NEW GENE(S) INVOLVED IN THE RESISTANCE OF PONCIRUS-TRIFOLIATA (L.) RAF. TO CITRUS TRISTEZA VIRUS

Citrus tristeza virus (CTV) causes important economic losses in the ci trus industry worldwide. Resistance to CTV is present in Poncirus trif oliata and is known to be controlled by a dominant gene at the Ctr loc us. Short-distance movement of CTV around the inoculum, as well as pas sive movement through the phloem vessels, were studied in segregant pl ants derived by self-pollination from P. trifoliata var. ''Flying Drag on'' in order to genetically analyze the mechanism of CTV resistance. Accumulation of CTV in the vicinity of the inoculum and in new flushes was studied by means of a direct tissue-blot immunoassay (DTBIA). CTV is able to passively move with the phloematic flux from inoculated re sistant genotypes Ctr-Rr and Ctr-RR up to a susceptible scion cultivar (Ctr-rr). Differences regarding CTV accumulation around the inoculum were found among Ctr-Rr individuals of the progeny. Bulked segregant a nalysis identified five RAPD markers linked to a locus (Ctm), or a gen omic region, involved in short-distance accumulation of CTV but locate d in a different linkage group from Ctr. This result indicates that Ct r is not the only locus responsible for resistance to CTV in P. trifol iata, and that at least one other gene is involved. Given that citrus is a perennial crop, breeding for durable disease resistance should ta ke into account selection at both the Ctl and Ctm loci.