Year of Publication

2004

Document Type

Dissertation

College

Agriculture

Department

Plant Pathology

First Advisor

Said A. Ghabrial

Abstract

Bean pod mottle virus (BPMV), a member of the genus Comovirus in the family Comoviridae, is widespread in the major soybean-growing areas in the United States. Soybean yield losses of 10-40% have been reported as a consequence of BPMV infection. The complete nucleotide sequences of two strains, K-Ha1 and K-Ho1, were determined. Field isolates of BPMV were classified into two distinct subgroups (I and II) based on slot blot hybridization and sequence analyses. Full-length cDNA clones from which infectious transcripts can be produced were constructed for strains K-G7, K-Ho1 and K-Ha1. Whereas strains K-Ha1 and K-G7 induced mild or moderate symptoms in infected soybean plants, strain K-Ho1 produced very severe symptoms. Symptom severity was mapped to RNA1. Chimeric RNA1 constructs were generated by exchanging full or partial coding regions of the five RNA1-encoded mature proteins between the full-length cDNA clones of the three RNA1s and the resultant transcripts were inoculated onto soybean. The results showed that the coding regions of the protease co-factor (Co-pro) and the putative helicase (Hel) are determinants of symptom severity. Although symptom severity correlated well with accumulation of viral RNA, neither the Co-pro nor Hel protein could be demonstrated as a suppressor of RNA silencing. Furthermore, separate expression of the Co-pro or Hel proteins from a PVX vector induced necrosis on the inoculated leaves of Nicotiana benthamiana. Characterization of BPMV K-Ho1 indicated that it is a diploid reassortant, containing two distinct types of RNA1s and one type of RNA2. Examination of field isolates from various locations in the United States and Canada revealed that diploid reassortants are of frequent occurrence in natural populations of BPMV. The vary severe symptoms induced by BPMV K-Ho1 can be mimicked by inoculation of plants with a mixture of RNA1 transcripts from two distinct strain subgroups and RNA2 transcript from either subgroup. Plants inoculated with a mixture of transcripts containing two types of RNA1 from the same strain subgroup did not produce very severe symptoms. These are due to interactions between two distinct types of RNA1s. At present, no soybean cultivars with resistance to BPMV are commercially available. Therefore, the feasibility of cross protection as an alternative disease management strategy was studied. Two mild strains of BPMV (K-Da1 and K-Ha1), belonging to subgroup II, were tested for their ability to protect infected plants against a severe strain (K-Ho1). Inoculation of the soybean cultivar Essex on the primary leaves with either of the two mild strains conferred complete protection against challenge inoculation with the severe strain K-Ho1, regardless of the timing of challenge inoculation. Cross-protection was evident regardless of whether virions or BPMV-RNA were used as inocula. Cross protection was independent of the soybean cultivar used and method of virus inoculation, sap-inoculation or by the bean leaf beetle, vector of BPMV. Protection was complete and durable.

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