Abstract
Wild maize (teosinte) has been reported to be less susceptible to pests than their modern maize (corn) relatives. Endophytes, defined as microbes that inhabit plants without causing disease, are known for their ability to antagonize plant pests and pathogens. We hypothesized that the wild relatives of modern maize may host endophytes that combat pathogens. Fusarium graminearum is the fungus that causes Gibberella Ear Rot (GER) in modern maize and produces the mycotoxin, deoxynivalenol (DON). In this study, 215 bacterial endophytes, previously isolated from diverse maize genotypes including wild teosintes, traditional landraces and modern varieties, were tested for their ability to antagonize F. graminearum in vitro. Candidate endophytes were then tested for their ability to suppress GER in modern maize in independent greenhouse trials. The results revealed that three candidate endophytes derived from wild teosintes were most potent in suppressing F. graminearum in vitro and GER in a modern maize hybrid. These wild teosinte endophytes could suppress a broad spectrum of fungal pathogens of modern crops in vitro. The teosinte endophytes also suppressed DON mycotoxin during storage to below acceptable safety threshold levels. A fourth, less robust anti-fungal strain was isolated from a modern maize hybrid. Three of the anti-fungal endophytes were predicted to be Paenibacillus polymyxa, along with one strain of Citrobacter. Microscopy studies suggested a fungicidal mode of action by all four strains. Molecular and biochemical studies showed that the P. polymyxa strains produced the previously characterized anti-Fusarium compound, fusaricidin. Our results suggest that the wild relatives of modern crops may serve as a valuable reservoir for endophytes in the ongoing fight against serious threats to modern agriculture. We discuss the possible impact of crop evolution and domestication on endophytes in the context of plant defense.
Highlights
Modern maize, belonging to the genus Zea, was domesticated in southern Mexico 9000 years ago from wild, annual tropical grasses called teosintes, with the primary ancestor being Parviglumis (Zea mays ssp. parviglumis) which survives today in the wild (Matsuoka et al, 2002)
The results revealed that four bacterial endophytes could consistently inhibit the growth of F. graminearum
Three of these endophytes were isolated from wild maize genotypes: strain 1D6 from Z. diploperennis and strains 4G12 and 4G4 from Parviglumis, the direct ancestor of modern maize (Figures 1A,2A)
Summary
Modern maize, belonging to the genus Zea, was domesticated in southern Mexico 9000 years ago from wild, annual tropical grasses called teosintes, with the primary ancestor being Parviglumis (Zea mays ssp. parviglumis) which survives today in the wild (Matsuoka et al, 2002). Modern maize, belonging to the genus Zea, was domesticated in southern Mexico 9000 years ago from wild, annual tropical grasses called teosintes, with the primary ancestor being Parviglumis There are additional species of teosintes that continue to grow in the wild in Mexico and Central America including the perennial Zea diploperennis (Iltis and Doebley, 1980). Though the increased disease susceptibility of modern maize has been attributed to changes in the plant genome, there may be additional explanations. Domestication, breeding and migration, some endophytes were lost (Johnston-Monje and Raizada, 2011; Johnston-Monje et al, 2014), and it is possible that endophytic genomes may have been modified—phenomena that might contribute to the increased disease susceptibility of modern maize
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