THE PROBLEM OF HYPHAL GROWTH IN STREPTOMYCETES AND FUNGI

Apical growth is the growth habit of both filamentous streptomycetes a nd mycelial fungi. It is also the tactic of higher plants, but only hy phal growth is considered here. The problem of apical hyphal growth is that the structure is only supported from its base giving little supp ort to the continuously elongating tip. Clearly, a different strategy is needed for the two classes of micro-organisms because the prokaryot e has no cytoskeleton to perform mechanical work, but instead has a pa ssive exoskeleton resisting the cell's turgor pressure; i.e. it has a covalently cross-linked fabric covering the entire cell-the sacculus-t hat resists the cell's turgor pressure. Conversely, the lower eukaryot e does have a cytoskeleton, but has no totally enclosing cross-linked fabric; rather, the hyphal wall of the eukaryotic fungi resembles fibe rglass in that the wall is composed of fibers embedded in a plastic ph ase that gradually sets to become more rigid. Very different models ar e considered here for apical growth of these two classes of micro-orga nisms. It is proposed that the prokaryotic streptomycetes carry out a rapid turnover of the tip wall, reminiscent of the well-established in side-to-outside growth of the side wall of bacilli. This process maint ains the integrity of the wall during growth because an intact covalen tly linked portion of the tip is always present. This model depends on the ability of the murein composing the sacculus to expand elasticall y. Two models are considered for the mycelial fungi, where the tip wal l is enlarged by the result of fusion of vesicles. The recent model of Bartnicki-Garcia et al. (1989, Protoplasma 153, 46-57) is critically discussed. Their model is that the rate of addition to a particular el ement of wall area depends on its distance from an autonomously moving Spitzenkorper (idealized as the vesicle supply center, VSC, in the th eoretical constructs). The three-dimensional version of their model is also analyzed. The favored model is the ''soft spot hypothesis''; it is based on ideas formulated more than a century ago and recently revi ved independently by F. M. Harold, J. G. M. Wessels, and A, L. Koch fo r different reasons. It assumes that the vesicles are fused (intussusc epted) only if they reach points in the wall that are sufficiently new to be still plastic. Thus newer wall will be preferentially incorpora ted into fresh wall. This could provide a way for turgor pressure to f acilitate the incorporation of wall materials and synthetic enzymes an d couple elongation to the success of the organism in converting resou rces into cytoplasm. It could also provide a way for vacuole formation to force elongation. For the efficient and safe elongation of fungal hyphae there may be a role for a combination of a ''vesicle supply cen ter'' and a ''soft spot'' model to work together.