Hyphal Branching

Although each hypha exhibits apical growth (i.e. extends at its tip), it doesn't continue growing as just a single filament - it will eventually BRANCH and as the branches become progressively longer they too will branch, as illustrated in this movie clip from the Fungal Cell Biology Group based at the University of Edinburgh.

Features:
  • Hyphal branching is necessary for efficient colonization and utilization of the substrate upon which the fungus is growing.
  • A branch arises when a NEW GROWTH POINT is initiated in the existing lateral wall of the hypha - this is accompanied by the ACCUMULATION OF VESICLES.
  • Branch formation almost certainly involves wall lytic enzymes (model 1), since the branch will emerge through a mature, rigid area of the hypha's lateral wall.
  • Branches normally EXTEND AWAY FROM ONE ANOTHER, filling the gaps between existing hyphae, because they're:
    • responding to nutrient gradients - growing out of areas where nutrients have become limited around existing hyphae, into areas where nutrients are more plentiful
    • growing away from areas which have become staled by the metabolic by-products of existing hyphae.
  • The extent of hyphal branching, i.e. the density of a fungal colony (number of hyphal branches formed per unit area), is directly related to the concentration of nutrients in the substrate or growth medium:
    • a sparsely branched colony (low hyphal density) will develop on a nutritionally weak substrate or growth medium
    • a densely branched colony will develop on a nutritionally rich substrate or growth medium.
BUT:
  • RADIAL GROWTH of the colony is NOT influenced by the concentration of nutrients (within limits).
  • So a colony will reach approximately the same diameter in a given time interval whether growing on a nutritionally rich or poor growth medium (again, within limits).
THE REASON? - BECAUSE:
  • Existing hyphal tips at the colony margin (which determine the diameter of a colony) have priority over all other hyphal tips (i.e. the branches) for the available nutrients.
  • Only nutrients in excess of those required by the marginal hyphal tips are available to support branching.
  • Therefore, the more nutrients that are surplus to the colony margin's requirements, the greater the hyphal density.

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