Abstract
Roots are essential for plant adaptation to changing environments, yet the role of roots in crop domestication remains unclear. This study examined the evolution of root phenotypes from teosinte to maize, a transition resulting in reduced nodal root number (NRN), multiseriate cortical sclerenchyma (MCS), and increased seminal root number (SRN). We reconstructed the root phenotypes of maize and teosinte, as well as the environments of the Tehuacán Valley over the last 18 000 yr using a combination of ancient DNA, paleobotany, and functional-structural modeling. Our models reveal that increasing Holocene atmospheric CO(2) concentrations favored the appearance of reduced NRN and MCS between 12 000 and 8000 yr before present (yrBP), promoting deeper root systems. The advent of irrigation by 6000 yrBP switched nitrogen distribution from topsoil to subsoil domains, a change that increased the utility of reduced NRN and MCS. Comparison of allelic frequencies among ancient samples ranging from 5500 to 500 yrBP suggests that increased SRN may have appeared c. 3500 yrBP, coinciding with a period of increased human population, agricultural intensification, and soil degradation. Our results suggest that root phenotypes that enhance plant performance under nitrogen stress are important for maize adaptation to changing agricultural practices in the Tehuacán Valley.