Résumé
A Mildew Resistance Locus O (MLO) cDNA was identified from a library of Theobroma cacao L. meristems (Catongo varieties) infected by Moniliophthora perniciosa, the fungus responsible for the witches' broom disease. In other plants, the MLO gene is characterized as a defense and programmed cell death (PCD) modulator, and for this reason may be a good candidate for functional studies aiming the increase of plant resistance. An in silico analysis of the cacao MLO (TcMLO) using the BLAST, Pfam, InterProScan and ORF-Finder programs, as well as a search on CocoaGenDB databank were performed. TcMLO belongs to a multigene family of proteins containing 19 sequences present in the cacao genome: 12, 5 and 2 of them showed one, two and three MLO domains, respectively. The complete TcMLO sequence (including UTRs and ORF) is 5712 bp in length with 13 exons and 12 introns, and is located on the chromosome 5. The TcMLO ORF is 1629 bp in length and encodes a protein with 542 amino acids containing 2 MLO domains. The expression of TcMLO was analyzed by quantitative PCR (qPCR) in resistant (TSH1188) and susceptible (Catongo) cacao varieties infected or not by Moniliophthora perniciosa. Plantlets of cacao were inoculated by the droplet method with a basidiospore suspension of M. perniciosa. After inoculation, the plantlets were kept for 24h at 25±2ºC and 100% humidity. Apical meristems were harvested in triplicates at 24, 48 and 72 hours after inoculation (hai), and 8, 15, 30, 45, 60 and 90 days after inoculation (dai). Non-inoculated plants (controls) were kept and harvested in the same conditions. The qPCR of Tc MLO was obtained using the standard settings of the ABI PRISM 7500 and using the System of Sequence Detection software. The TcMLO expression was analyzed with the comparative Ct method (2-??Ct) using malate dehydrogenase and actin as endogenous reference genes, and non-inoculated plants (control) as calibrator. The results showed that TcMLO was more expressed in Catongo than in TSH1188 at the early and final stages of disease. In TSH1188, the highest expression of MLO was observed at 15 dai. The expression of TcMLO at the final stage of the disease in the susceptible infected plants may be related to the PCD events occurring in this variety as a signal for the finalization of the fungus life cycle. Funding Agency: FAPESB, CNPq, CAPES, EMBRAPA, FINEP/Renorbio and CIRAD. (Texte intégral)