Auxin is a plant hormone that positively regulates the initiation and emergence of lateral roots. In this study we used several approaches to reduce auxin transport moving either from the shoot toward the root apex or from the root tip toward the base and examined the effect of these treatments on lateral root initiations and emergence. Our results are consistent with shoot derived auxin driving lateral root initiation, while we could detect no role for root tip auxin in lateral root initiation or elongation. We examined how environmental variables regulate root formation with a focus on day length and growth temperature and found that elevated temperature and increased day length positively regulate root formation. We explored several mechanisms by which root initiation is enhanced under these conditions including elevated auxin synthesis, transport, and signaling. Both auxin accumulation and auxin dependent gene expression increase during these treatments, suggesting a mechanistic basis for regulation of root branching. We also asked whether populations isolated from different latitudes might exhibit differences in root architecture. Environmental variables across a latitudinal gradient have been shown to both transiently affect growth, development, and physiological responses, as well drive the evolution of ecotypes with unique genotypes that increase fitness. We hypothesized that early root architecture as well as resource allocation would differ for populations across latitude based on the latitudinal provenance. We observed differences in lateral root initiation, elongation, and developmental patterns under higher UV that followed a latitudinal cline. In populations from higher latitude, there were fewer lateral root primordia and emerged lateral roots, as well as less shoot biomass. We observed that in mature plants grown at lower UV levels, the photosynthetic rate and shoot biomass were lower in populations from higher latitudes leading to a different carbon balance. Root respiration and root biomass did not change between these populations isolated from different latitudes. Together these results provide insight into the hormonal, environmental, and genetic controls of root developmental patterning
