The A.thaliana receptor-like kinase PERK13 guides root hair tip growth and controls cell wall composition during pH-related stress
AffiliationSchool of BioSciences
Document TypePhD thesis
Access StatusThis item is embargoed and will be available on 2020-12-05. This item is currently available to University of Melbourne staff and students only, login required.
© 2018 Dr. Amelie Mendrinna
Plant root hairs are tubular shaped cells emerging from the outermost cell layer of the root. They substantially extend the root surface, thereby optimise the plant’s capacity to absorb water and nutrients. Root hairs, furthermore, serve as a docking station for nitrogen fixing bacteria and help to anchor the plant in the soil. These features make root hairs very important for plant-soil interactions and research in this area holds future implications for agriculture in changing soil environments in the course of climate change. Our research is focused on root hair growth and factors that help to maintain the integrity of the dynamic cell wall that surrounds the hairs, and that changes during growth. We are interested in how various extra- and intracellular factors, including calcium ions, reactive oxygen species (ROS), pH, the cytoskeleton and vesicular trafficking, are coordinated to sustain cell wall changes and polar growth. In this study we characterise the Arabidopsis thaliana receptor-like kinase PERK13 of the subfamily proline-rich extensin-like receptor kinases (PERK). Members of the PERK family play a role in cell wall integrity sensing due to their extracellular domain that resembles cell wall extensins and that is possibly responsible for an association with pectins in the cell wall. We found that PERK13 is specifically localised to the root hair plasma membrane and guides root hair growth from initiation to full elongation. PERK13 orchestrates intracellular processes to sustain root hair growth under pH-related stress. Plants that lack the kinase protein cannot maintain root hair polarity, which results in uncontrolled expansion of the root hair tip when they are grown in unstable pH environments. Our results suggest that this phenotype is due to overactive plasma membrane H+-ATPases that lead to an over-acidified apoplast in the perk13 mutant. Supporting these findings, we discovered structural changes in the cell wall, particularly in pectic polysaccharides and xyloglucan; both of which are known to be susceptible to pH changes. Pectin is not only regulated by the apoplastic pH but also by calcium ion oscillations that regulate root hair growth. Indeed, the mutant showed a decrease in the frequency of intracellular calcium oscillations and bursting root hairs in the mutant could be rescued by manipulating the calcium ion permeability of the plasma membrane. Addressing the challenging question how pH, calcium and cell wall dynamics are connected with PERK13 we investigated a possible interaction with Rho of plants (ROPs), molecular switches that act as major regulatory hubs of root hair growth. Here, we found that PERK13 and ROP constitutively active lines interact genetically and exhibit an additive root hair phenotype. Summarising our results, we propose that PERK13 controls cell wall composition under pH stress conditions by integrating important drivers of root hair growth such as pH and calcium into a cell wall integrity feedback mechanism. This project may be of great significance to agricultural applications that improve nutrient uptake and plant growth on soils with aberrant pH levels.
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