Root Traits and Phenotyping Strategies for Plant Improvement

Abstract

Roots are crucial for nutrient and water acquisition and can be targeted to enhance plant productivity under a broad range of growing conditions. A current challenge for plant breeding is the limited ability to phenotype and select for desirable root characteristics due to their underground location. Plant breeding efforts aimed at modifying root traits can result in novel, more stress-tolerant crops and increased yield by enhancing the capacity of the plant for soil exploration and, thus, water and nutrient acquisition. Available approaches for root phenotyping in laboratory, greenhouse and field encompass simple agar plates to labor-intensive root digging (i.e., shovelomics) and soil boring methods, the construction of underground root observation stations and sophisticated computer-assisted root imaging. Here, we summarize root architectural traits relevant to crop productivity, survey root phenotyping strategies and describe their advantages, limitations and practical value for crop and forage breeding programs.

Keywords: abiotic stress; breeding; phenotyping; root; root breeding; root ideotype; root plasticity.

Figure 1

Root architectural, structural and anatomical traits for breeding crops with improved water and nutrient acquisition. (A) Root gravitropism in maize primary roots; (B) Extensive root hair formation on a wheat seedling; (C) Origin of root branches in wheat. New roots emerge from the leaf and coleoptile nodes (white and red arrows), seed (green arrow) and from primary/seminal roots (yellow arrow).

Figure 2

Overview of root phenotyping strategies in the greenhouse, laboratory and field and their application for basic, translational and applied research. The description of images is clockwise within each of the three categories. Laboratory: alfalfa (Medicago sativa L.) roots in clear vials and growth media; wheat (Triticum aestivum L.) seedlings growing on germination paper in plastic trays; alfalfa seedling imaged using flatbed scanner; Arabidopsis thaliana roots stained with propidium iodide to observe cell wall and green fluorescent protein (GFP) labeling the actin cytoskeleton; A. thaliana root clarified showing lateral root initiation; alfalfa seedlings growing in glass cylinders with growth media; Brachypodium distachtyon (model grass) seedlings growing in growth media in plates. Greenhouse: EnviroKing^® (Harrington Industrial Plastics, Albuquerque, NM, USA) UV clear PVC piping at a slanted angle; black deepots; semi-cylindrical mesocosm fronted with clear plexiglass; mesocosms with plastic liners; individual EnviroKing^® UV clear PVC piping for real-time observation of RSA including root depth. Field: overview of an alfalfa field trial; shovelomics approaches digging wheat roots; washed roots from field-grown alfalfa plants; tractor for obtaining core samples; outline of root area for harvest.