Dual Roles for Calcium Ions in Apical Growth of Neurospora crassa

Abstract

We report initial attempts to define the role of Ca2+ in the polarized extension of Neurospora crassa. Growth of the organism was diminished in media containing less than 1 mM-Ca2+; extension was more severely impaired than biomass synthesis, resulting in the formation of stubby, bulbous hyphae, even of spherical cells. Reduced extension and abnormal morphology were correlated with the loss of surface-bound Ca2+, probably associated with the cell wall. Intracellular Ca2+ may be represented by material that fluoresces brightly in the presence of chlortetracycline. Punctate fluorescent bodies and diffuse fluorescence were both arrayed in a longitudinal gradient, maximum apically. Addition of the calcium ionophore A23 187 induced dissipation of the fluorescence; concurrently, the hyphae lost as much as one half of their Ca2+ content. Extension continued almost unabated, but multiple branches quickly emerged from the apex. The observations suggest that a cytoplasmic Ca2+ gradient is not required for polarized extension, but may play a role in ensuring the dominance of the apex.

Branching of fungal hyphae: regulation, mechanisms and comparison with other branching systems

“…Because these observations were made using rapidly growing N. crassa hyphae, they may not apply to all fungal hyphae. Nevertheless, they are consistent with the idea that the hyphal tip harbors factors that maintain its integrity and suppress branching (i.e., apical dominance; Schmid andHarold 1988, Semighini andHarris 2008). Lateral branching would only occur when a potential site is far enough removed from the tip so as to escape the effects of these factors.…”

“…The characteristic pattern of mycelial organization implies that individual fungal hyphae exhibit a phenomenon known as apical dominance, whereby the growing tip is dominant and suppresses the formation of lateral branches in its vicinity (Schmid andHarold 1988, Semighini andHarris 2008). This phenomenon likely reflects the exclusive targeting of exocytic vesicles laden with components required for cell surface expansion and cell wall deposition to the hyphal tip at the expense of potential branching sites.…”

An IP3-activated Ca2+ channel regulates fungal tip growth

“…The vesicular Ca 2+ gradient has been visualized by CTC fluorescence in growing tips of pollen tubes (Reiss et al, 1985), the oomycetes S. ferax (Yuan and Heath, 1991) and N. crassa (Schmid and Harold, 1988). In N. crassa the CTC gradient is very similar to the steep gradient of apical vesicles (Collinge and Trinci, 1974) (L.B.S.-G. and R.R.L., unpublished) suggesting that vesicular Ca 2+ originates from vesicles at the growing apex.…”

Fungal Morphogenesis

“…Although not well understood, the enforcement of apical dominance in some filamentous fungi requires the accumulation of re-active oxygen species at the hyphal tip (Tanaka et al 2006;Semighini and Harris 2008). It seems likely that calcium gradients would also play a role in this process (Schmid and Harold 1988). Because of apical dominance, lateral branches often emerge from subapical hyphal compartments separated from the tip by a septum.…”