Effect of severe and mild Citrus Tristeza Virus isolates on selected genes expression of different citrus hosts.

Seven genes, involved in plant defence response (metallothionein, MT and peroxidase, PRX, alcool-dehydrogenase ADH), signal transduction (CONSTANS-like, CLP and ethylene-responsive binding protein, EREBP), aminoacid transport (aminoacid permease, AP) and regulation of gene silencing (rgs-CaM), were selected to evaluate their modulation in different citrus hosts inoculated by CTV isolates. Plants with different susceptibility to CTV, as mexican lime, sour orange, pineapple sweet orange, Duncan grapefruit and Etrog citron were tested 18 months after inoculation with a severe isolate (SY568 like), whereas sour orange and mexican lime seedlings were also challenged with a mild isolate (T30 like) to compare relative expression levels. Real-time RT-PCR results showed that selected genes exhibited different patterns of expression depending on citrus host. ADH and PRX genes were down regulated in all citrus hosts. Rgs-CaM, AP and CLP were down-regulated in Etrog citron, Duncan grapefruit and sweet orange but were over-expressed in sour orange. Moreover, seedlings of sour orange and mexican lime, the most susceptible hosts, showed a similar expression pattern for ADH, EREBP and PRX (down-regulated), as well as for MT, rgs-CaM and AP (up-regulated) when infected with either SY568 or T30 like isolates. Nevertheless, changes induced in rgs-CaM gene by the severe isolate were 11 and 6 times higher than the mild, respectively in sour orange and mexican lime. Over expression of rgs-CaM gene occurred only in highly susceptible plants showing severe symptoms. This gene appears to act as host specific suppressor of RNA silencing, enabling CTV to evade host RNA silencing activity.