A homeostatic set point for branching in Neurospora crassa

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The mechanisms responsible for controlling hyphal extension and branching are still poorly understood. We have investigated these processes by studying their dependence on temperature and nutrient concentration. Tip growth is highly responsive to temperature change, increasing linearly from 4 to 37 °C. Over this range of temperatures the branching pattern shows virtually no response. Likewise, varying nutrient concentration does not affect branch distribution. Colonies subjected to rapid extreme temperature downshifts (for example from 25 to 4°) display a strong and highly predictable branching response. There are three stages to this response. First there is an initial lag phase of growth without branching. After this, the growing tips display a series of tightly spaced, dichotomous branches dubbed starbursting. Following continued growth at 4°, tips enter a recovery phase, returning to normal branching frequencies with lateral branches. The strength of the response to cold is correlated with the ratio of growth rates before and after the downshift. The combined observations point to a homeostatic set point for branch distribution that compensates for temperature and nutrient concentration.