Inheritance of resistance to potyviruses in Phaseolus vulgaris L. II. Linkage relations and utility of a dominant gene for lethal systemic necrosis to soybean mosaic virus.

A single dominant factor, Hss, that conditions a rapid lethal necrotic response to soybean mosaic virus (SMV) has been identified in Phaseolus vulgaris L. cv. 'Black Turtle Soup', line BT-1. Inoculated plants carrying this factor developed pinpoint necrotic lesions on inoculated tissue followed by systemic vascular necrosis and plant death within about 7 days, regardless of ambient temperature. BT-1 also carries dominant resistance to potyviruses attributed to the tightly linked or identical factors, I, Bcm, Cam, and Hsw, so linkage with Hss was evaluated. No recombinants were identified among 381 F3 families segregating for potyvirus susceptibility, thus if Hss is a distinct factor, it is tightly linked to I, Bcm, Cam, and Hsw. BT-1 was also crossed reciprocally with the line 'Great Northern 1140' ('GN 1140') in which the dominant gene, Smv, for systemic resistance to SMV was first identified. Smv and Hss segregated independently and are co-dominant. The ('GN 1140' x BT-1) F1 populations showed a seasonal shift of the codominant phenotype. Evaluation of the ('GN 1140' x BT-1) F2 population under conditions where Smv is partially dominant allowed additional phenotypic classes to be distinguished. Pathotype specificity has not been demonstrated for either Smv or Hss. Genotypes that are homozygous for both dominant alleles are systemically resistant to the virus and in addition show undetectable local viral replication or and no seed transmission. This work demonstrates that a gene which conditions a systemic lethal response to a pathogen may be combined with additional gene(s) to create an improved resistant phenotype.

Specific infectivity and host resistance have predicated potyviral and pathotype nomenclature but relate less to taxonomy.

The names of potyviruses and viral-strains have represented the occurrence of predominant pathotypes on predominant crop genotypes. Thus virus nomenclature, but not viral taxonomy, has been decisively influenced by plant-genotype susceptibility and indirectly by host genetic resistance. REsistance to infection (i.e., host range) continues to serve a practical role in differentiating recognized viruses. Plant genes that confer disease tolerance or viral resistance remain a principal means of viral pathotype differentiation, as well as a principal control measure against major viral pathogens. Degrees of genetic diversity among isolates of recognized viruses should not be underestimated, and any system of viral taxonomy should be prepared for flexibility at the species level.

Sources of resistance to viruses in the Potyviridae.

Resistance to 56 viruses in the family Potyviridae in 334 plant species was tabulated. Studies conducted in the last 60 years have elucidated the genetics and usefulness of 135 resistance genes, but no reports on the heritability of other sources of resistance are available. In most of the plant species, resistance to species of Potyviridae was simply inherited, either dominantly (60 genes) or recessively (39 genes). In some cases resistance was conferred by two or more genes. Symbols have been assigned to 86 genes, of which very few are duplicate entities. Resistance genes can be useful in determining relationships among these viruses, as well as for their identification. The role of conventional breeding and biotechnology in transferring genes from one species to another is discussed.

Inheritance of resistance to potato y viruses in Phaseolus vulgaris L : 1. Two independent genes for resistance to watermelon mosaic virus-2.

Resistance to watermelon mosaic virus-2 in Phaseolus vulgaris L. is conferred by two distinct dominant alleles at independent loci. Based on segregation data one locus is designated Wmv, the other, Hsw. The dominant allele Wmv from cv. Great Northern 1140 prevents systemic spread of the virus but viral replication occurs in inoculated tissue. In contrast, Hsw confers both local and systemic resistance to WMV-2 below 30C. At higher temperatures, plants that carry this allele in the absence of modifying or epistatic factors develop systemic veinal necrosis upon inoculation with the virus that results in rapid death. Patho-type specificity has not been demonstrated for either allele; both factors confer resistance to every isolate tested. A temperature-sensitive shift in epistasis is apparent between dominant alleles at these loci. Because Hsw is very tightly linked if not identical to the following genes for hypersensitivity to potyviruses I, (bean common mosaic virus), Bcm, (blackeye cowpea mosaic virus), Cam, (cowpea aphid-borne mosaic virus) and Hss (soybean mosaic virus), parental, reciprocal dihybrid F1 populations, and selected F3 families were inoculated with each of these viruses and held at 35 C. F1 populations developed vascular necrosis completely or primarily limited to inoculated tissue, while F3 families from WMV-2-susceptible segregates were uniformly susceptible to these viruses. The relationship between Hsw, Wmv and other genes for potyvirus resistance suggest patterns in the evolution of resistance and viral pathogenicity. Characterization of the resistance spectrum associated with each factor provides an additional criterion to distinguish genes for plant virus resistance.