LETHAL NECROSIS, FRUIT NECROSIS AND TOP STUNTING: MOLECULAR- BIOLOGICAL ASPECTS OF THREE CUCUMBER MOSAIC VIRUS-INDUCED DISEASES OF PROCESSING TOMATOES IN ITALY

Three isolates of cucumber mosaic cucumovirus (CMV) named CMV-PG, CMV-Tfn and CMV-TTS and their satellite RNAs (designated CARNA5 for CMV Associated RNAS) were identified as principal etiological agents in three different alterations affecting processing tomato crops in Italy. The disease patterns were defined as lethal necrosis (CMV-PG), tomato fruit necrosis (CMV-Tfn) and tomato top stunting (CMV-TTS). CMV-TTS and CMV-Tfn belong to the WT subgroup (or subgroup I) whereas CMV-PG belongs to the S subgroup (or subgroup II). In the absence of their natural satellite RNAs, all CMV isolates induce typical filimorphism in tomato, however this condition proved to be dramatically affected by the biological expression of their respective satellite RNAs. In particular, PG- CARNA5 was characterized by a necrogenic phenotype. Tfn-CARNAS expressed an ameliorative phenotype on tomato leaves but it was unable to prevent internal fruit necrosis, and TTS-CARNA5 induced the top stunting condition, also with severe effects on the production. In order to determine which factors or sequence domains can affect biological expression of Tfn and TTS-CARNA5, biologically active transcripts were prepared from cDNA clones either using wild sequences or recombinant molecules in which the 5' half was substituted with the 5' half of a different variant using the common Nhe I site. The biological activity of Tfn and TTS satellite RNAs transcribed from respective cDNA clones was found to correlate with phenotypes expressed by wild molecules in tomato. Expression of recombinant molecules demonstrated that the top stunting determining sequence is located within the 5' half of the TTS variant and it is dependent upon functions coded by genomic RNAs 1 and 2 of subgroup WT CMV strains. A domain determining a stunting phenotype is probably present also in the 5' half of the Tfn variant but its expression is dependent upon functions specifically coded by CMV-Tfn strain.