D. L. Seifers

Temperature sensitivity and efficacy of wheat streak mosaic virus resistance derived from Agropyron intermedium.

Agronomically promising wheat (Triticum aestivum) lines are now available that have the short arm of chromosome 4Ai-2 from Agropyron intermedium translocated onto the long arm of wheat chromosome 4D. This translocation confers a high level of resistance to wheat streak mosaic virus (WSMV). In growth chamber tests, we demonstrated that, when the translocation is present, the resistance is effective at 20 but not at 25°C. Lines with the entire Ai-2 chromosome remained symptom-free at both temperatures. In field tests, both naturally infested and mechanically inoculated lines carrying the 4Ai2-S translocation were WSMV symptom free, and grain yields, test weights, and plant height were not reduced by WSMV. The grain yields of WSMV-susceptible cultivars were reduced by 21 to 45% in the same test. Although the WSMV resistance carried on the translocation is high-temperature sensitive, it was effective in the field and continues to be a very promising source for the development of WSMV-resistant cultivars.

Phylogenetic relationships, strain diversity and biogeography of tritimoviruses

North American and Eurasian isolates of Wheat streak mosaic virus (WSMV; genus Tritimovirus) and Oat necrotic mottle virus (ONMV; genus Rymovirus) were examined. Nine WSMV isolates differentially infected oat, barley, inbred maize line SDp2 and sorghum line KS56. The WSMV isolates clustered into groups based on phylogenetic analyses of the capsid protein (CP) cistron and flanking regions. WSMV isolates from the United States (US) and Turkey were closely related, suggesting recent movement between continents. Although more divergent, WSMV from Iran (WSMV-I) also shared a most recent common ancestor with the US and Turkish isolates. Another group of WSMV isolates from central Europe and Russia may represent a distinct Eurasian population. Complete genome sequences of WSMV from the Czech Republic (WSMV-CZ) and Turkey (WSMV-TK1) were determined and comparisons based on complete sequences yielded relationships similar to those based on partial sequences. ONMV-Pp recovered from blue grass (Poa pratensis L.) in Germany displayed the same narrow host range as ONMV-Type from Canada. Western blots revealed a heterologous relationship among CP of WSMV and ONMV. Phylogenetic analyses of the capsid protein cistron and flanking genomic regions indicated that WSMV and ONMV are related species sharing 74.2-76.2% (nucleotide) and 79.2-81.0% (amino acid) identity. Thus, ONMV should be removed from the genus Rymovirus and designated a definitive member of the genus Tritimovirus. Phylogenetic analyses further suggest that Sugarcane streak mosaic virus is not a tritimovirus, and may represent a new genus within the family Potyviridae.

Temperature-Sensitive Wheat streak mosaic virus Resistance Identified in KS03HW12 Wheat

Wheat streak mosaic virus (WSMV) infection reduces seed yield and quality in wheat. These losses can be alleviated significantly by exploiting genetic host plant resistance. A new source of temperature-sensitive resistance to WSMV, KS03HW12, and its parental lines (KS97HW29/ KS97HW131//KS96HW100-5) were evaluated in both greenhouse and field conditions. Parental wheat lines were exposed to WSMV pressure under different temperatures in growth chambers to determine the stability of the resistance, and 2 years of field yield trials were conducted to confirm effectiveness. To determine the effectiveness of its resistance against a spectrum of isolates, KS03HW12 was tested against six different WSMV isolates of different geographic origins. Among the three pedigree parents, only one, KS97HW29, was resistant. The parental lines of KS97HW29 are not available for testing; therefore, the presumed origin of the resistance could not be further confirmed. None of the six tested WSMV isolates systemically infected KS03HW12 at 18°C. Yield of KS03HW12 in field tests was not different from healthy controls. Thus, the elite winter wheat KS03HW12 appears to be a stable and effective source of temperature-sensitive resistance to WSMV and should be useful for wheat breeding programs.

Occurrence and Distribution of Triticum mosaic virus in the Central Great Plains

Wheat curl mite (WCM)-transmitted viruses-namely, Wheat streak mosaic virus (WSMV), Triticum mosaic virus (TriMV), and the High Plains virus (HPV)-are three of the wheat-infecting viruses in the central Great Plains of the United States. TriMV is newly discovered and its prevalence and incidence are largely unknown. Field surveys were carried out in Colorado, Kansas, Nebraska, and South Dakota in spring and fall 2010 and 2011 to determine TriMV prevalence and incidence and the frequency of TriMV co-infection with WSMV or HPV in winter wheat. WSMV was the most prevalent and was detected in 83% of 185 season-counties (= s-counties), 73% of 420 season-fields (= s-fields), and 35% of 12,973 samples. TriMV was detected in 32, 6, and 6% of s-counties, s-fields, and samples, respectively. HPV was detected in 34, 15, and 4% of s-counties, s-fields, and samples, respectively. TriMV was detected in all four states. In all, 91% of TriMV-positive samples were co-infected with WSMV, whereas WSMV and HPV were mainly detected as single infections. The results from this study indicate that TriMV occurs in winter wheat predominantly as a double infection with WSMV, which will complicate breeding for resistance to WCM-transmitted viruses.

The complete genome sequence of Triticum mosaic virus, a new wheat-infecting virus of the High Plains

The genome of Triticum mosaic virus (TriMV), a recently discovered mite-transmitted wheat potyvirus, was sequenced, characterized, and compared to other members of the family Potyviridae. TriMV has a single mRNA strand of 10,266 nucleotides with a predicted polyprotein consisting of 3,112 peptides. Protein alignments of the coat protein demonstrate that TriMV has 45.9% identity to Sugarcane streak mosaic virus strain AP (SCSMV-AP), but shares only 23.2% identity to Wheat streak mosaic virus. Although TriMV is mite-transmitted and could be placed in the genus Tritimovirus, it is significantly divergent and should be placed in the newly proposed genus Susmovirus.

Survival of five wheat curl mite, Aceria tosichilla Keifer (Acari:Eriophyidae), strains on mite resistant wheat

The survival of the wheat curl mite (WCM), Aceria tosichilla Keifer, on five sources of resistant wheat (Triticum aestivum L.) was determined for collections of mites from Kansas (including a strain adapted to ‘TAM 107’), South Dakota and Texas, USA and Alberta, Canada. Sources of resistance to Aegilops squarrosa L. and Agropyron elongatum (Host) were resistant to WCMs from South Dakota and Alberta, but susceptible to WCMs from Kansas and Texas. Two wheats with resistance to rye (Secale cereale L.), PI 475772 and TAM 107, were resistant to all WCM collections except the strain from Kansas that was selected for adaptation to TAM 107. A common wheat (PI 222655) was resistant to all WCM collections except the one from Alberta, Canada. Because WCMs have overcome the resistance of TAM 107 in Kansas, the only resistance now available in commercial cultivars may be lost. Results indicate that PI222655 is the best source of resistance to replace TAM 107 in the USA but it may not be effective in Canada. Resistance to Ae. squarrosa and A. elongatum could be deployed against WCMs in Alberta and South Dakota but these sources may not be effective in Kansas and Texas. However, one WCM collection from each location may not represent the general mite population of an area. Therefore, any new sources of resistance should be evaluated fully against WCMs from areas where they are likely to be used in commercial cultivars.

Characterization of a distinct Johnsongrass mosaic virus strain isolated from sorghum in Nigeria

Summary.A virus isolated from sorghum in Nigeria has been partially characterized. It was tested by enzyme-linked immunosorbent assay (ELISA) using antisera to Maize dwarf mosaic virus, Johnsongrass mosaic virus (JGMV), Sugarcane mosaic virus strain-MDB, Sorghum mosaic virus, and Zea mosaic virus. A partial host range, symptom phenotypes for selected sorghum lines, and the mass of the coat protein (CP) subunit was analyzed by sodium-dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and its amino acid (aa) sequence determined by time-of-flight mass spectrometry (TOFMS). The Nigerian isolate was positive in ELISA to only JGMV antiserum. It infected sorghum and smooth brome but not oat or johnsongrass. It caused necrosis in 12 of 13 tested sorghum lines, while the USA JGMV isolate caused necrosis in only one sorghum line. In SDS-PAGE, the mass of the Nigerian virus CP was 3,000 Da smaller than that of JGMV-MDO. Moreover, TOFMS analyses showed that, while residues 1–7 of the CP aa sequence were identical to those of JGMV (GenBank #A27631), and residues 57–293 were almost identical to residues 67–303 of JGMV, the intermediate region exhibited significant differences, including a 10 aa deletion. These data indicate that the virus should be considered a distinct isolate of JGMV (JGMV-N) and expands the known range of JGMV to Africa.

Occurrence and symptom expression of American wheat striate mosaic virus in wheat in Kansas.

Extracts of winter wheat with viruslike symptoms from Comanche County, Kans., did not react with antisera to viruses previously found infecting wheat in the state, nor was the virus mechanically transmissible to wheat, maize, or sorghum. Leaves of infected wheat developed thin, chlorotic striations that were more severe on the abaxial than adaxial surface. The virus reacted with American wheat striate mosaic virus (AWSMV) antiserum and was vectored by the painted leafhopper (Endria inimica), confirming the virus as AWSMV. A protein of 59 kDa, consistently detected in extracts of infected leaves, reacted with antiserum to AWSMV in Western blots. This is the first report of AWSMV in the Southern Great Plains region. Experiments showed that, on some cultivars, AWSMV caused brown necrotic streaking of culms and glumes, a symptom not described previously for wheat infected by this virus. Hard red winter wheat cultivars Ike, Karl, and TAM 107 had positive enzyme-linked immunosorbent assay values, but exhibited few or no symptoms.

Heritable, De Novo Resistance to Leaf Rust and Other Novel Traits in Selfed Descendants of Wheat Responding to Inoculation with Wheat Streak Mosaic Virus

Stable resistance to infection with Wheat streak mosaic virus (WSMV) can be evolved de novo in selfing bread wheat lines subjected to cycles of WSMV inoculation and selection of best-performing plants or tillers. To learn whether this phenomenon might be applied to evolve resistance de novo to pathogens unrelated to WSMV, we examined the responses to leaf rust of succeeding generations of the rust- and WSMV-susceptible cultivar ‘Lakin’ following WSMV inoculation and derived rust-resistant sublines. After three cycles of the iterative protocol five plants, in contrast to all others, expressed resistance to leaf and stripe rust. A subset of descendant sublines of one of these, ‘R1’, heritably and uniformly expressed the new trait of resistance to leaf rust. Such sublines, into which no genes from a known source of resistance had been introgressed, conferred resistance to progeny of crosses with susceptible parents. The F1 populations produced from crosses between, respectively, susceptible and resistant ‘Lakin’ sublines 4-3-3 and 4-12-3 were not all uniform in their response to seedling inoculation with race TDBG. In seedling tests against TDBG and MKPS races the F2s from F1 populations that were uniformly resistant had 3∶1 ratios of resistant to susceptible individuals but the F2s from susceptible F1 progenitors were uniformly susceptible. True-breeding lines derived from resistant individuals in F2 populations were resistant to natural stripe and leaf rust inoculum in the field, while the ‘Lakin’ progenitor was susceptible. The next generation of six of the ‘Lakin’-derived lines exhibited moderate to strong de novo resistance to stem rust races TPMK, QFCS and RKQQ in seedling tests while the ‘Lakin’ progenitor was susceptible. These apparently epigenetic effects in response to virus infection may help researchers fashion a new tool that expands the range of genetic resources already available in adapted germplasm.

YIELD REDUCTION BY PLANT VIRUS INFECTION:A POSSIBLE CAUSE OF A PHYSIOLOGICAL DISORDER

Viruses are intercellular parasites with a limited genome, and therefore dependent on the host cell for multiplication and proliferation. When viruses infect plants the usual response is formation of a mosaic symptom, considered to reflect perturbations in the chloroplasts. However, in some instances, plants remain symptomless. In either case, no cellular disruption is documented. This is in contrast to animal viruses, which shut off cellular protein synthesis, culminating in cellular disruption. Yet, generally, infection of plants by viruses results in reduction of plant size and yield. Since most plant virus infection is not accompanied by clear pathogenic damage, there sulting yield reduction caused by it remains an enigma. Most investigations of plant viruses are oriented towards preventing infection, with little attention to host pathogenic damage. However, a better understanding of plant and virus interactions may help explain the mode by which virus infection causes yield reduction. The scientific ...