The responses of soybean (Glycine max (L.) Merr.) to soybean mosaic virus (SMV)
include mosaic, necrosis and no symptoms, and vary with virus strain and allele of the
gene for resistance (Rsv) carried by the host. Genetic studies have shown that plants
giving mosaic are susceptible, and that plants giving either the necrotic or symptomless
response are resistant. The objective of this study was to examine the mechanism of
resistance by tracking SMV replication and movement in susceptible responses, and its
restriction in necrotic and symptomless responses. Two SMV strains were inoculated at
a single spot to Rsv-containing genotypes, selected to give each response. The leaf
imprint immunoassay was developed and used to track the rate and extent of invasion by
SMV from the tip of a primary, unifoliolate leaf to regions within that leaf and to the
stem and younger trifoliolate leaves. In susceptible responses, SMV was detected at the
site of inoculation in 6-7 days, and throughout the mid-rib and in the first trifoliate leaf
in 8-9 days. In necrotic resistance responses, SMV was detected at and around the
inoculation site in 8-9 days and in leaf veins, midrib and the first trifoliolate in 15-17
days, but was restricted to necrotic areas. In symptomless resistance responses, no virus
was detected at any time. Greater virus replication and movement was found in
unifoliolate and trifoliolate leaves of younger (15 days-old) than of older (18-days-old) plants.
It was concluded that: (i) necrotic ally-responding cultivars manifest resistance
by reduction and delay in replication, and by restriction of virus movement; (ii) virus
replication is restricted in resistant cultivars; and (iii) rate and extent virus replication and
movement is affected by stage of plant development.
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