MOLECULAR DIFFENTIATION OF WEAK AND SEVERE CITRUS TRISTEZA VIRUS ISOLATES IN MÉXICO

Abstract

The Citrus Tristeza Closterovirus (CTV) is the more important viral disease of citrus trees in the world. Over 100 million trees were lost in the past 20 years in South America, U.S.A, Israel and Spain. The CTV is transmitted by the aphid Toxoptera citricida and by infected vegetative material. The presence of the virus and the aphid in the Mexican territory constitute a real menace, since most of the commercial plants (> 90 %) are grafted on C. aurantium which is susceptible to CTV, so there could be an epidemic. The molecular characterization of different races, including the complete genome sequence of the CTV, has allowed progresses in obtaining transgenic citric cultivars resistant to this virus. This study was aimed to develop an method for the differentiation of CTV races, based on the molecular characterization of the viral genome. For that purpose we amplified the coat protein gene, and we examined its polymorphisms with restriction enzymes that might allow differentiation between weak and severe races. Our results show that using the enzyme Hae III in PCR products of the gene p25, was possible to relate its genetic profiles to the different CTV races. The use of Hae III allowed to distinguish the weak (CBG-T2, CBG-V2) from the severe (CBG-NL1, CBG-NL2, CBG-V1, H33) races. Furthermore, the enzyme Kpn I allowed to differentiate those races causing stem pitting from those inducing decline. The analysis of the sequence of the gene p25 showed three possible amino acids (glycine, threonine and phenylalanine) in positions 49 and 63 and 124, respectively, that could be involved in the pathogenicity of the CTV, since they are conserved in the severe races.