Assembly of a polar domain - the role of electrostatic interactions at the plasma membrane during the initiation of tip growth in Arabidopsis roots
Final Report Abstract
Form follows function – the famous principle in product design and architecture is a concept that humans have adapted from nature. Shape, of cells, organs or the body, is being optimized through processes of evolution to fulfill a certain function, maximize efficiency and, thereby, improve overall fitness. Since evolution acts through genetics, information on shape must be, to some extent, encoded in our genome. The big question is, how the genetic code – a series of nucleotides – is translated into a functional shape. Our research aims at identifying mechanisms that (1) robustly define the shape of cells and (2) integrate environmental conditions in the regulation of cellular growth. To study these problems, we are using a cell type of the outer tissue layer of plant roots that produces a certain shape in a highly reliable manner: the root hair. These specialized cells form finger-like protrusions at a specific site of the cell periphery. On the one hand, the position of this site, as well as its diameter, show only little variation or high robustness. On the other hand, the growth rate and final length of a root hair is highly dependent on growth conditions. During this project, both aspects of root hair formation, shaping and growth regulation have been studied. 1. Root hair shape. To form this protrusion, numerous components of the cellular growth machinery have to be targeted to this particular site. This site can be imagined like a construction site, where the architect’s plan (i.e., genetic information) is also translated into a large three-dimensional structure. However, in the case of the site of root hair outgrowth, it has remained unclear how this site is chosen, who takes the lead and marks this site, and what the specific timings of the subsequently arriving components are. In our project we addressed these questions by first performing a temporal analysis of the assembly process in root hair cells of the reference plant Arabidopsis thaliana. Taking advantage of fluorescent markers that we fused to many known and expected components of the growth machinery, we could produce a detailed “schedule” of events at the future growth site. Importantly, we identified a protein, named ROPGEF3, that is not only the earliest known component to arrive at the growth site, but is also required for the formation of the growth site and the recruitment of subsequent components. Interestingly, when too much of ROPGEF3 is produced by the cell, the protein has the ability to induce the formation of additional sites in other areas of the cell periphery or even in cells that usually do not show such local accumulations. This demonstrates that ROPGEF3 is necessary and (in certain cellular contexts) sufficient to initiate a polar domain that determines cellular shape. 2. Root hair growth. Once the growth site is defined, regulatory mechanisms initiate growth that are very sensitive to environmental conditions. To be able to observe root hair formation over longer periods of time, or to manipulate these conditions in a controlled manner, we developed two devices that consist of microchannels that serve as growth chambers for roots of Arabidopsis seedlings. The first device consists of straight channels that allow for predictable growth under defined conditions, which enabled us to perform high resolution imaging of protein recruitment to the future hair growth site. The second device enabled us to address the question whether the root takes over a coordinating function that controls hair growth in all root hair cells or whether each root hair cell senses it’s local environment and responds to it independently. In this device, a root could be treated differently on both sides, creating an asymmetric growth environment. Our experiments showed that root hairs on each side acted autonomously. This means that roots growing in heterogeneous environments can adapt their shape to local conditions, which is likely a benefit when growing in complex substrates like soil. “Arabidopsis perfusion platform RootChip reveals local adaptations of roots.” https://plantae.org/arabidopsis-perfusion-platform-rootchip-reveals-local-adaptationsof-roots/ “GTPasen im Takt. So entstehen Wurzelhaare.” https://www.pflanzenforschung.de/de/pflanzenwissen/journal/gtpasen-im-takt-so-entstehen-wurzelhaare-11072 “Distinct RopGEFs successively drive polarization and outgrowth of root hairs.” https://plantae.org/distinct-ropgefs-successively-drive-polarization-and-outgrowth-ofroot-hairs-curr-biol/
Publications
- (2018) Dual-flow-RootChip reveals local adaptations of roots towards environmental asymmetry at the physiological and genetic levels. New Phytol. 217(3):1357-69
Stanley, CE, Shrivastava, J, Brugman, R, van Swaay, D, and Grossmann, G
(See online at https://doi.org/10.1111/nph.14887) - (2018). Fabrication and Use of the Dual-Flow-RootChip for the Imaging of Arabidopsis Roots in Asymmetric Microenvironments. Bio-protocol 8(18): e3010
Stanley, CE, Shrivastava, J, Brugman, R, Heinzelmann, E, Frajs, V, Bühler, A, van Swaay, D, and Grossmann, G
(See online at https://doi.org/10.21769/bioprotoc.3010) - (2019) Distinct ROPGEFs successively drive polarization and outgrowth of root hairs. Curr. Biol.
Denninger, P, Reichelt, A, Schmidt, VAF, Mehlhorn, DG, Asseck, LY, Stanley, CE, Keinath, NF, Evers, J-F, Grefen, C, and Grossmann G
(See online at https://doi.org/10.1016/j.cub.2019.04.059) - (2019) Multiple cyclic nucleotide-gated channels coordinate calcium oscillations and polar growth of root hairs. Plant J.
Brost, C, Studtrucker, T, Reimann, R, Denninger, P, Czekalla, J, Krebs, M, Fabry, B, Schumacher, K, Grossmann, G, and Dietrich P
(See online at https://doi.org/10.1111/tpj.14371) - (2020) Chapter: Microfluidic Systems for Plant Root Imaging. Methods in Cell Biology: Plant Cell Biology, eds. Dixit, R, Haswell, E, Anderson, C; Vol. 160; Academic Press
Guichard, M, Bertran Garcia de Olalla, E, Stanley, CE and Grossmann G
(See online at https://doi.org/10.1016/bs.mcb.2020.03.012)
