Effects of domestication and nitrogen on root growth and fine scale architecture in maize (Zea mays L.)

This thesis is an investigation of the hidden half of maize (Zea mays L.). I investigated maize root growth and architecture relative to shoot traits in response to domestication and low nitrogen. A customized aeroponics growth system was engineered, in which roots were grown in the air, misted with a nutrient solution. Aeroponics allowed dynamic phenotyping of the post-embryonic maize root system at a fine scale and in a homogeneous rhizosphere environment. In modem maize, novel root responses to low nitrogen were found, including an increase in second order lateral root branching and root hair length plasticity. The data also suggested that stochastic variation in lateral root length may be an adaptation to low nitrogen stress or itself a stress response. These results help to fill in the gap that existed in the otherwise well characterized nitrogen-stress response in modem maize. Examination of wild ancestors can identify which traits have been altered by selection as possible targets for future genetic improvement. I investigated the adaptations to low nitrogen in the wild ancestor of modem maize, Balsas teosinte ( Zea mays ssp. parviglumis). Surprising conservation was found in root responses to low nitrogen in modem maize and teosinte. However, the mechanisms involved were often different and likely unavoidable consequences of selection for decreased tillering by ancient agriculturalists. Finally, I investigated the effects of domestication on root morphology. Ancient selection on the Teosinte branched 1 (Tb1) locus was responsible for decreased tillering during domestication. In modem maize, a mutant allele of Tb1 (tb1-ref) that restores the ancestral shoot phenotype, increased crown root number, perhaps as an indirect effect of increased tillering. Homozygous tb1-ref mutants had increased first and second order lateral root branching suggesting that TB1 might act as a repressor of root branch meristems. Though the root system of modem maize is different than teosinte, the tb1-ref allele alone was sufficient to restore the ancestral root phenotype. These results suggest that selection at the Tb1 locus for shoot apical dominance by ancient agriculturalists had a major effect, albeit unintentional, on the maize root system. These results have implications for water and nutrient uptake efficiency, limiting factors for maize production in the century ahead.