EVALUATION OF SWEET ORANGE TRANSGENIC LINES CHALLENGED WITH PSOROSIS ISOLATES

ZANEK MC; REYES CA; VELÁZQUEZ K; COSTA N; PLATA MI; GARCIA ML

Campinas, Brazil

Conferencia; XVIII Conference of the International Organization of Citrus Virologists (IOCV); 2010

IOCV

Citrus psorosis, first described by Swingle & Webber (1896), is a serious viral disease affecting citrus trees in many countries (Roistacher, 1993). In Argentina, it causes important losses (Danós, 1990), and seems to be spread by an unknown vector (Beñatena and Portillo, 1984). Fawcett and Klotz (1938) described two types of psorosis diseases called Psorosis A (Ps A) and Psorosis B (Ps B), both systemic in all infected trees, and graft-transmitted. Ps A induces bark scaling restricted to the main trunk and limbs, and chlorotic flecks and spots in young leaves. Ps B is the more aggressive type showing extensive bark scaling, affecting thin branches and limbs, blotching in old leaves, with brownish gum-impregnated eruptions on the leaf underside and pustules on old leaves (Aliotto et al., 2007). Roistacher (1993) proposed a specific diagnostic test for Ps A using a cross-protection assay with Ps B as the challenger. In this test sweet orange seedlings are graft-inoculated with tissue of the candidate tree, and later grafted with a Ps B infected tissue. Ps A-free plants reveal Ps B symptoms after 3-6 months, whereas Ps A-infected plants will be protected from the second inoculum. Cross-protection occurs because the two types of symptoms are produced by different isolates of the same virus. The causal agent, Citrus psorosis virus is tripartite and its genome consists of three ssRNAs of negative polarity. Pathogen-derived resistance (PDR) has been applied to generate resistant plants to viral diseases. For this purpose, we have obtained sweet orange lines expressing three different CPsV genes, the coat protein, the 54K protein and 24K protein. The transgenic lines were challenged with a Ps A isolate (CPV-4) and with a Ps B isolate from Argentina (CPsV 189-34) but the plants were not tolerant to CPsV infection. However, the percentage of infected plants with Ps B isolate was slightly lower than the infected with the Ps A isolate along the analyzed flushes, besides it was observed a delay in the appearance of the symptoms compared with the controls, indicating a very weak protection.