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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.

Both MDMV-A and -B were transmitted by natural infection and mechanical inoculation on South Dakota adapted sorghum hybrids and occurred at the Brookings area. Grain sorghum hybrids were found to react differently to mechanical inoculation with MDMV-A and -B, respectively. Strain A usually induced a systemic mosaic symptom, while strain B usually produced local and necrotic symptoms. However, a few cultivars infected with strain B exhibited systemic symptoms. During a comparative ultrastructural study of MDMV infection, there was an obvious difference in inclusion bodies between MDMV-A and MDMV-B. Electron microscope revealed the presence of pinwheels, bundles, tubes, scrolls, and laminated aggregates in cells of sorghum leaf tissue systemically infected with MDMV-B, while no laminated aggregates were found in cells of sorghum leaf tissue infected with MDMV-A. Ultrastructural aspects of HOK sorghum plants in the primary acute stage of symptom development and MDMV-A interaction, exhibited obvious ultrastructural changes in host cell morphology such as cell wall abnormalities. The cell wall abnormalities involved cell wall thickenings and protrusions associated with paramural bodies, extended plasmodesmata, and extra protoplasmic sacs. The objectives of this study are to determine the response of South Dakota adapted sorghum cultivars to mechanical inoculation, to identify potential reservoirs for MDMV, to compare sorghum cultivars reaction to MDMV-A and -B, and to describe the symptom development at different stages of sorghum cultivars inoculated with MDMV. Also, to observe the cytological modification of HOK sorghum cultivar inoculated with MDMV-A mechanically, and finally to compare the difference of inclusion bodies between strain A and B.