Microtubule Nucleation in Plant Cortical Array Patterning

Plants maximize resource capture by sensing their environment and carefully adjusting their morphology during growth. Unlike animal cells, plants construct their form through the manufacture of semirigid cell walls, layers of crosslinked cellulose polymers with special material properties. Cellulose is the most abundant biopolymer on the planet, critical for food supply, fuel, and as renewable construction materials.

By carefully templating how these cell wall fibers are deposited during growth, the plant cell can expand into shapes as simple as a cube or as complicated as stars or puzzle pieces. This project examines the primary mechanisms that plants use for templating cellulose fibers into different pattern types to ultimately determine their morphology. Using a simple and genetically tractable plant system, the project investigates how microtubule polymers on the inside surface of the cell form patterns that determine how cellulose is deposited on the outside surface of the cell.

The work uses advanced imaging methods and computational approaches to map the temporal and spatial changes in microtubule patterning in living cells through time. The molecular and genetic mechanisms will be pursued through the investigation of newly discovered mutations that impair pattern formation. The imaging data and genetic analysis are coupled to computational models with the goal of relating microtubule patterning to cell expansion and its relation to plant morphology.

The Broader Impacts of the work include its intrinsic merit as the data produced will be applicable to all plants. In addition, computational and imaging tools will be made available for use by the plant research community. The experiments for this project, including molecular genetic analyses and live-cell imaging, will be performed by undergraduate students in a class setting, bringing students into the world of real scientific discovery as a broader impact of this work. Additionally, the PI participates in a local summer research program for High School students.

Flowering plants organize microtubule arrays at the cell cortex to template the deposition of cellulose microfibrils, creating the material properties required for correct cell expansion. This project examines the proposal that acentriolar plant cells control where, and in what orientation, microtubules are nucleated to affect array patterns. Focusing on axially growing hypocotyl cells, the work will test the hypothesis that AUGMIN8 proteins regulate microtubule nucleation from the side of existing microtubules as a specific nucleation type with a major role in setting array pattern.

The working model posits that cortical microtubule arrays oriented along the plant growth axis depend upon continuous anti-parallel microtubule-dependent microtubule nucleation where transversely aligned arrays use parallel microtubule-dependent microtubule nucleation. Investigation of the larger AUGMIN8 gene family will ask if different members determine different types of microtubule-dependent microtubule nucleation for different array patterns.

The model will be further evaluated genetically in a series of new Arabidopsis mutants that appear blocked for a signaling step that would normally turn off anti-parallel microtubule nucleation, preventing the formation of correct transverse co-aligned patterns. The outcomes of this work will contribute directly to our understanding of the rules of life with impacts on plant cell and developmental biology.

Hypocotyl elongation through soils is a critical trait for crop development with impacts on plant breeding, crop engineering and food supply.

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.