Strain-specific resistance conditioned by a single dominant gene in soybean cv. York inoculated with SMV-G1, revealed no symptoms and no detectable viral replication (R). Unlike the hypersensitive response (HR), the R response did not result in localized virus and induction of a series of defense responses, but in inhibition of virus replication. However, this resistance was overcome by the resistance- breaking strain, SMV-G4, which induced lethal necrosis (N). Unlike HR, G4 strain was not restricted but spread ina restricted pattern along the vein, stem and into upper un-inoculated leaves where it induced necrosis. Like HR, PR proteins were found to accumulate in the N response and were named SPR (soybean pathogenesis-related) proteins. On the basis of major similarities in molecular weight characteristics and enzyme-substrate specificities, SPRs are proposed to be classed into four groups: SPR1; 1a, 1b, 1c, 1d; SPR2: 2a, 2b, 2c; SPR3: 3a, 3a’, 3b, 3c, 3d, 3e; SPR4. The functions of SPR1 and SPR4 groups have not yet been determined. The SPR2 group was identified as β-1,3-glucanases (GLN) and classed two subgroups. The SPR3 group was identified as chitinases (CHN) and classed three distinct subgroups. Like HR, the N response induced a series of defense responses with marked temporal increases, in a bimodal pattern, of phenylalanine ammonia-lyase (PAL) and peroxidase (POX) activities, bimodal pattern of PAL mRNA and chalcone synthase (CHS) mRNA induction, a single pattern of chitinase mRNA induction, and glyceollin and lignin accumulation. These marked increases occurred at or just before the time of initial appearance of necrotic lesions, or following necrosis development for localization of virus. Total RNA isolated from soybean inoculated with SMV-G4 were used for synthesis of cDNA by reverse transcription, and amplified using 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMGR)-specific primer sets by polymerase chain reaction. Amplified cDNAs were cloned into Bluescript vector (pKS⁻), transformed into E. coli, isolated pSOY HMGR and used as a probe for Northern analysis. We demonstrate that expression of HMGR mRNA is correlated with strain-specific resistance (R). SMV coat protein (CP) degraded in vitro by proteolysis during purification. The CP of SMV purified from infected leaves had an major size of 34 kD in SDS-PAGE with minor peptides of 32 and 31 kD. The minor peptides increased during the storage at 4°C or with trypsin treatment, and reacted with antiserum to intact virions. This heterogeneity of protein was not removed by alkaline phosphatase treatment for varying time intervals, and was not related with phosphorylation and dephosphorylation, or with virus maturation. These studies provided additional evidence of N- or C-terminal exposure on the particle’s surface. / Ph. D.