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A soil-plate based pipeline for assessing cereal root growth in response to polyethylene glycol (PEG)-induced water deficit stress

Sven K. Nelson, Melvin J. Oliver
Frontiers in plant science 2017 v.8 no. pp. 1272
gas exchange, seedlings, wheat, drought, paper, crop yield, polyethylene glycol, roots, shoots, photographs, water stress, hydroponics, solutes, Triticum aestivum, agar, quantitative analysis, lighting, soil, root growth, digital images, seed germination, green light
Drought is a serious problem that causes losses in crop-yield every year, but the mechanisms underlying how roots respond to water deficit are difficult to study under controlled conditions. Methods for assaying root elongation and architecture, especially for seedlings, are commonly achieved on artificial media, such as agar, moistened filter paper, or in hydroponic systems. However, it has been demonstrated that measuring root characteristics under such conditions does not accurately mimic what is observed when plants are grown in soil. Morphological changes in root behavior occur because of differences in solute diffusion, mechanical impedance, exposure to light (in some designs), and gas exchange of roots grown under these conditions. To address such deficiencies, we developed a quantitative method for assaying seedling root lengths and germination in soil using a plate-based approach with wheat as a model crop. We also further developed the method to include defined water deficits stress levels using the osmotic properties of polyethylene glycol (PEG). Seeds were sewn into soil-filled vertical plates and grown in the dark. Root length measurements were collected using digital photography through the transparent lid under plant-safe green lighting. Photographs were analyzed using the cross-platform ImageJ plugin, SmartRoot, which can detect root edges and partially automate root calling. This allowed for quick measurements and straightforward and accurate assessments of non-linear roots. Other measurements, such as root width or angle, can also be collected by this method. An R function was developed to collect exported root length data, process and reformat the data, and output plots depicting root/shoot growth dynamics. For water deficit experiments, seedlings were transplanted side-by-side into well-watered plates and plates containing PEG solutions to simulate precise water deficits.