The cell walls of plants and fungi are thought to provide the strength
required to resist turgor and thus maintain the integrity and morphol
ogy of these cells. However, during growth, walls must undergo rapid e
xpansion which requires them to be plastic and therefore weak. In most
tip-growing cells there is an apical concentration of F-actin associa
ted with the rapidly expanding cell wall. Disruption of F-actin in the
growing tips of hyphae of Saprolegnia ferax by a localized irradiatio
n, beginning 2-6 mum behind the apex, with actin-selective 270 nm uv l
ight caused the hyphae to burst, suggesting that actin supports the we
ak apical wall against turgor pressure. Bursting was pH dependent and
Ca2+ independent at neutral pH. Hyphae burst in the very tip, where th
e cell wall is expected to be weakest and actin is most concentrated,
as opposed to the lower part of the apical taper where osmotic shock i
nduces bursting when actin is intact. When hyphae were irradiated with
a wavelength of light that is less effective at disrupting actin, gro
wth was slowed but they failed to burst, demonstrating that bursting w
as most likely due to F-actin damage. We conclude that F-actin reinfor
ces the expanding apical wall in growing hyphae and may be the prime s
tress bearing structure resisting turgor pressure in tip growing cells
.