| The growth of plant, fungal, and bacterial cells depends critically on two processes: the deposition of cell wall material and the mechanical deformation of this material by forces developed within the cell. To understand how these two processes contribute to cell growth, I undertook an experimental and theoretical investigation of polar morphogenesis, also known as "tip growth," in pollen tubes, fungal hyphae and tip-growing water molds. Using a novel wall labeling technique, I was able to provide the first detailed maps of surface expansion across a range of species. The results reveal interesting material properties of the polymeric cell walls of these species. Both the viscosity and the Poisson's ratio are highly inhomogeneous, as the growing cell controls the structure and chemistry of the polymer matrix. A comparative analysis of the expansion profiles across three kingdoms suggests that many basic aspects of cell morphogenesis are preserved in all polarly growing cells, while anatomical differences may account for the characteristics unique to each species. Focusing on the rich phenomenology in the angiosperm pollen tubes, I developed an experimentally motivated, dynamical model of cell expansion that incorporates 1) the microscopic architecture and rheology of the polymeric wall, and 2) the exocytosis of wall material. The model shows two regimes corresponding to the observed behaviors of pollen tubes: steady and pulsatile growth. It can account for the frequency, amplitude and waveform of pulsatile cells, and the scaling relationships between these variables. By solving the dynamical system on a three-dimensional, thin-shell geometry, the model can also explain the surface expansion patterns and morphologies of cells. |
