Both observations point towards an adaptive response which is mediated most probably via Ca2+ signalling. First, high extracellular Ca2+ concentrations trigger chitin synthesis in A. niger and thereby confer increased protection LY294002 against antifungal proteins as shown for AFP [15]. Second, it primes the Ca2+ homeostatic machinery to better maintain a low [Ca2+]c

resting level when challenged with the antifungal protein, e.g. by (i) the increase of the activity of existing Ca2+ pumps/transporters to counteract the AFPNN5353-specific intracellular Ca2+ perturbation, or (ii) the modulation of the expression of Ca2+ channels/pumps/exchangers [17]. The former hypothesis (i) might be supported by the observation that the addition of CaCl2 only 10 min before A. niger was challenged with AFPNN5353 restored the low [Ca2+]c resting level. However, the perturbation of the Ca2+ homeostasis by a sustained elevation of the [Ca2+]c resting level indicates that A. niger is not able to restore the low [Ca2+]c resting level after exposure to AFPNN5353 and this might trigger programmed cell death (PCD) on the long term as it was shown to occur in A. nidulans in response to the P. chrysogenum

PAF [34]. Since AFP was shown to cause membrane permeabilization [21], the influx of Ca2+ might be due to changes in membrane permeability for this ion, if not the formation of pores. However, our staining experiments with CMFDA and PI exclude this possibility at least in the first 10 min of exposure to AFPNN5353 when the [Ca2+]c resting level reaches its maximum. This result is further corroborated by the fact that higher external concentrations this website of Ca2+ reduced the AFPNN5353 specific

rise in [Ca2+]c resting level which – in our opinion – would not occur with leaky membranes. However, we do not exclude changes in membrane permeability at longer exposure times to this antifungal protein and more studies are needed to answer this question. Finally, we observed that the internalization of AFPNN5353 is characteristic for sensitive but not resistant moulds. A lack of binding of AFPNN5353 to insensitive fungi might point towards the absence or inaccessibility of a putative interacting molecule at the cell surface. AFPNN5353 localized to the cytoplasm of target TCL fungi only when actin filaments were formed. This is in agreement with the endocytotic uptake and intracellular localization of the P. chrysogenum antifungal protein PAF in sensitive filamentous fungi [14, 45]. Importantly, we observed that AFPNN5353 was internalized by hyphae even under sub-inhibitory concentrations (0.2 μg/ml for A. nidulans) which suggests that a threshold concentration is required to cause severe growth defects in target fungi. The presence of high concentrations of extracellular Ca2+ counteracted AFPNN5353 uptake. This finding parallels well with the report of [20] that the presence of cations, such as Ca2+, interfered with the binding of AFP to the surface of F.