Living organisms come in all shapes and sizes. Most grow in thickness (animals, plants), but some develop in only one dimension (1D), such as filaments with cells stacked on top of each other, or in two dimensions (2D), such as a disc made of a single layer of cells. In contrast, multicellular algae, mosses, and fungi are organisms that can limit their growth to one or two axes.
During embryogenesis, brown algae develop according to specific spatio-temporal patterns. In space, they grow first in one dimension, then in two, before some of them shift to three dimensions. In time, the stages during which the cells divide along the same axes last several days. This way of developing 3D embryos is unique among multicellular organisms. and brown algae show a high diversity in this mode of 3D body construction: the time at which the cell division orientations are switched, the duration of the 1D or 2D growth periods, and the order in which the body axes are set all differ between species.
The aim of the project is to identify the mechanisms that control 3D growth. In particular, the project will focus on the mechanisms allowing cells to maintain the same division axis and those that trigger cell division in another spatial dimension. It will also investigate whether these mechanisms are similar between four different algae, and to what extent they can be compared with those developed by plants, animals or fungi.
Three lines of research will be developed. In line 1, the growth of brown algae embryos needs to be observed over time and in 3D. This will require 4D microscopy technologies such as light sheet and multiphoton microscopy, together with the development of fluorescent vital probes to mark the outlines of cells specifically involved in establishing 3D growth. It will also require a better understanding of cytoskeletal dynamics and the mapping of anisotropic markers, such as chemical or mechanical markers of the cell wall. In line 2, cell growth and division will be modelled based on the cellular data obtained in line 1. We will use microtubule dynamics models to simulate the position of the cell division plane and viscoplastic models of the cell wall to simulate cell growth. In line 3, the models will be tested by changing the experimental conditions, e.g. by applying mechanical forces (turgidity, compression) or by modifying the dynamics of cell growth and division (e.g. by genetic mutation or drug-mediated inhibition).
