Discovery and Testing of Bacteria from Pollen and Unpollinated Silks of Pan-American Maize to Combat Fusarium graminearum
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 unpollinated 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 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. Preliminary qualitative observations are consistent with the hypothesis that these anti-Fg microbiome traits were under selection in the Americas, but further statistical evidence is required.