Probing the effect of tip pressure on fungal growth: Application to Aspergillus nidulans

The study of fungal cells is of great interest due to their importance as pathogens and as fermenting fungi and for their appropriateness as model organisms. The differential pressure between the hyphal cytoplasm and the bordering medium is essential for the growth process, because the pressure is correlated with the growth rate. Notably, during the invasion of tissues, the external pressure at the tip of the hypha may be different from the pressure in the surrounding medium. We report the use of a method, based on the micropipette-aspiration technique, to study the influence of this external pressure at the hyphal tip. Moreover, this technique makes it possible to study hyphal growth mechanics in the case of very thin hyphae, not accessible to turgor pressure probes. We found a correlation between the local pressure at the tip and the growth rate for the species Arpergillus nidulans. Importantly, the proposed method allows one to measure the pressure at the tip required to arrest the hyphal growth. Determining that pressure could be useful to develop new medical treatments for fungal infections. Finally, we provide a mechanical model for these experiments, taking into account the cytoplasm flow and the wall deformation.

Figure 1

(a) Sketch of the experimental procedure developed to perform tip-pressure experiments on A. nidulans. The differential pressure $\mathrm{\Delta }P$ during the tip-pressure tests is proportional to the differential height of the water reservoir connected to the micropipette, $\mathrm{\Delta }P=\rho g\mathrm{\Delta }h$, where $\rho g$ is the specific weight of water. (b) Representative image of a tip-pressure experiment carried out on A. nidulans. The hyphal tip was tracked to quantify the growth rate in each pressure level.

Figure 2

Representative tip-pressure experiment of one hyphal tip. (a) Curve differential pressure $\mathrm{\Delta }P$ versus time. The nine steps at different differential pressure are shown. (b) Microscopy images showing, for the nine steps of differential pressure, the length of the hypha inside the microcapillary, $L_{\mathrm{p}}$. (c) Curve length inside the microcapillary, $L_{\mathrm{p}}$, versus time; the slope of the curve during each loading step has been obtained by linear fitting.

Figure 3

Growth rate $d{L}_{\mathrm{p}}/dt$ versus differential pressure for the tip-pressure experiments. First (i) and last (ix) steps (both with $\mathrm{\Delta }P=0)$ are shown separately. Average values were obtained for $N=10$ experiments. Error bars represent standard error.

Figure 4

Mechanical model. (a) Scheme of the hypha during the growth into the microcapillary, showing the main variables used in the analysis of the process. The three types of water flow are (1) internal water flow due to the pressure gradient (modeled using the Poiseuille equation), (2) flow through the hypha wall outside the microcapillary, and (3) inside the microcapillary, in both cases by osmosis. (b) The function for the internal pressure $P_{\mathrm{int}}\left(x\right)$ varies exponentially; λ, represented schematically, is the characteristic length of the exponential decay (see Appendix). (c) Relationship between the growth rate $dl/dt$ and the differential pressure $\mathrm{\Delta }P$ predicted by the model. It is found that $dl/dt\approx 0$ for $\mathrm{\Delta }{P}_{\mathrm{stop}}$ (see Discussion section).