Location: Wheat, Sorghum and Forage Research2012 Annual Report
1a. Objectives (from AD-416):
Objective 1: Elucidate the underlying mechanism of virus resistance in the resistant wheat variety Mace, its derivatives, and other sources to identify new disease-resistant lines with increased yield potential. Objective 2: Develop and evaluate transgenic wheat for disease resistance by expression of viral genome sequences in various forms. Objective 3: Define the role of Triticum mosaic virus proteins in semi-persistent transmission by the wheat curl mite, vector of both Wheat streak mosaic virus and Triticum mosaic virus. Objective 4: Identify and characterize Triticum mosaic virus gene functions, host interactions and rate of evolution, and strain differentiation during horizontal transmission.
1b. Approach (from AD-416):
Wheat streak mosaic virus (WSMV) is common throughout the American Great Plains and is responsible for significant and recurring economic losses in wheat (losses over the past five years are estimated to average $460 million annually). Triticum mosaic virus (TriMV) is a recently emerged virus in the same geographic region and likely poses the same risks as WSMV. Wheat varieties with temperature-dependent genetic resistance to WSMV recently have become available. One wheat variety, Mace, is resistant to both WSMV and TriMV but the mechanism(s) of virus resistance is unknown. Knowledge of resistance mechanisms can suggest techniques for rapidly screening wheat lines for WSMV and TriMV resistance. New screening methods will be essential to track the Wsm1 resistance gene in wheat lines that have lost closely linked PCR markers. In addition, other sources of resistance to WSMV and TriMV to augment natural resistance and mitigate against the evolution of resistance-breaking strains of these viruses is desirable. Virtually nothing is known about the molecular biology of TriMV and the roles of viral proteins in replication, vector transmission, and virus-plant host interactions. Improved understanding of the genetic basis of these basic viral functions will facilitate efforts to ameliorate the effects of viral infection in wheat and other cereal crops. The objectives here are designed to fill these knowledge gaps with WSMV and TriMV resistance, TriMV molecular biology and vector transmission. In addition, we will develop transgenic wheat with resistance likely to be effective against a broad spectrum of WSMV and TriMV strains.
3. Progress Report:
Wheat streak mosaic virus (WSMV) and the recently found Triticum mosaic virus (TriMV) cause significant wheat yield loses in the Great Plains region. Both viruses are transmitted by wheat curl mites and co-infection of these two viruses in wheat fields has been reported. The effects of single and double infections with these two viruses were determined for Millennium (a WSMV-susceptible cultivar) and for Mace (a WSMV-resistant cultivar). Single and double infections had more negative effects on growth and yield determinants for Millennium than for Mace and confirmed that Mace has resistance to both viruses. Thus, Mace provides a new means for virus disease management in addition to current cultural practices. To understand how the host defense mechanisms operate against viral infections, virus-specific small RNA (sRNA) sequences were obtained from virus-infected Arapahoe (a susceptible wheat cultivar) and Mace (a resistant wheat cultivar) plants. sRNAs from infected wheat plants covered the entire length of viral genomes, which indicate that the basal anti-virus defense of wheat targets the entire genomic RNAs of WSMV and TriMV. Recent studies indicated that RNA-based transgenic plants using tandem hairpin RNA sequences from several viruses provided a robust and broad-spectrum resistance to multiple viruses. A tandem hairpin RNA construct from conserved regions of both WSMV and TriMV genomes was developed. This hairpin construct was cloned into a binary vector for wheat transformation which may provide new sources of resistance to both viruses in transgenic wheat. Viral RNA translation into proteins is an important step in establishing virus infection. The non-coding region at the beginning of viral RNAs often plays a critical role in the translation process. TriMV has an unusually long 739 nucleotide (nt) non-coding region and its functional role was investigated. This non-coding region of TriMV was firmly established as an enhancer of RNA translation. Precisely, TriMV sequence between nucleotides 300 and 829, extending 90 nucleotides into the TriMV P1 gene was identified to be an optimal translational enhancer. It is important to know the epidemiology and bio-diversity of viruses to develop effective management strategies. The diversity of TriMV was examined by sequencing the P1 and coat protein coding regions of 14 virus isolates from Colorado and 18 isolates from Nebraska, and found to have only 0.4% and 0.2% sequence diversity for P1 and coat protein regions, respectively. Sequence analysis of TriMV field isolates suggests that TriMV was introduced into the Great Plains region as recently as 10-17 years ago. Plants are known to have innate defenses against viruses. In turn, viruses have evolved proteins to overcome host defense mechanisms. In contrast to the helper component-proteinase of Potyvirus species, the P1 protein of TriMV was identified as a suppressor of RNA silencing. This established that the TriMV protein is an important pathogenicity determinant and that it has major roles in virus invasion and disease development in wheat and other plant hosts.
Byamukama, E., Tatineni, S., Hein, G., Graybosch, R.A., Baenziger, P., French, R.C., Wegulo, S. 2012. Effects of single and double infections of winter wheat by Triticum mosaic virus and Wheat streak mosaic virus on yield determinants. Plant Disease. 96:859-864.