Walking around campus, we are surrounded by plants of various sizes — pines, grass, bushes, mosses. Despite the variety of size and characteristics, all these plants share a similar structure: their cytoskeleton. The cytoskeleton is the protein fibers found within the liquid cytoplasm of plant cells that maintain and modify their physical structure. It performs the same function in plants as the bone skeleton does in animals. But how does it function? How does a tiny seed develop into a large, sturdy tree?
On October 11th I met with Dr. Luis Vidali, a scientist researching mechanisms in plant cell growth and reproduction, with a focus on studying the cytoskeleton of the moss species Physcomitrium patens. Born in Mexico before moving to the US to continue his studies after college, Dr. Vidali received his doctorate at the University of Massachusetts, Amherst, and is currently Associate Professor of Biology and Biotechnology at the Worcester Polytechnic Institute in Worcester, MA.
*At the time of the interview verbatim quotes could not be recorded. This transcript is based on notes taken during the interview and the transcript was submitted to Professor Vidali prior to publication to make sure his words were accurately represented.
Luke Taylor: If you were to explain the implications of your research to the general public in a few words or sentences, what would you say?
Dr. Luis Vidali: I study how plant cells grow, especially how plant cells take up more space as they grow. Studying the growth of plant cells is important because plants are integral to our everyday lives- from providing food, fibers, and fuels. Plants are responsible for all of these and all plants are made of microscopic cells with defined shapes. If you want to understand how plants grow, you need to understand plant cell growth.
LT: Why is moss such a good model?
LV: The model I use is Physcomitrium patens: spreading earth moss. We want plant models that grow fast and have a short reproductive cycle, to expedite the pace of researching the cells. Additionally, plants have a reproduction cycle that consists of alternation of generations, where the gametophyte is haploid (has one copy of each chromosome in its cells) and the sporophyte is diploid (has double the number of chromosomes in its cells). The generation we use is primarily the gametophyte, so it having fewer chromosomes in its cell for each generation allows us to identify mutations in its genome and find a demonstrated phenotype much faster than with diploid cells. The moss cells will eventually become identical to each other, allowing for easy control of experimentation without self-breeding techniques.
LT: How do you circumvent the alternation of generations cycle with moss cells if you primarily work with the gametophyte?
LV: The diploid sporophyte of the moss we work with makes brown capsules. We are not interested in these capsules; we are interested in the more dominant gametophyte. The reason we can circumvent the alternation of generations is because the spores develop protonemata, which are the filaments of cells growing from the moss gametophyte. We grind the moss every week to prevent the sporophyte from developing and propagating spores. The tools we use to perform this grinding are blenders not too unlike the ones in a kitchen blender, but could use a two-shaft, two-probe homogenizer as well.
LT: You state that the cytoskeleton is one of the most conserved structures in plants, animals, and fungi. From what you have researched in plant cytoskeletal structure and function, which functions in plant cytoskeletons do you think may be conserved (paralleled) by fungi and animals, and which structures and functions do you believe diverged?
LV: Conserved structures in all eukaryotic cells include the separation of chromosomes by the microtubules in the mitotic spindle, and the polarized transport of vesicles mediated by actin. In evolution, cell division divergence includes the use of the phragmoplast exclusively in some green algae and plants. The phragmoplast is a complex including microtubules and actin which mediates the production of the cell plate during cytokinesis of the plant cell. In contrast, fungi and animal cells use actin and myosin to make the contractile ring, which squeezes the two daughter cells apart.
LT: Myosin is one of the proteins of study in your lab. From my understanding, myosin is associated with animal muscle cells. How does myosin in plant cells relate to myosin use in animal cells?
LV: Plant cells only have two classes of myosin proteins, whereas animal cells have several more classes, the most abundant one is myosin II, (Which explains why animal muscle contraction may be the first thing to come to mind when one hears of myosin. Myosin class II in animal cells make contractile filaments with actin, whereas myosin class I mediate vesicle transportation with actin. These myosins in animal cells are related to stress fibers and their contractile nature. Plant cells lack these myosins: they only have myosin class VIII and XI. Myosin class XI is functionally homologous with myosin class V. Myosin V was present within the last common ancestor between plants and animals and mediates the transport of vesicles. The presence of myosin XI in plant cells shows the conserved nature of vesicle transportation in eukaryotic cells. In fungi, class V, I, and II myosins are present. And class II has a function like the one in the contractile ring seen in animal cells. This is an example of the phylogenetic closeness of the fungi kingdom to the animal kingdom in comparison to the plant kingdom to the animal kingdom. Myosin VIII in plant cells specifically mediates vesicle transportation of the phragmoplasts and plasmodesmata, a function specific to plant cells.
LT: You have a collaboration with the department of physics at WPI. What does this entail in terms of your research and methods? Do you find the interdisciplinary nature of your research to be more enlightening about phytological research? How do you apply the principles of physics in your research?
LV: In my lab we use biophysical and mathematical techniques to model the diffusion of vesicles and molecules in the cell. To do this, we first need to measure the diffusion coefficient of the particles. The diffusion coefficient provides information of how fast particles are moving in space and has units of µm2/s. Because the motion of particles is difficult to measure directly, we instead use the diffusion coefficient to estimate how fast particles may cover a given area in space. By using the plane of coverage as a function of time, we know it takes longer for the molecule or vesicle with a larger diffusion coefficient to cover a larger space. In our experiments, we use reaction-diffusion calculations to measure how long vesicles bind to myosin, and we see that the vesicles bind to the myosin for very brief periods of time. These mathematical and physical models of diffusion allow us to understand the systems better and model changes in the rate of vesicle transport and secretion.
We also use physics and mathematics to study the mechanical properties of plant cell walls. For our purposes, we model plant cell walls as a thin shell of a complex polysaccharide matrix, which behaves like a balloon. Having osmotic pressure of the plant cell will apply turgor pressure to the wall of the cells, causing it to expand and assume a more rigid form. The level we study the cell wall at is at the material rather than molecular level, generally speaking. We use an elastic model for the material properties of the cell wall, including considering tensions, stressors, and strains from the turgor pressure on the wall from osmosis. The purpose of our model is to make predictions about how the plant cell behaves, and as we continue to test these predictions, we update our model’s parameters accordingly.
Related papers by Dr. Vidali and colleagues:
Bibeau, J.P., Furt, F., Mousavi, S.I., Kingsley, J.L., Levine,M.F., Tüzel, E. and Vidali, L. (2020) In vivo interactions between myosin XI, vesicles and filamentous actin are fast and transient in Physcomitrella patens. J. Cell Sci. (2020) 133, jcs234682 doi: 10.1242/jcs.234682
Chelladurai, D., Galotto, G., Petitto, J., Vidali, L., and Wu, M. (2020). Inferring lateral tension distribution in wall structures of single cells. Eur Phys J Plus 135, 662. https://doi.org/10.1140/epjp/s13360-020-00670-8.
Galotto, G., Abreu, I., Sherman, C.A., Liu, B., Gonzalez-Guerrero, M., and Vidali, L. (2020) Chitin triggers calcium-mediated immune response in the plant model Physcomitrella patens. Molecular Plant-Microbe Interactions. doi: 10.1094/MPMI-03-20-0064-R
Kingsley, J.L., Bibeau, J.P., Mousavi, S.I., Unsal, C., Chen, Z., Huang, X., Vidali, L., and Tüzel, E. (2018) Characterization of cell boundary and confocal effects improves quantitative FRAP analysis. Biophysical Journal. 114:1153-1164. doi:10.1016/j.bpj.2018.01.01.