RESISTANCE OF TRANSGENIC NICOTIANA-BENTHAMIANA PLANTS TO TOMATO SPOTTED WILT AND IMPATIENS NECROTIC SPOT TOSPOVIRUSES - EVIDENCE OF INVOLVEMENT OF THE N-PROTEIN AND N-GENE RNA IN RESISTANCE

Transgenic Nicotiana benthamiana plants containing the nucleocapsid (N ) protein gene of the lettuce isolate of tomato spotted wilt virus (TS WV-BL) have been produced. Analyses of these transgenic plants showed that plants producing small amounts of the N protein were highly resis tant to both the homologous isolate and closely related isolates, wher eas plants producing large amounts of the N protein possessed moderate levels of protection against both the homologous isolate and two isol ates of distantly related impatients necrotic spot virus (INSV). Howev er, the latter protection was highly dependent on inoculum strength. D espite a delay in symptom appearance in the high expressors, none of t he transgenic plants were protected against the more closely related, newly named groundnut ringspot virus originally from Brazil. These res ults substantiated and extended previous observations in transgenic to bacco. Comparative analyses of transgenic plants expressing either the translatable or the untranslatable N gene showed that protection agai nst the homologous isolate and closely related isolates in plants expr essing low levels of the N gene was due to the presence of N gene RNA, whereas protection against the homologous isolate and distantly relat ed INSV isolates in plants expressing high levels of the N gene was du e to the accumulation of N protein. Thus, different mechanisms appeare d to be involved in protection against infection by tospoviruses that share different levels of N gene sequence identities. Interestingly, b oth N gene RNA- and N protein-mediated protections are effective again st the homologous isolate. These mechanisms cannot operate simultaneou sly in the same transgenic plant, but they must operate in different p lants, because a low level of N gene transcription is required for RNA -mediated protection, whereas a high level of N protein is required fo r protein-mediated protection. We also compared the nucleotide sequenc e of the N gene among the test tospovirus isolates, and the informatio n obtained from this comparative analysis was used to develop hypothes es that may account for the two protection patterns.