Location: Corn Insects and Crop Genetics Research
2015 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
Cereal crop loss caused by disease remains a significant agricultural challenge in both developed and developing countries. Hence, controlling existing and emerging cereal diseases is vital to food security. 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 pathogen signaling were obtained in FY2015; ARS scientists utilized the powdery mildew pathogen of barley to investigate components of pathogen virulence and plant disease resistance. Effector proteins secreted by pathogens suppress host defenses. This makes the crop host susceptible to yield reducing diseases. As part of Objective 1A and 1B, they discovered and characterized a predicted metalloprotease that is required for pathogen virulence, suppresses host defenses and is evolutionarily conserved among nearly half of all sequenced fungal genomes, including economically important plant pathogens, animal pathogens, and free-living non-pathogens. This novel effector represents an ancient, broadly important fungal protein family, members of which have evolved to function as effectors in plant and animal hosts.
The DNA of the powdery mildew genome encodes 540 predicted secreted effectors. In a parallel project, genome-wide, next generation RNA-sequencing of powdery mildew-infected barley indicates that distinct subsets of these effector candidates are differentially expressed at penetration, or during development of fungal haustorial feeding structures. This suggests that powdery mildew is able to sense compromised resistance functions in its host and modify expression of its effector repertoire accordingly.
To discover and functionally characterize corn genes and pathways that respond to leaf blight infections in maize, the final year of replicated, multi-location genetic mapping experiments were executed for both Southern corn leaf blight (Raleigh NC only) and Northern corn leaf blight (Ithaca, NY and Ames, IA) using a high resolution mapping resource curated by ARS scientists in Ames, IA. Seeds and experimental designs were provided to collaborators at Cornell University and USDA-ARS in Raleigh, NC. Two replicates of 760 corn genotypes were inoculated with local pathogen isolates and phenotyped for disease symptoms in order to identify genetic factors involved in quantitative disease defense. It is expected that these experiments will provide short and actionable lists of genes that can be tested for roles in defense, as well as insights into how corn defends differently against specific strains of these pathogens.
To identify contrasting types of signaling interactions between corn and pathogens, additional local isolates of Northern corn leaf blight were collected, purified, and raised for artificial inoculation of a select panel of lines to reveal differences. A subset of these isolates that captures the diversity of disease interactions has been raised for artificial inoculation of both gene expression and disease resistance experiments, offering opportunities to dissect pathogen signaling effects on a specific host defense mechanism.
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. The interaction of barley with the powdery mildew fungus is a well-developed model to investigate the molecular communications among plants and plant pathogens. Scientists at USDA-ARS in Ames, Iowa led an International team from the USA (ARS, Iowa State University, Cornell University and Indiana University), and the United Kingdom (Imperial College, London and RHUL, Egham) to uncover the mysteries of how pathogens colonize their hosts to cause disease. Novel gene-silencing and overexpression technologies were developed to demonstrate that a protein from the powdery mildew fungus promotes pathogen virulence and suppresses host cell death in the grain crop, barley. Homologs encoding this particular protein are present in 96 of 241 sequenced fungal genomes representing plant pathogens, as well as human and insect pathogens. This broadly conserved protein family provides the opportunity to investigate mechanisms and fungal pathogenesis that are important in both medicine and agriculture, which will lead to preventative treatments.
Review Publications
Liu, J., Cheng, X., Liu, D., Xu, W., Wise, R.P., Shen, Q. 2014. The miR9863 family regulates distinct Mla alleles in barley to attenuate NLR receptor-triggered disease resistance and cell-death signaling. PLoS Genetics. 10(12):e1004755. DOI:10.1371/journal.pgen.1004755.
Whigham, E., Qi, S., Mistry, D., Surana, P., Xu, R., Fuerst, G.S., Pliego, C., Bindschedler, L., Spanu, P., Dickerson, J., Innes, R.W., Nettleton, D., Bogdanove, A.J., Wise, R.P. 2015. Broadly conserved fungal effector BEC1019 suppresses host cell death and enhances pathogen virulence in powdery mildew of barley (Hordeum vulgare L.). Molecular Plant-Microbe Interactions. 28(9): 968-983. DOI: http://dx.doi.org/10.1094/MPMI-02-15-0027-FI.
Xu, W., Meng, Y., Surana, P., Fuerst, G.S., Nettleton, D., Wise, R.P. 2015. The knottin-like Blufensin family regulates genes involved in nuclear import and the secretory pathway in barley-powdery mildew interactions. Frontiers in Plant Science. 6:409. DOI:10.3389/fpls.2015.00409.
Teixeira, J.E., Weldekidan, T., de Leon, N., Flint Garcia, S.A., Holland, J.B., Lauter, N.C., Murray, S.C., Xu, W., Hessel, D., Kleintop, A.E., Hawk, J., Hallauer, A.R., Wisser, R. 2015. Hallauer’s Tusón: a decade of selection for tropical-to-temperate phenological adaptation in maize. Heredity. 114:229-240.
