Molecular Genetic Investigations of Auto-straightening Using Brachypodium Roots as Model

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Plants are anchored in place but can move and grow in response to the physical environment. One such remarkable response is to gravity: regardless of the position of the plant, whether turned upside down or sideways, young plant roots will grow in the direction of gravity, towards the Earth and shoots will grow away from gravity, in the direction of light. How the plant perceives and responds to gravity stimulation is a question that has intrigued scientists a century and a half, since the time of Darwin. It is now known that variations in cell growth drive the movement of roots, but the growth patterns are subtle and complicated. Recently it was discovered that during the gravity response, roots grow down by curving and also self-straightening, a mechanically puzzling but essential phenomenon related to how the roots acquire water and nutrients from the soil. This project investigates how self-straightening occurs using interdisciplinary methods, including novel imaging techniques, mathematics, molecular genetics and genomics. The long-term goal is to identify the genes that control the curving root response to gravity. Understanding how roots grow into and through the soil will open the door to manipulating and selecting for more efficient root growth. As the world faces changing water availability and deficits, this research could uncover root responses that can be harnessed for crop improvements. The project also brings the wonder of plant movement to the general public through establishing museum displays complete with images and information about root growth. Graduate and undergraduate students will be trained in inter-disciplinary biological research to help move these studies into the future.Plant organs have the ability to sense a curvature and respond by straightening, a process that is evocative of proprioception. This auto-straightening response contributes to organ's posture, an important determinant of overall plant architecture, productivity and market value. When gravistimulated, Brachypodium roots initially respond by developing a strong downward curvature whose rate decreases after the tip reaches a 45-degree angle from the vertical. At this point, the root straightens before undergoing a new round of curvature. This process reiterates several times before the tip reaches a vertical posture. From then on, the root tip continues to oscillate around the vertical. A mathematical model was built to recapitulate these complex behaviors, and used to estimate explanatory parameters of the movements for a total of 46 distinct Brachypodium accessions. This allowed the development of a genome-wide association study (GWAS) aimed at identifying contributing genes. Several DNA polymorphisms were found to be associated with the amplitude of the oscillations. The main objective of this project is to investigate the cellular characteristics of these oscillations, and study the molecular mechanisms controlling them. The specific aims are: 1) Characterize the spatio-temporal distribution of curvatures and accompanying auto-straightening phases during a graviresponse; 2) Investigate the involvement of auxin gradients in their regulation, and 3) Initiate a molecular characterization of these oscillations, using transcriptomics and GWAS approaches to identify candidate regulators. These experiments will lead to a better understanding of the molecular and cellular mechanisms that govern auto-straightening and proprioception in plants.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.