Determination of resistance/susceptibility of important citrus species to Citrus chlorotic dwarf associated virus (CCDaV) and expression analyses of some R genes

Citrus chlorotic dwarf associated virus (CCDaV) is one of the virus diseases adversly effecting yield and quality in citrus production. The disease emerged first in Turkey in the late 1980s and spread to citrus producing areas of the Eastern Mediterranean Region of Turkey. The disease is transmitted by Parabemisia myricae (Homoptera: Aleyrodidae) and virus was also reported in China. In this thesis study CCDaV-BATEM-M isolate was identified, biological and molecular characteristics of this isolate was determined. The result of the multiple sequence comparision of the CP gene at amino acid and nucleotide level revealed that the isolates share 99-100% similarity to other isolates from Turkey and other countries in the world. Then, resistance status of commonly grown citrus scpecies in Turkey to CCDaV were determined. In this respect, six different citrus species were inoculated with CCDaV and virus load at 3, 6, 9 and 12 months after inoculation was determined by absolute quantitation using real-time PCR. By comparing results obtained, resistance levels of these citrus species to CCDaV were determined and it was shown that while lemon was the most sensitive, trifoliate orange the most resistant species to CCDaV. In addition, expression of some R genes were analyzed in the most resistant and susceptible citrus species to determine R genes associated with CCDaV resistance. For this, first 15 R genes encoded in genome of sweet orange were selected and gene spesific primers were designed for each R gene. Then lemon and trifolaite orange plants were inoculated with CCDaV by grafting and total RNA was isolated from leaf samples collected from the infected and control plants at at 3, 6 and 9 months after inoculation. The expressions of the fifteen selected R genes in the infected and controls plants were determined by real-time RT-PCR method. As a result, while the expression of six R genes increased in response to CCDaV infection in resistant plants the expression of the others did not change. The expression analyses revealed some R genes are differentially expreesed in resistant trifoliate orange and susceptible lemon in reponse to CCDaV infection. The results showed that seven differentily expressed R genes identified in this study may be important for CCDaV resistance.