Imaging of total calcium in urediospore germlings of Uromyces by ion microscopy

Phospholipase C-mediated calcium signalling is required for fungal development and pathogenicity in Magnaporthe oryzae

Calcium signalling has profound implications in the fungal infection of plants and animals, during which a series of physiological and morphological transitions are required. In this article, using a model fungal pathogen, Magnaporthe oryzae, we demonstrate that the regulation of the intracellular calcium concentration ([Ca(2+)](int)) is essential for fungal development and pathogenesis. Imaging of [Ca(2+)](int) showed that infection-specific morphogenesis is highly correlated with the spatiotemporal regulation of calcium flux. Deletion of the fungal phospholipase C gene (M. oryzae phospholipase C 1, MoPLC1) suppressed calcium flux, resulting in a fungus defective in developmental steps, including appressorium formation and pathogenicity. Surprisingly, the PLC-delta1 gene of mouse was able to functionally substitute for MoPLC1 by restoring the calcium flux, suggesting the evolutionary conservation of the phospholipase C-mediated regulation of calcium flux. Our results reveal that MoPLC1 is a conserved modulator of calcium flux that is essential for the regulation of key steps in fungal development and pathogenesis.

Antifungal effects and mechanism of action of viscotoxin A3

Viscotoxins are cationic proteins, isolated from different mistletoe species, that belong to the group of thionins, a group of basic cysteine-rich peptides of approximately 5 kDa. They have been shown to be cytotoxic to different types of cell, including animal, bacterial and fungal. The aim of this study was to obtain information on the cell targets and the mechanism of action of viscotoxin isoform A3 (VtA3). We describe a detailed study of viscotoxin interaction with fungal-derived model membranes, its location inside spores of Fusarium solani, as well as their induced spore death. We show that VtA3 induces the appearance of ion-channel-like activity, the generation of H2O2, and an increase in cytoplasmic free Ca2+. Moreover, we show that Ca2+ is involved in VtA3-induced spore death and increased H2O2 concentration. The data presented here strongly support the notion that the antifungal activity of VtA3 is due to membrane binding and channel formation, leading to destabilization and disruption of the plasma membrane, thereby supporting a direct role for viscotoxins in the plant defence mechanism.

Involvement of calcium/calmodulin signaling in cercosporin toxin biosynthesis by Cercospora nicotianae

Cercosporin is a non-host-selective, perylenequinone toxin produced by many phytopathogenic Cercospora species. The involvement of Ca(2+)/calmodulin (CaM) signaling in cercosporin biosynthesis was investigated by using pharmacological inhibitors. The results suggest that maintaining endogenous Ca(2+) homeostasis is required for cercosporin biosynthesis in Cercospora nicotianae. The addition of excess Ca(2+) to the medium slightly increased fungal growth but resulted in a reduction in cercosporin production. The addition of Ca(2+) chelators [EGTA and 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid] also reduced cercosporin production. Ca(2+) channel blockers exhibited a strong inhibition of cercosporin production only at higher concentrations (>2 mM). Cercosporin production was reduced greatly by Ca(2+) ionophores (A23187 and ionomycin) and internal Ca(2+) blocker [3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester]. Phospholipase C inhibitors (lithium, U73122, and neomycin) led to a concentration-dependent inhibition of cercosporin biosynthesis. Furthermore, the addition of CaM inhibitors (compound 48/80, trifluoperazine, W-7, and chlorpromazine) also markedly reduced cercosporin production. In contrast to W-7, W-5, with less specificity for CaM, led to only minor inhibition of cercosporin production. The inhibitory effects of Ca(2+)/CaM inhibitors were partially or completely reversed by the addition of external Ca(2+). As assessed with Fluo-3/AM (a fluorescent Ca(2+) indicator), the Ca(2+) content in the cytoplasm decreased significantly when fungal cultures were grown in a medium containing Ca(2+)/CaM antagonists, confirming the specificity of those Ca(2+)/CaM antagonists in C. nicotianae. Taken together, the results suggest that Ca(2+)/CaM signal transduction may play a pivotal role in cercosporin biosynthesis in C. nicotianae.

Expression of recombinant aequorin as an intracellular calcium reporter in the phytopathogenic fungus Phyllosticta ampelicida

Conidia of Phyllosticta ampelicida germinate only after they have made contact with a substratum. Previous work has shown that external free calcium must be available to the spore for germination to be initiated. Transgenic strains of P. ampelicida expressing apo-aequorin, a calcium-sensitive luminescent protein, were developed to monitor cytoplasmic free Ca(2+) ([Ca(2+)]c). Transformants were verified by PCR and Southern hybridization. Apo-aequorin production was quantified for each of 21 transformants. The transformant that emitted the most light per unit of protein was found to contain 0.59 mg apo-aequorin/g total protein. To ascertain the feasibility of aequorin-based [Ca(2+)]c quantification, [Ca(2+)]c changes were measured in mycelia during various physiologically perturbing treatments: exposure to high concentrations of external Ca(2+), hypoosmotic shock, and mechanical perturbation. This is the first report of a plant pathogenic fungus for which aequorin-based Ca(2+) measurement protocols have been developed.

Ca(2+) shuttling in vesicles during tip growth in Neurospora crassa

Tip-growing organisms maintain an apparently essential tip-high gradient of cytoplasmic Ca(2+). In the oomycete Saprolegnia ferax, in pollen tubes and root hairs, the gradient is produced by a tip-localized Ca(2+) influx from the external medium. Such a gradient is normally dispensable for Neurospora crassa hyphae, which may maintain their Ca(2+) gradient by some form of internal recycling. We localized Ca(2+) in N. crassa hyphae at the ultrastructural level using two techniques (a) electron spectroscopic imaging of freeze-dried hyphae and (b) pyroantimoniate precipitation. The results of both methods support the presence of Ca(2+) in the wall vesicles and Golgi body equivalents, providing a plausible mechanism for the generation and maintenance of the gradient by Ca(2+) shuttling in vesicles to the apex, without exogenous Ca(2+) influx. Ca(2+) sequestration into the vesicles seems to be dependent on Ca(2+)-ATPases since cyclopiazonic acid, a specific inhibitor of Ca(2+) pumps, eliminated all Ca(2+) deposits from the vesicles of N. crassa.

Cytoskeletal and Ca2+ regulation of hyphal tip growth and initiation

Hyphal tip growth is a complex process involving finely regulated interactions between the synthesis and expansion of cell wall and plasma membrane, diverse intracellular movements, and turgor regulation. F-actin is a major regulator and integrator of these processes. It directly contributes to (a) tip morphogenesis, most likely by participation in an apical membrane skeleton that reinforces the apical plasma membrane, (b) the transport and exocytosis of vesicles that contribute plasma membrane and cell wall material to the hyphal tips, (c) the localization of plasma membrane proteins in the tips, and (d) cytoplasmic and organelle migration and positioning. The pattern of reorganization of F-actin prior to formation of new tips during branch initiation also indicates a critical role in early stages of assembly of the tip apparatus. One of the universal characteristics of all critically examined tip-growing cells, including fungal hyphae, is the obligatory presence of a tip-high gradient of cytoplasmic Ca2+ that probably regulates both actin and nonactin components of the apparatus, and the formation of which may also initiate new tips. This review discusses the diversity of evidence behind these concepts.

Ca2+ regulation of Phyllosticta ampelicida pycnidiospore germination and appressorium Formation

Phyllosticta ampelicida conidia germinate only after making contact with and attaching to a substratum. Previous studies suggested a role for Ca2+ in this process. A Ca2+ buffering system was used to control the external free Ca2+ concentration. Both germination and appressorium formation were reduced or abolished with low Ca2+ (less than or equal to nanomolar levels) but were nearly 100% at millimolar levels of Ca2+. Germination initiation required Ca2+ within 10-25 min after the spore made contact with the substratum. Appressorium initiation required Ca2+ 90-120 min following initial contact. Ca2+ channel blockers nicardipine and lanthanum abated spore development. TMB-8, a blocker of internal Ca2+ channels, reduced both developmental events. Gadolinium, a putative stretch-activated Ca2+ channel blocker, abolished both developmental events at nanomolar levels. Calmodulin antagonists, compounds R-24751 and 48/80, abated spore development at micromolar levels. Together, these results suggest that Ca2+ signaling is involved in both germination and appressorium formation in P. ampelicida pycnidiospores.