (CBB) caused by Xanthomonas axonopodis pv. manihotis (Xam). Recently, a resistance gene called RXAM1 was identified, which codes for a protein that has an extracellular LRR (Leucine-Rich Repeat) domain and a cytoplasmic STK (Serine Threonine Kinase) domain. RXAM1 colocalizes with a QTL explaining 13 % of the resistance to the CIO136 strain of Xam. In this work, the response to infection with the strain XamCIO136 in ten different varieties of cassava was evaluated, which allowed the classification of the cassava varieties TMS60444, SG10735, MCOL1522, MCOL1505 and MCOL2215 as susceptible, while CM6438-14, CM523-7, and MBRA902 were cataloged as resistant. Also, SNPs (Single Nucleotide Polymorphism) polymorphisms were identified in the RXAM1 gene in the same group of cassava varieties. SG10735, CM6438-14, TMS6044, and MBRA685 presented the highest level of polymorphisms, while the varieties CM523-7, CM2177-2 and MCOL1522 were less polymorphic for this gene. The statistical analyses did not allow to identify a significant association between the phenotype and the identified polymorphisms. This study represents a first effort towards the establishment of associating between allelic variants and the cassava response to bacterial blight.