USE OF INTRAMOLECULAR CHIMERAS TO MAP MOLECULAR DETERMINANTS OF SYMPTOM SEVERITY OF POTATO SPINDLE TUBER VIROID (PSTVD)

Nucleotide sequence comparison shows that sequence variations are most ly clustered in the P (pathogenicity) and V (variable) domains of the potato spindle tuber viroid (PSTVd) molecule. Although these compariso ns suggest the P domain as the primary determinant of PSTVd symptom se verity, the potential contribution of the V domain has never been anal ysed in detail. To investigate the relationship between the structure of these domains and pathogenicity, six intraspecific chimeric PSTVd v ariants were constructed by exchanging P and V domains between a mild and two different severe PSTVd isolates. Infectivity studies showed th at the P domain is directly responsible for the severity of symptoms i nduced in tomato. The four recombinants containing a P domain from a s evere isolate caused severe symptoms including severe epinasty, stunti ng and veinal necrosis: while the two chimeras containing the mild iso late P domain induced only mild symptoms. Quantitation of viroid accum ulation in plants infected with the various recombinants suggests that , with the constructions used, symptom severity did not correlate with viroid accumulation, indicating that the P domain did not influence s ymptom production through this simple mechanism.