Differential display analysis of gene expression in Etrog citron leaves infected by Citrus viroid III

Citrus are natural hosts of several viroids, which are plant pathogens composed exclusively of a non-protein-coding, small single-strandedcircular RNA that is able to replicate autonomously in susceptible hosts. They are responsible for symptoms such as stunting, leaf epinasty, andchlorosis. Citrus viroid III (CVd-III) has been long regarded as a possible dwarfing agent of citrus grafted on trifoliate orange and its hybrids. Toinvestigate molecular mechanisms involved in pathogenesis, the messenger RNA (mRNA) differential display technique was here applied toidentify genes whose transcription was significantly altered in leaves of Etrog citron (Citrus medica) infected by CVd-III (variant b). Of eighteengenes identified, thirteen were up-regulated by viroid infection, while five were down-regulated. Except for two genes that encode proteins ofunknown function, the remaining genes are mainly involved in plant defence/stress responses, signal transduction, amino acid transport, and cellwall structure. Among the up-regulated genes, it is noteworthy a suppressor of RNA silencing that might be involved in viroid and viruspathogenicity. The functions of these genes are discussed.