Open Access paper available here.
Roots are mysterious organ systems that inhabit a world hidden by a veil of soil. Methods that lift this veil often sacrifice physiological relevance in exchange for visualization capability and experimental control. Such compromises may be incompatible with our desire to understand the adaptive mechanisms plants use to thrive despite environmental changes. We have devised a new method named GLO-Roots for Growth and Luminescence Observatory for Roots. GLO-Roots comprises an integrated solution for the growth, visualization and quantification of root systems in a diverse set of plant species including the eudicots Arabidopsis and tomato as well as grasses Brachypodium distachyon and Setaria viridis (unpublished). Our method enables the visualization of plant roots in thin sheets of soil using luminescence-based reporters. We codon-optimized a wide array of luciferase reporter genes and tested which isoforms worked best to reveal the root and, in dual color reporter assays, enabled structure and gene expression to be visualized simultaneously. Using the GLO-Roots system, we were able to simulate drought conditions and showed that Arabidopsis roots exhibit enhanced gravity responses that direct growth downwards towards deep-water resources. We also show that GLO-Roots is ideal for studying root-microbe interactions through the use of bacterial luminescence reporter systems. Working with Guillame Lobet at the University of Liege, we designed GLO-RIA, an image analysis package that automatically quantifies root systems imaged using GLO-Roots. Importantly, this software is able to relate root architectural features with gene expression and soil moisture to generate a true multi-dimensional understanding of root biology. This work is of particular significance as it provides a comprehensive method to study root growth and environmental response at late stages of the lifecycle and with physiological relevance unparalleled by other published systems. The GLO-Roots method is also the subject of a patent application filed in 2013.