Bicarbonate gradients modulate growth and colony morphology in Aspergillus nidulans.

Fungal colonies expand through apically extending peripheral hyphae. These long and relatively unbranched leaders leave behind cells that show a higher degree of branching and lower growth rates. Towards the centre of the colony, quiescent, but viable cells sustain processes such as transport of water, solutes and organelles, and support the concerted genesis of specialized reproductive structures. A survey of chemical signals controlling changes in hyphal growth and branching of Aspergillus nidulans in solid minimal medium revealed that progressive accumulation of bicarbonate from the periphery to the centre of the colony effected a dual modulation of both parameters. At areas immediately behind the periphery, bicarbonate induced progressive branching without a significant change in the specific growth rate. At distal and central regions, increased branching was accompanied by progressive inhibition of the specific growth rate. The results provide new evidence that chemical cues participate in the control of colony morphology and function, which are likely to vary with changing environmental conditions.