Long lasting rubber tree resistance to Microcyclus ulei

The resistance of the MDF180 Hevea brasiliensis cultivar to South American leaf blight (SALB) caused by the fungus Microcyclus ulei has been studied under both controlled conditions of inoculation and natural infestation in the Brazilian state of Bahia. Inoculated isolates on this cultivar in most cases developed sporulating lesions and produced conidiospores, but at a moderate level, characteristic of a race non-specific, partial resistance. The teleomorph of M. ulei has never been reported on this cultivar, either in controlled conditions or in natural infestation, whereas it is commonly observed on other susceptible and partially resistant cultivars. This type of resistance has seldom been described for other pathosystems. MDF180 resistance now enduring for more than 30 years in areas very conducive to the disease, it can therefore be described as a durable resistance. To initiate a comprehensive approach of the resistance, five cDNA libraries were built and analyzed comparing up-regulated genes of MDF180 and PB314 (a susceptible genotype) from 6 h post-infection (hpi) to 58 days post-infection (dpi). We sequenced 8,027 expressed sequence tags (EST). After control of their quality, trimming and assembling, we obtained 1,165 singlets and 458 contigs. Based on sequence similarity 49% of the sequences could be assigned to functional categories, whereas 21% corresponded to already described sequences but with unknown function and 30% did not show any significant similarity with other sequences. Macroarray expression analysis allowed the identification of 311 genes differentially regulated during the infection process of both genotypes. Focusing on candidate genes associated with stress and defense, twelve genes were up-regulated in young lesions of MDF180 (e.g. NBS-LRR, cf5, Peroxydase, Pathogenesis-Related-Protein, cystatin) while seven genes appeared up-regulated in older lesions of PB314 (e.g. SOD, LRR, chitinase). These results provided a first contribution to the understanding of molecular mechanisms of the MDF180 resistance to South American leaf blight and provide information for the further exploration of signalization pathways involved in this durable resistance. (Résumé d'auteur