Location: Corn, Soybean and Wheat Quality Research2009 Annual Report
1a. Objectives (from AD-416)
Objective 1: Develop corn and soybean virus disease control strategies. • Sub-objective 1.A. Characterize the nature of host resistance to virus disease. • Sub-objective 1.B. Identify, map, and clone virus-resistance genes. • Sub-objective 1.C. Determine the effect of combining quantitative virus resistance with insect resistance on virus disease severity. • Sub-objective 1.D. Characterize insect vector/virus relationships. Objective 2: Identify and characterize emerging virus diseases in corn and soybeans. Objective 3: Develop virus-based gene vectors in corn. Soybeans and corn are the two highest value crops grown in the U.S. Although research from this laboratory and others has led to significant improvements in their management, virus diseases continue to be annual threats. Furthermore, history has shown that unanticipated, and often unknown, new virus disease problems can rapidly emerge at any time. In soybeans, the threat of virus diseases has increased even more following the introduction of the soybean aphid into the U.S. in 1999. This is the first soybean-colonizing aphid in the U.S., and the consequences for future virus disease problems in soybeans are unknown. A long-term objective of this program is to reduce corn and soybean losses attributable to virus diseases. Our strategy to do so is to identify corn and soybean viruses when they arise and to characterize their biology and epidemiology (Objective 2), and to then use this information develop practical, effective methods and strategies for minimizing crop losses (Objective 1). Our final objective (Objective 3) is to use our knowledge and expertise in maize virology to develop new tools for forward and reverse genetic analysis of maize gene function. We will develop gene silencing and expression vectors based on selected maize viruses, and use these vectors immediately to characterize the functions of candidate maize genes thought to be important for virus infection and/or resistance. Because reliable vectors are lacking for monocotyledenous plants such as corn, vectors developed under this objective would be of benefit both to corn geneticists and to those seeking to use corn for the production of non-endogenous materials.
1b. Approach (from AD-416)
Our overall approach is to: 1) identify existing and emerging viruses; 2) understand their biology; and 3) develop effective disease control strategies. Known viruses will be identified using existing diagnostics. For previously uncharacterized viruses, we will culture them in healthy plants using mechanical or arthropod transmission, determine their characteristics, and develop diagnostic assays. This knowledge will be used to formulate disease control strategies, although, usually, the most effective and economic control strategy is to use virus-resistant crop varieties and cultivars. Therefore, a primary focus of this project is to identify, characterize, and map virus resistance in maize and soybean germplasm. To develop an understanding of how resistance genes work, they will be isolated and characterized, and their role in the molecular and biochemical changes associated with virus resistance will be examined. Also, factors affecting virus transmission by arthropod vectors will be characterized so that alternative disease control methods can be developed. Maize virus-based gene expression and silencing vectors will be developed to facilitate functional analysis of plant resistance genes using forward and reverse genetics. Such vectors should have broad impact as few are available for cereals. The impact of this research will be to advance our knowledge of virus diseases of corn and soybeans and provide vital information for the development of control strategies to reduce disease losses.
3. Progress Report
The majority of our research effort was on the objective to develop corn and soybean virus disease control strategies. We showed that, BPMV incidence was lower in semi-dwarf insect-resistant soybeans than in standard insect-susceptible cultivars, but found it was also lower in a semi-dwarf, insect-susceptible cultivar. We demonstrated that contributions from ‘modifier genes’ derived from the resistant parent Pa405 are important for resistance of maize to Maize dwarf mosaic virus (MDMV) and Sugarcane mosaic virus. We showed that Graminella nigrifrons is an inefficient vector of Maize fine streak virus (MFSV), and identified two populations of MFSV-infected insects after feeding on infected plants, insects with high virus titer that could transmit the virus and insects with low virus titer that did not transmit the virus. We are in the third year of trials to evaluate the effects of BPMV on Ohio, North Dakota and Wisconsin soybean cultivars with partial BPMV resistance; microarray analysis of the response of resistant and susceptible maize 1 and 3 days post-MDMV inoculation were done; molecular markers were used to identify cross-over events in the chromosomal regions near two WSMV resistance genes and several thousand progeny have been analyzed for these cross-overs. We completed a project with Ohio State University and Serbian collaborators to define the disease cycle for maize redness caused by stolbur phytoplasma and the life cycle of the disease vector, Reptalus panzeri. For our objective to develop virus-based gene vectors in corn, we are working with Oklahoma State University collaborator to make a gene-silencing vector using Maize necrotic streak virus (MNeSV), and with OSU and John Innes Centre collaborators to develop gene expression vectors using Maize fine streak virus (MFSV) and Maize mosaic virus. We tested constructs with maize phytoene desaturase (pds) sequences inserted into engineered cloning sites. Although some plants became infected with these constructs, none of the expected photobleaching associated with pds expression was observed. After re-examining the viral genome sequence for secondary RNA structures, a problem with disrupting the sequences near the inserted cloning sites was identified. New cloning sites were selected and constructs were made that are being tested at ARS and OkSU. Previously, we showed that the MFSV nucleoprotein and phosphoprotein can be expressed in Drosophila S2 cells using commercially available plasmids. We demonstrated expression of a replicon construct carrying an antisense GFP construct flanked by the viral 5’ and 3’ UTR in S2 cells. A construct for the L gene, which encodes the viral RNA dependent RNA polymerase was made and expression of these constructs in S2 cells is currently being tested.
1. Vectors of stolbur phytoplasma transmission to maize and the maize redness (MR) disease cycle in Serbia. Maize redness, caused by stolbur phytoplasma, can produce yield losses of up to 90% in the Banat regions of Serbia, Romania and Hungary. MR incidence and intensity has increased dramatically in the past few years. We established that the cixiid planthopper R. panzeri is the major vector of stolbur phytoplasma to maize in Serbia, and identified two new plant hosts of stolbur phytoplasma, johnsongrass and wheat. We showed that large populations of R. panzeri carrying the MR pathogen may overwinter on autumn-planted wheat and emerge to infect maize the following summer. The information gained in this research is critical for understanding an emerging disease of an important crop. The results will be used directly for designing and testing cultural approaches for controlling the disease, such has alternative crop rotations and more aggressive weed control.