Accelerated ion fluxes during differentiation in zoospores of Phytophthora palmivora

Potassium homeostasis influences the locomotion and encystment of zoospores of plant pathogenic oomycetes

Zoospores of plant pathogenic oomycetes exhibit distinct swimming speeds and patterns under natural conditions. Zoospore swimming is influenced by ion homeostasis and changes in the ionic composition of media. Therefore, we used video microscopy to investigate swimming patterns of five oomycete species in response to changes in potassium homeostasis. In general, zoospore speed tended to be negatively correlated with zoospore size. Three Phytophthora species (Phytophthora palmivora, Phytophthora megakarya, and Phytophthora infestans) swam in straight patterns with speeds ranging from 50 to 250 microm/s whereas two Pythium species (Pythium aphanidermatum and Pythium dissotocum) swam at similar speeds ranging from 180 to 225 microm/s with a pronounced helical trajectory and varying amplitudes. High external concentrations of potassium salts reduced the swimming speed of Ph. palmivora and induced encystment. This was not observed for Py. aphanidermatum. Application of the potassium ionophores gramicidin, nigericin and valinomycin resulted in reduced swimming speeds and changes in the swimming patterns of the Phytophthora species. Therefore, potassium ions play a key role in regulating zoospore behavior.

Soybean isoflavones trigger a calcium influx in Phytophthora sojae

Both the motile zoospores and the hyphal germ tubes of Phytophthora sojae respond chemotropically to the soybean isoflavones daidzein and genistein. The role of Ca(2+) in the cellular response to these host signals was investigated by using X-ray microanalysis of cells to monitor net changes in cellular levels of Ca(2+) and by quantifying the effects of exogenous Ca(2+) and daidzein on the developmental fate of encysted zoospores. Confirmation that isoflavones trigger a net influx of Ca(2+) into the cell was demonstrated by X-ray microanalysis of individual encysted zoospores. Zoospores exposed to 10 mM Ca(2+) and 1 microM daidzein at the time of encystment formed cysts that contained more Ca(2+) than zoospores exposed to Ca(2+) alone. The magnitude of internal Ca(2+) stores appears to be a determining factor affecting the developmental fate of P. sojae cysts.

Transmembrane Ca2+ fluxes associated with zoospore encystment and cyst germination by the phytopathogen phytophthora parasitica

Phytophthora spp. can infect plants within 30 min of a zoospore arriving at a host surface. The infection sequence from zoospore encystment to cyst germination and host penetration seems to be largely preprogrammed and dependent on calcium signaling. Here we present evidence that the encystment process leading to autonomous germination in Phytophthora parasitica may be coordinated by sequential net Ca2+ fluxes across the cell membrane, which we assessed by fluorimetry of the probe fura-2 in zoospore bathing medium. Encystment was associated with a large initial net Ca2+ influx, which was abolished by the channel blockers La3+ and verapamil. The net influx was followed by a larger, progressive net Ca2+ efflux over 20-30 min, which was associated with cyst germination but was inhibited by TMB-8, implicating Ca2+ release from intracellular stores. The effects of inhibitors were overcome by high (1 mM) exogenous Ca2+, enabling encystment and germination. Copyright 1998 Academic Press.

Amiloride inhibition of angiogenesis in vitro

Angiogenesis is important to such processes as normal embryonic development and tissue growth, and is also a central feature of diseases such as diabetic retinopathy and the growth of solid tumors. Understanding the basic events governing angiogenesis has therefore attracted great interest. The ion channel blocking agent, amiloride, has been shown to inhibit angiogenesis in an in vivo model (Lansing et al., '91). This suggested a vital role for Na(+)-coupled transport processes in angiogenesis. A large number of structural analogues of amiloride have been synthesized (Kleyman and Cragoe, '88), and many of these are well characterized with respect to biological activity. These analogues present an opportunity to dissect the process of angiogenesis and identify potentially important physiological events. In this report we describe the effects of amiloride on an in vitro model for angiogenesis employing vascularized tissue explants. Amiloride inhibits capillary morphogenesis completely and reversibly at concentrations as low as 134 microM. It appears to act by blocking endothelial cell proliferation, but not migration. Inhibition is heightened by the introduction of hydrophobic groups on the terminal guanidino nitrogen atom, or on the 5-amino position. An analogue substituted at both of these positions is 30-fold more potent than the parent compound. Of amiloride's known biological activities, these results most closely correlate with the inhibition of Ca2+ transport processes, and thereby suggest an important role for Ca2+ transport in capillary morphogenesis.

Nonchemical signals used for host location and invasion by fungal pathogens

Not all aspects of the interactions between fungal pathogens and their hosts are mediated by specific chemical signals, but until recently the functional significance of nonchemical cues (such as local electrical fields, pH gradients and host surface topography) was obscure. Recent studies of the mechanisms underlying fungal tropisms and taxes to such cues have revealed novel architectural and physiological properties of the fungal cells that contribute to their success as pathogens.