To study the host plant-viroid interaction, green bark samples of Troyer citrange, sour orange and Alemow seedlings inoculated with a Citrus viroid-IIIb (CVd-IIIb) source were processed by a rapid and sensitive RT-SYBR Green I-based real time assay to assess molecular events taking place at an early stage of infection. The messenger RNA (mRNA) differential display technique was applied for the identification and isolation of genes whose transcription was altered significantly in Etrog citron

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... ves infected by CVd-IIIb. Seven of them, known to be related to plant stress defense, were selected to evaluate the variation of the expression level in Troyer citrange, sour orange, and alemow seedlings by Northern Blot analysis and reverse-transcriptase quantitative real time PCR (RT-qPCR). Both methods were valuable, but the latter was more sensitive. The seven genes were detected, in the three rootststocks, as up-or down-regulated. The inoculation of CVd-IIIb in Troyer citrange seedlings modified the expression of metallothionein (MT), alcohol dehydrogenase (ADH), ethylene-responsive binding protein (EREBP), regulator of gene silencing (RGS), peroxidase (PRX) and CONSTANS-like at a lower level than in Etrog citron but with a similar pattern. Alemow seedlings showed a slight over-expression of MT and ADH; in contrast, PRX was severely down regulated. Sour orange seedlings did not reveal any signal with the exception of PRX and CONSTANS, which were up regulated as in Etrog citron. RT-qPCR allowed the detection of the genes, which escaped the Northern Blot analysis, showing in sour orange the same pattern (over-expression) as Etrog citron for ADH, EREBP and AP; whereas MT and RGS were down regulated. In alemow seedlings, EREBP, RGS, AP and CLP showed a reduction of the expression level as well as AP in Troyer citrange.