Seminal roots constitute the initial wheat root system and provide the main route for water absorption during early stages of development. Seminal root number (SRN) varies among species. However, the mechanisms through which SRN is controlled and in turn contribute to environmental adaptation are poorly understood. Here, we show that SRN increased upon wheat domestication from 3 to 5 due to the activation of 2 root primordia that are suppressed in wild wheat, a trait controlled by loci expressed in the germinating embryo. Suppression of root primordia did not limit water uptake, indicating that 3 seminal roots is adequate to maintain growth during seedling development. The persistence of roots at their primordial state promoted seedling recovery from water stress through reactivation of suppressed primordia upon rehydration. Our findings suggest that under well-watered conditions, SRN is not a limiting factor, and excessive number of roots may be costly and maladaptive. Following water stress, lack of substantial root system suppresses growth and rapid recovery of the root system is essential for seedling recovery. This study underscores SRN as key adaptive trait that was reshaped upon domestication. The maintenance of roots at their primordial state during seedling development may be regarded as seedling protective mechanism against water stress.

Root axial conductance, which describes the ability of water to move through the xylem, contributes to the rate of water uptake from the soil throughout the whole plant lifecycle. Under the rainfed wheat agro-system, grain-filling is typically occurring during declining water availability (i.e., terminal drought). Therefore, preserving soil water moisture during grain filling could serve as a key adaptive trait. We hypothesized that lower wheat root axial conductance can promote higher yields under terminal drought. A segregating population derived from a cross between durum wheat and its direct progenitor wild emmer wheat was used to underpin the genetic basis of seminal root architectural and functional traits. We detected 75 QTL associated with seminal roots morphological, anatomical and physiological traits, with several hotspots harbouring co-localized QTL. We further validated the axial conductance and central metaxylem QTL using wild introgression lines. Field-based characterization of genotypes with contrasting axial conductance suggested the contribution of low axial conductance as a mechanism for water conservation during grain filling and consequent increase in grain size and yield. Our findings underscore the potential of harnessing wild alleles to reshape the wheat root system architecture and associated hydraulic properties for greater adaptability under changing climate.

Root‐system hydraulic conductivity (RSHC) is an important physiological characteristic that describes the inherent ability of roots to conduct water across a water‐potential gradient between the root and the stem xylem. RSHC is commonly used as an indicator of plant functioning and adaptability to a given environment. A simple, fast, and easy‐to‐use protocol is described for the quantification of RSHC at the seedling stage in two important monocot species grown in hydroponic solution: Setaria viridis, a C4 model plant, and wheat, a C3 crop plant. This protocol can also be easily modified for use with almost any grass species and environmental treatments, such as salinity or hormone treatments. © 2020 by John Wiley & Sons, Inc.Basic Protocol: Setaria hydrostatic root‐system hydraulic conductivityAlternate Protocol: Measuring the root conductivity of young plants with soft stems

Root axial conductance contributes to the rate of water uptake from the soil throughout the whole plant lifecycle. In a rainfed wheat agro-system, grain-filling is typically occurs during declining water availability (i.e. terminal drought). Therefore, preserving soil water moisture during grain filling could serve as a key adaptive trait. We hypothesized that lower wheat root axial conductance can promote higher yields under terminal drought. A segregating population derived from a cross between durum wheat and its direct progenitor wild emmer wheat was used to underpinning the genetic basis of seminal root architectural and functional traits. We detected 75 QTL associated with seminal roots morphological, anatomical and physiological traits, with several hotspots harboring co-localized QTL. We further validated the axial conductance and central metaxylem QTL using wild introgression lines. Field-based characterization of genotypes with contrasting axial conductance, suggested the contribution of low axial conductance as a mechanism for water conservation during grain filling and consequent increase in grain size and yield. Our findings underscore the potential of introducing wild alleles to reshape the wheat root system architecture for greater adaptability under changing climate.