Lost phenotypes: Exploring the role of maize history and breeding in rhizosphere nitrification suppression

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Sierra Raglin and Angela Kent, Natural Resources & Environmental Sciences, University of Illinois at Urbana-Champaign, Urbana, IL, Esther N. Ngumbi, Department of Entomology, University of Illinois at Urbana-Champaign, Urbana, IL

Background/Question/Methods
Maize (Zea mays subsp. mays) domestication occurred ~9000 years ago. Substantial modification to the maize genome has since occurred, including introgressions with sympatric Zea species and population bottlenecks. Simultaneously, maize reproductive and vegetative architecture was heavily selected to maximize yields. In the 20th century, selective breeding was conducted in agroecosystems artificially fertilized with nitrogenous fertilizers, removing the selective pressures of nitrogen deficiencies from maize germplasm development. In doing so, the directed evolution of maize may have inadvertently selected against microbial nutritional symbioses important for growth and fitness. Nitrification, the chemolithoautotrophic oxidation of ammonium to nitrate, is an important microbial biogeochemical process with the potential to increase rhizosphere nitrogen loss by promoting nitrate leaching and/or nitrous oxide emissions. Numerous grass species, including landrace rice (Oryza sativa), perennial wild rye (Leymus racemosus), and sorghum (Sorghum bicolor), possess the biological nitrification inhibition (BNI) phenotype, allowing plants to enzymatically inhibit nitrification, and increasing rhizosphere nitrogen availability. We hypothesize that maize breeding in replete N conditions has selected against maintenance of the BNI phenotype in modern maize germplasm. The effect of maize genotype and demography on rhizosphere nitrification potential and microbial communities was assessed using rhizosphere nitrification potential enzyme assays and 16S rRNA amplicon sequencing.
Results/Conclusions
Maize genotypes were categorized based on history (demographics): teosinte (Zea mays subsp. parviglumis) the progenitor of modern maize (WildParent); wild upland Zea mays subsp. mexicana, which introgressed with early domesticated maize (WildIntro); landrace maize native to Mexico (MexLand); landrace maize native to southwestern US (AmLand), selected to represent the migration of maize into North America; inbred landrace maize varieties (InbredLand), selected to represent the transition of maize breeding from open pollination to selfing/inbreeding; Ex-PVP modern inbreds used for the development of elite hybrids (InbredModern). Each demographic group was composed of four genotypes, totaling 24 genotypes, with bulk soil and bulk+allythiourea (a synthetic nitrification inhibitor) as controls. Linear mixed effects models revealed that demographic group significantly influenced rhizosphere nitrification potential (F(7,157) = 12.424, P < 0.0001), with the lowest rhizosphere nitrification potential within the Mexican landraces, and the highest in the Ex-PVP modern inbred cultivars. The Mexican landraces did not have significantly different nitrification potential than the allythiourea-inhibited control, suggesting these genotypes may possess a nitrification inhibition phenotype. Nitrification potential significantly influenced (F(1,141) = 35.532, P < 0.0001) leaf SPAD readings (used as a proxy for leaf nitrogen content), suggesting rhizosphere nitrification rates influence maize N-assimilation. Microbiome data is being processed.