Many organisms grow by extending filaments. This phenomenon is common in fungi, mosses and algae, where the most distal cell of the filament (the apical cell) is responsible for growth: it elongates and divides, leaving a string of daughter cells behind.
The mechanism of elongation has been extensively studied in the apical cells of plants, particularly in roots (root hairs) and in the pollen grain which forms a tube in order to convey the male gamete nucleus to the egg. In these systems, the driving force of growth is the internal pressure inside the cell. This pressure applies equally in all directions and on the entire surface of the cell wall. The fact that this global pressure promotes localized elongation (only at the tip), is due to the "softer" wall at this point. This variation in mechanical properties (viscoplasticity) results from local and temporary biochemical changes in the wall, which are usually controlled by enzymes.
Using a biophysical modeling approach, we have demonstrated a completely different strategy in the filamentous and microscopic brown alga Ectocarpus: in this organism, growth is possible thanks to a very localized thinning of its wall at the tip of the cell (10 times thinner than on the flanks). This increases the wall stress at this location, allowing growth. Thus, by precisely controlling the thickness of the wall along the cell - not its rigidity as in plants - this alga manages to control both its growth rate and the shape of its apical cell. The brown seaweed Ectocarpus would therefore grow using the same biophysical laws as plants (turgor, stress, cell curvature, rheology of materials), but would reach the same objective by playing with another key parameter: the thickness of the wall, and not the nature of its materials. This strategy might be more advantageous in a marine context, or in the case of very slow growth (Ectocarpus grows 300 times slower than the pollen grain tube), but these hypotheses remain to be explored.
Article: Rabillé H, Billoud B, Tesson B, Le Panse S, Rolland É, Charrier B (2019) The brown algal mode of tip growth: Keeping stress under control. PLoS Biol 17(1): e2005258. https://doi.org/10.1371/journal.pbio.2005258
Photo: Ectocarpus filaments (cell wall stained in green) made of cylindrical, elongated cells and ended by a dome-shape apical cell.