Local lesions and induced resistance.

The local lesion phenomenon is one of the most notable resistance mechanisms where virus after multiplying in several hundred cells around the point of entry, does not continue to spread and remains in a local infection. Several types of local lesions are known, inter alia, necrotic, chlorotic, and starch lesions. Cells inside the lesion generally contain much less virus than cells in a systemic infection. Cytopathic changes accompany the local lesion development. Proteases that may have properties similar to caspases, which promote programmed cell death (PCD) in animals, seem to participate in PCD during the hypersensitive response. Salicylic acid seems to be associated with the HR and may play a role in localizing the virus. The functions and properties of the N gene of Nicotiana, which was the first plant virus resistance gene to be isolated by transposon tagging, are discussed and compared with other plant genes for disease resistance. The Inhibitor of Virus Replication (IVR) associated with the local lesion response is mainly a tetratricopeptide repeat (TPR) protein. TPR motifs are also present in inducible interferons found in animal cells. Transformation of N. tabacum cv. Samsun nn, in which Tobacco mosaic virus (TMV) spreads systemically, with the NC330 gene sequence, encoding an IVR-like protein, resulted in a number of transgenic plant lines, expressing variable resistance to TMV and the fungal pathogen Botrytis cinerea. Transformation of tomato plants with the IVR gene became also partially resistant to B. cinerea (Loebenstein et al., in press). IVR-like compounds were found in the interspecific hybrid of N. glutinosa x N. debneyi that is highly resistant to TMV, and in the "green island" tissue of tobacco, cv. Xanthi-nc, infected with Cucumber mosaic virus (CMV). Infection in one part of the plant often induces resistance in other non-invaded tissues. Local (LAR) or systemic (SAR) acquired resistance can be activated by viruses, bacterial, and fungal pathogens or other natural and synthetic compounds. Accumulation of salicylic acid accompanies the induction of resistance. Possible mechanisms are outlined. Synthetic compounds, as for example, acibenzolar-S-methyl (ASM) were developed for use in a novel strategy for crop protection through abiotic induction of SAR. For example, ASM protected cantaloupes against a fungal pathogen and CMV. Additional attempts to protect crops by inducing SAR are outlined and it is hoped that future research and its application will find its use in plant protection.