IDENTIFICATION OF TOMATO BUSHY STUNT VIRUS HOST-SPECIFIC SYMPTOM DETERMINANTS BY EXPRESSION OF INDIVIDUAL GENES FROM A POTATO-VIRUS-X VECTOR

In this study, we analyzed the influence of two nested genes (p19 and p22) of tomato bushy stunt virus (TBSV) on disease symptoms in systemi cally infected plants and in local lesion hosts. The contribution of i ndividual genes was determined by bioassays with an infectious clone o f wild-type TBSV, with p19/p22 mutant derivatives, and by expression o f individual TBSV genes from a heterologous potato virus X (PVX) vecto r. Our results showed that TBSV genes could be expressed at high level s from the PVX vector. The subcellular localization of these proteins as well as the ability of PVX-expressed p22 to trans complement TBSV c ell-to-cell movement defective mutants indicate that the exogenously e xpressed proteins are functionally active. Inoculation studies with TB SV mutants and the PVX derivatives demonstrated that p19 induced a gen eralized necrosis upon systemic infection of Nicotiana benthamiana and N. clevelandii. In addition, p19 elicited the formation of local necr otic lesions in N. tabacum; however, in N. glutinosa and N. edwardsoni i, the local lesion response was activated by p22. These results show that the pig and p22 proteins of TBSV are important symptom determinan ts and that closely related plant species may contain different resist ance genes that selectively respond to individual TBSV proteins.