Nucleotide sequences were determined for two hop stunt viroid-related Group II citrus viroids characterized as either a cachexia disease non-pathogenic variant (CVd-IIa) or a pathogenic variant (CVd-IIb). Sequence identity between the two variants of 95.6% indicated a conserved genome with the principal region of nucleotide difference clustered in the variable (V) domain. Full-length viroid RT-PCR cDNA products were cloned into plasmid SP72. Viroid cDNA clones as well as derived RNA transcripts were transmissible to citron (Citrus medica L.) and Luffa aegyptiaca Mill. To determine the locus of cachexia pathogenicity as well as symptom expression in Luffa, chimeric viroid cDNA clones were constructed from segments of either the left terminal, pathogenic and conserved (T1-P-C) domains or the conserved, variable and right terminal (C-V-T2) domains of CVd-IIa or CVd-IIb in reciprocal exchanges. Symptoms induced by the various chimeric constructs on the two bioassay hosts reflected the differential response observed with CVd-IIa and -IIb. Constructs with the C-V-T2 domains region from clone-IIa induced severe symptoms on Luffa typical of CVd-IIa, but were non-symptomatic on mandarin as a bioassay host for the cachexia disease. Constructs with the same region (C-V-T2) from the clone-IIb genome induced only mild symptoms on Luffa, but produced a severe reaction on mandarin, as observed for CVd-IIb. Specific site-directed mutations were introduced into the V domain of the CVd-IIa clone to construct viroid cDNA clones with either partial or complete conversions to the CVd-IIb sequence. With the introduction of six site-specific changes into the V domain of the clone-IIa genome, cachexia pathogenicity was acquired as well as a moderation of severe symptoms on Luffa.


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