ROOT HAIR MORPHOGENESIS AND FUNCTION IN THE TEMPERATE GRASS BRACHYPODIUM DISTACHYON

Miguel Angel Rosas

doi:10.7273/000007225

The root epidermis is essential for interactions with the rhizosphere and facilitates water and nutrient assimilation. Root hairs increase the surface area of the root and show dynamic growth responses to environmental cues. The molecular signals that govern root hair initiation and growth have been well documented in Arabidopsis thaliana, whereas less is known about the molecular cues that regulate root hair growth in the temperate C3 grass model Brachypodium distachyon. Despite the importance of root systems for plant growth, the molecular and genetic regulation of root hair tip growth has been understudied in monocots, which makeup the majority of agricultural biomass. In order to study the regulatory mechanisms that drives root hair growth in grasses we used the nitrate-responsive buzz mutant in Brachypodium distachyon, which lacks root hairs and has an increased root growth rate as a tool to determine the genetic and physiological underpinnings of two related growth processes in the root: root hair outgrowth and the regulation of root growth rate. We found that BUZZ encodes a cyclin dependent kinase (CDK)-like gene that is an essential regulator of post-initiation root hair growth and a modulator of root architecture. Additionally, we found nitrate transporters were upregulated in buzz which provides a link between CDK-mediated root hair growth and nitrate. To investigate this connection, we quantified root architectural parameters in response to various nitrate concentrations in the mutant and observed insensitive and stimulatory effects on primary and lateral root growth, respectively. Interestingly, under low nitrate conditions the buzz mutants accumulated more nitrate in shoot tissue and had a higher nitrate uptake rate relative to wild type. Moreover, proteins that interact with BUZZ to drive root developmental plasticity in response to nitrate and root hair growth were identified. Mutations in these BUZZ interacting proteins resulted in plants with abnormal root hair phenotypes, including shorter root hairs and arrested tip growth post-initiation. These data provide a framework for how BUZZ coordinates tip-mediated cell elongation in root hairs. In this model BUZZ interacts with components of the transcriptional regulatory apparatus to control the expression of genes involved in intracellular trafficking and cell wall biosynthesis. BUZZ orchestrates these seemingly distinct processes to coordinate vesicle-mediated transport of cellular components to the expanding tip, thereby supporting root hair growth. These findings deepen our understanding of root hair growth in grasses and the role of BUZZ in tip expansion. Additionally, this work provides insights into how root hairs sense environmental changes and initiate plant-wide physiological responses.

ROOT HAIR MORPHOGENESIS AND FUNCTION IN THE TEMPERATE GRASS BRACHYPODIUM DISTACHYON

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