Understanding Differences in Alfalfa Root Systems and Their Importance for Abiotic Stress Tolerance.

The root system architecture (RSA) of plants impacts their capacity for efficient water and nutrient uptake. Root phenes have been associated with productivity under stress conditions and these stress factors can also impact the persistence of perennial species. The objectives of this study were to identify root traits that increase productivity under abiotic stress and determine genetic determinants underlying these traits in alfalfa (Medicago sativa L.). Alfalfa is a perennial, auto-tetraploid, out-crossing species and a major forage crop. Field-based evaluations were conducted to identify individuals contrasting for root morphology (branched vs. tapped roots) and drought tolerance. Seedlings from multiple alfalfa populations were further characterized in the greenhouse for biomass production and root growth, including the root penetration ability under drought stress conditions. The amount of tertiary roots was significantly different between plants with mainly tapped vs. branched roots. Anatomical traits of predominately tapped vs. branched roots including cortical cell size and number, and diameter of the root vasculature were also evaluated. Root traits associated with increased biomass production were identified. In parallel, individuals from breeding and mapping populations contrasting for root growth were genotyped using genotyping-by-sequencing to identify genetic determinants associated with RSA in alfalfa. We also evaluated shifts in allele frequencies in key genes associated with root development. Identifying the ideal RSA for different soil types and abiotic stress conditions and unravelling key genes most relevant for root growth provides the foundation to implement genomics-based breeding approaches to increase alfalfa biomass yield and persistence.