Mushroom morphogenesis

When Agaricus bisporus grows and colonises compost it exists as a network (mycelium) of fine filamentous cells known as hyphae. These cells are largely involved in extracting nutrition from the compost by releasing extracellular enzymes to break down the polymers. If certain environmental triggers are activated the mycelial cells undergo a phase change from vegetative to reproductive growth, resulting in the formation of the edible mushroom fruitbodies. The regulation and physiology of this phase change has been the subject of a collaborative research project involving EMR scientists. To stimulate mushroom production in commercial cultivation, colonised compost is first covered by a casing soil; and after approximately one week when the casing is colonised with A. bisporus mycelium, the aerial environment is changed by lowering the temperature (from 25° to 18 °C) and the ventilation of the growing room. Three separate physiological switches have been identified which control mushroom formation by environmental change. Two of the switches are essential while the third has a quantitative effect, producing more or fewer mushrooms. The first essential switch is activated by the lowering the levels of 1-octen-3-ol (a natural volatile produce by the mycelium) by ventilation and allows the initial change in cellular behaviour when the hyphae grow towards each other and form hyphal knots which later grow into undifferentiated primordia. These primordia will only differentiate into distinct tissues and grow into mushrooms when the second switch is activated, which is the lowering of the temperature. Reduction of CO2 levels (due to increased ventilation) activates the third but non-essential switch regulating the numbers of mushrooms produced. The ecological explanation of how this control system evolved in the wild is that the mycelium is detecting late autumn (when high leaf fall provides new substrate for spores to grow) by perceiving temperature and CO2 (fall in microbial activity after initial decomposition of leaf), and the 1-octen-3-ol switch is detecting the depth of covering soil and possible cracks through which the developing mushrooms can emerge.

Microarray analysis of A. bisporus mycelium has identified 45 genes showing significant changes in transcript levels during the phase change. These genes can be sorted into three clusters based on similarity of expression profiles. Common and specific gene promoter motifs are found within each cluster. These findings have enhanced our understanding of the cellular changes, physiological switches and molecular mechanisms associated with the reproductive phase change in mushrooms.

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