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.

New Sources of Temperature-Sensitive Resistance to Wheat streak mosaic virus in Wheat.

Expressing temperature-sensitive resistance (TSR) protects wheat against yield losses from infection with Wheat streak mosaic virus (WSMV). In examining how 2,429 wheat accessions from the National Small Grains Collection responded to inoculation with the Sid81 isolate of WSMV, 20 candidate TSR sources were discovered. To differentiate their relative effectiveness, accession responses over 21 days to inoculation with GH95, Sid81, and PV57 virus isolates in regimes of 18 and 20°C were observed. At 18°C, all 20 candidate TSR sources were uniformly or nearly uniformly asymptomatic 21 days after inoculation with the PV57 isolate, resistance indistinguishable from resistant checks KS96HW10-3 and RonL. By contrast, the Sid81 isolate induced symptoms in low but significant proportions of plants of two candidates, and the GH95 isolate in high proportions for four candidates and low but significant proportions for two others. In the more stringent 20°C regime, the uniform or near-uniform induction of symptoms in response to inoculation with GH95 failed to differentiate among the 20 candidate TSR sources and two resistant checks, while PV57 and Sid81 identified several candidates that performed similarly to KS96HW10-3 and significantly better than RonL. By identifying new sources of resistance, this study contributes to the control of WSMV.

Triticum mosaic virus: A New Virus Isolated from Wheat in Kansas.

In 2006, a mechanically-transmissible and previously uncharacterized virus was isolated in Kansas from wheat plants with mosaic symptoms. The physiochemical properties of the virus were examined by purification on cesium chloride density gradients, electron microscopy, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), sequencing of the nucleotides and amino acids of the coat protein, and immunological reactivity. Purified preparations contained flexuous, rod-shaped particles that resembled potyviruses. The coat protein was estimated from SDS-PAGE to have a mass of approximately 35 kDa. Its amino acid sequence, as deduced from DNA sequencing of cloned, reverse-transcribed viral RNA and separately determined by time-of-flight mass spectrometry, was most closely related (49% similarity) to Sugarcane streak mosaic virus, a member of the Tritimovirus genus of the family Potyviridae. The virus gave strong positive reactions during enzyme-linked immunosorbent assays using polyclonal antibodies raised against purified preparations of the cognate virus but gave consistent negative reactions against antibodies to Wheat streak mosaic virus (WSMV), other wheat potyviruses, and the High Plains virus. When the virus was inoculated on the WSMV-resistant wheat cv. RonL, systemic symptoms appeared and plant growth was diminished significantly in contrast with WSMV-inoculated RonL. Taken together, the data support consideration of this virus as a new potyvirus, and the name Triticum mosaic virus (TriMV) is proposed.

Association of a Virus with Wheat Displaying Yellow Head Disease Symptoms in the Great Plains.

Wheat with yellow head disease (YHD) (yellow heads and mosaic leaf symptoms) has been observed in Kansas since 1997. A pathogen was transmitted from the infected wheat to maize by vascular puncture inoculation and to Nicotiana benthamiana by rub inoculation. The original infected wheat and infected maize and N. benthamiana test plants all produced a unique 32- to 34-kDa protein when analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Time-of-flight mass spectrometry analysis of the unique 32- to 34-kDa protein showed that the amino acid sequence was most closely related to the nucleoprotein of Rice hoja blanca virus, indicating that the virus causing YHD symptoms in wheat is a tenuivirus. Antiserum made to this protein failed to react with extracts made from healthy wheat or wheat infected with Wheat streak mosaic virus or the High Plains virus. The antiserum did react to extracts made from symptomatic wheat, maize, and N. benthamiana, shown by SDS-PAGE to contain the unique protein, and to extracts of wheat with YHD symptoms from Kansas, North Dakota, South Dakota, and Oklahoma. The name Wheat yellow head virus is proposed for this virus.

Characterization of the agent of "high plains disease": mass spectrometry determines the sequence of the disease-specific protein.

The "32-kDa" protein specifically associated with high plains disease was characterized by time-of-flight mass spectrometry, after the agent had been isolated in pure culture by "vascular puncture inoculation," a novel mechanical means of transmission. Two isolates from different geographic locations each consisted of a mixture of subpopulations that were highly homologous to an amino acid sequence derived from a nucleotide sequence (U60141) deposited in GenBank trade mark by the Nebraska group as "the probable N-protein of high plains virus." However, the U60141 sequence was found to be incomplete; de novo sequencing of peptides produced by proteolytic digestions of the 32-kDa band from an SDS-PAGE separation showed that an additional 18 amino acid residues were present at the N terminus. BLAST (basic local alignment search tool) examination of the sequence showed no significant homology with any protein in the databases, indicating that the infectious agent of high plains disease is likely a member of a hitherto unclassified virus group.

Natural Infection of Sorghum by Foxtail Mosaic Virus in Kansas.

Sorghum (Sorghum bicolor) was infected by a mechanically transmissible, flexuous, rod-shaped virus. Antiserum made against the purified virus reacted specifically in enzyme-linked immunosorbent assay to the virus and to the potexvirus foxtail mosaic virus (FoMV), indicating that the sorghum virus was an isolate of FoMV. Comparison of the sorghum isolate (H93) to FoMV PV 139 showed that H93 differed biologically by causing severe symptoms in sorghum, not readily infecting certain barley lines, and causing only faint symptoms in barley. At the molecular level, the capsid of H93 had a mass of 23.9 kDa and 217 amino acid residues compared with 23.7 kDa and 215 residues previously reported for the nucleic acid sequence of FoMV. The amino acid sequences of the two viruses were greater than 96% identical. They varied by having four substitutions, one deletion, and three insertions between residues 66 and 67. This is the first report of natural infection of sorghum by FoMV, thus extending its host range among cereal crops.