Location: Corn Insects and Crop Genetics Research
2017 Annual Report
Objectives
Objective 1: Discover and functionally characterize host genes and pathways that respond to pathogen infection in barley and maize.
Sub-Objective 1A: Identification of barley Blufensin1 (Bln1)-mediated response pathways.
Sub-Objective 1B: Functional confirmation via integrated reverse genetic analysis.
Sub-Objective 1C: Identify and functionally characterize mechanisms of defense against leaf blights with hemi-biotrophic and necrotrophic lifestyles in maize.
Objective 2: Identify and analyze the role of pathogen effectors that influence host response in barley and maize.
Sub-Objective 2A: Isolation of Blumeria graminis f. sp. hordei (Bgh) effectors to identify plant targets that promote or suppress defense in host and nonhost interactions.
Sub-Objective 2B: Identification of candidate effectors produced by S. turcica and C. heterostrophus during interactions with maize.
Approach
Large scale sequencing of pathogen genomes, as well as their plant hosts, has provided unprecedented access to the genes and gene networks that underlie host-pathogen interactions. Regulatory focal points critical to these interactions will be investigated. Determination of these focal points will enable the molecular dissection of important disease resistance pathways, as well as the creation of a molecular toolbox from which to apply modern plant breeding methodologies.
Progress Report
Fungal pathogens are the greatest threats to cereal grain production worldwide. Effector proteins secreted by these pathogens manipulate host processes in order to create an ideal environment for colonization. To defend themselves, plants have evolved a battery of receptors that activate immune responses. While many aspects of plant defense have been studied using a reductionist approach, a comprehensive view of the cellular changes that actually render a plant resistant to pathogens has been a major challenge for plant-microbe research.
Using plant-pathogen interaction systems of barley and corn, this project aims to identify both host disease defense components and pathogen signaling molecules that suppress them. By understanding how plants and pathogens manipulate each other during complex interactions, geneticists and breeders can tip the scales in favor of the crop plants to promote more stable and more efficient production.
Significant new insights into disease resistance signaling were obtained in FY2017. ARS scientists used the cereal grain crop, barley, and its powdery mildew pathogen, to perform a genome-wide analysis of plant disease resistance and, as part of Objective 1, continued the discovery of new disease resistance mechanisms.
A new approach, designated Next Generation Interaction Screening, was employed where secreted effector proteins from the pathogen were utilized as ”bait” to fish for interacting targets in the host. Briefly, this approach combines a protein-protein interaction screen in yeast with next generation DNA sequencing. The outcome is a series of high-confidence host targets that can be used in molecular breeding programs. Several new barley proteins were identified that function in vesicle trafficking, a key component of host resistance to pathogens. Thus, analogous to human disease, converging evidence points to the secretory system as a key mediator of barley resistance to powdery mildew.
Novel results were obtained from experiments conducted using corn and two of its pathogens that attack leaves and thereby reduce crop yields. Using a pair of lines genetically differing for only a handful of genes to study specific gene expression responses to Southern leaf blight, it was observed that the gene of interest, an amino acid transferase, was strongly differentially expressed. A cascade of other gene expression changes was also associated with carrying the version of the gene that confers quantitative disease resistance. Transgenic studies on this gene were also conducted and validate both the genetic and transcriptomic observations, offering opportunities to functionally elucidate the mechanistic basis of this defense response, which at present remains unclear. Genetic, transcriptomic, and functional studies of corn’s defense responses to Southern leaf blight and Northern leaf blight were conducted and preliminary analyses indicate that for several other cases, the candidate gene of interest is indeed responsible for the observed defense response. Specific mechanistic experiments are ongoing.
The molecular mechanisms by which pathogen effectors manipulate the host are poorly understood and disease epidemics are accelerated during shifting climate conditions. Identification of barley and corn proteins targeted by multiple pathogen effectors would indicate that these proteins are critical to conserved defense responses and would be priority targets for resistance breeding, enabling advanced cultivar development. Knowledge gained from this project will promote broadly applicable disease control strategies.
Accomplishments
1. A corn gene conferring multi-disease resistance offers a new kind of crop protection. Several economically important fungal diseases of corn that specifically attack leaves offer a unique multi-disease system in which to discover genes that confer broad-spectrum disease resistance in plants. ARS researchers in Raleigh, North Carolina and Ames, Iowa led a multi-institutional, multi-disciplinary team in characterizing the function of a corn gene, in resistance to both gray leaf spot and Southern leaf blight. Research showed that increases in the expression of a corn gene, are associated with increased production of the structural organic polymer lignin as well as with increased resistance to both diseases. These findings offer opportunities to better protect corn crops and may serve as a model for protecting other crops against multiple diseases.
Review Publications
Lu, X., Kracher, B., Saur, I., Bauer, S., Ellwood, S.R., Wise, R.P., Yaeno, T., Maekawa, T., Schulze-Lefert, P. 2016. Allelic barley MLA immune receptors recognize sequence-unrelated avirulence effectors of the powdery mildew pathogen. Proceedings of the National Academy of Sciences. 113(42):e6486-6495. doi:10.1073/pnas.1612947113.
Loneman, D.M., Peddicord, L., Al-Rashid, A., Nikolau, B.J., Lauter, N.C., Yandeau-Nelson, M.D. 2017. A robust and efficient method for the extraction of plant extracellular surface lipids as applied to the analysis of silks and seedling leaves of maize. PLoS One. 12(7):e0180850. doi:10.1371/journal.pone.0180850.
