Interested Members of the University Community are invited to attend the Final Oral Examination for the Degree of Doctor of Philosophy of Anuja Shrestha of the Department of Plant Agriculture 

Date: Thursday, May 4th, 2023
Time: 1:00 PM (Virtual) 
Method: Via Zoom
Join Zoom Meeting: https://zoom.us/j/97385778463?pwd=RkJLK3lpb2NXSXBJMUJadW5XOEZwUT09
Meeting ID: 973 8577 8463
Passcode: 330940 

Thesis Title: “Discovery and Testing of Bacteria from Pollen and Unpollinated Silks of Pan-American Maize to Combat Fusarium graminearum” 

Advisory Committee
Dr. Manish N. Raizada, Advisor
Dr. Art Schaafsma
Dr. Katerina Jordan
Dr. Joseph Colasanti                                   

Examination Committee
Dr. Helen Booker, Chair 
Dr. Jason Wallace, External Examiner
Dr. Manish N. Raizada
Dr. Joseph Colasanti
Dr. Jennifer Geddes-McAlister 

ABSTRACT 

The process of fertilization in maize (Zea mays L.) makes its progeny susceptible to the fungal pathogen, Fusarium graminearum (Fg), that causes the devastating grain disease Gibberella ear rot (GER). Fg deposits mycotoxins in seeds, reduces progeny fitness, and ultimately causes significant economic losses to farmers globally. Maize is a wind-pollinated crop. Fg spores are airborne and enter developing seeds through the maternally derived style (silk) which must be exposed to the environment to capture pollen and transmit the male gametes to the ovule. Pollen and style tissues of other plants have been previously explored for their microbiomes; however no prior studies have reported the functions of pollen-associated bacteria in any plant species, and no silk-derived bacteria have been tested for suppression of Fg. I hypothesized that the male gametophyte (pollen) and the maternal transmission route for male gametes (unpollinated silk) possess beneficial bacteria that can actively defend the silk channels against Fg, protecting progeny seeds and ultimately the genetic contribution of both parents. Additionally, I hypothesized that there may have been long-term natural and/or farmer selection for pollen and silk-associated bacteria to suppress Fg along the silk passage and in progeny seeds. I cultured bacteria from pollen and unpollinated silks from diverse wild and farmer-selected maize landraces spanning the Americas. These bacteria were taxonomically classified using 16S rRNA sequencing, then tested against Fg in vitro. The five best bacterial strains were further sprayed onto silks and then challenged with Fg in replicated greenhouse trials, followed by mycotoxin testing: the strains suppressed disease and mycotoxins. The best anti-Fg strain from both pollen and silk were further studied using confocal fluorescent imaging of live silks, revealing their ability to pre-emptively colonize known Fg entry points on silks, with evidence for fungicidal activity after infection. Overall, I gained support for the hypotheses that maize pollen and silk possess beneficial bacteria that can defend the male migration route inside the exposed maternal silk passage against airborne Fg. I also found evidence to suggest these anti-Fg microbiome traits were under natural and human selection in the Americas, including by specific indigenous peoples.