http://www.fgsc.net/fus.htm, http://www-genome.wi.mit.edu/annotation/fungi/fusarium/, http://www.cbs.knaw.nl/fusarium/database.html.

Fungal pathogens cause disease in plant and animal hosts. The extent to which infection mechanisms are conserved between both classes of hosts is unknown. We present a dual plant-animal infection system based on a single strain of Fusarium oxysporum, the causal agent of vascular wilt disease in plants and an emerging opportunistic human pathogen. Injection of microconidia of a well-characterized tomato pathogenic isolate (isolate 4287) into the lateral tail vein of immunodepressed mice resulted in disseminated infection of multiple organs and death of the animals. Knockout mutants in genes encoding a mitogen-activated protein kinase, a pH response transcription factor, or a class V chitin synthase previously shown to be implicated in virulence on tomato plants were tested in the mouse model. The results indicate that some of these virulence factors play functionally distinct roles during the infection of tomato plants and mice. Thus, a single F. oxysporum strain can be used to study fungal virulence mechanisms in plant and mammalian pathogenesis.

Glycosylphosphatidylinositol-anchored (beta)-1,3-glucanosyltransferases play active roles in fungal cell wall biosynthesis and morphogenesis and have been implicated in virulence on mammals. The role of beta-1,3-glucanosyltransferases in pathogenesis to plants has not been explored so far. Here, we report the cloning and mutational analysis of the gas1 gene encoding a putative beta-1,3-glucanosyltransferase from the vascular wilt fungus Fusarium oxysporum. In contrast to Candida albicans, expression of gas1 in F. oxysporum was independent of ambient pH and of the pH response transcription factor PacC. Gene knockout mutants lacking a functional gas1 allele grew in a way similar to the wildtype strain in submerged culture but exhibited restricted colony growth on solid substrates. The restricted growth phenotype was relieved by the osmotic stabilizer sorbitol, indicating that it may be related to structural alterations in the cell wall. Consistent with this hypothesis, deltagas1 mutants exhibited enhanced resistance to cell wall-degrading enzymes and increased transcript levels of chsV and rho1, encoding a class V chitin synthase and a small monomeric G protein, respectively. The deltagas1 mutants showed dramatically reduced virulence on tomato, both in a root infection assay and in a fruit tissue-invasion model, thus providing the first evidence for an essential role of fungal beta-1,3-glucanosyltransferases during plant infection.

Rhodococcus sp. RB1 was able to thrive in media with up to 0.9 M NaCl or KCl and in the presence of high concentrations of nitrate (up to 0.9 M) and nitrite (up to 60 mM), but only under oxic conditions. An adaptation period was not required for salt tolerance, but a rapid extrusion of K+ and intake of Na+ was observed after addition of 0.5 M NaCl. Nitrate assimilation was limited by the carbon supply, but nitrite was not accumulated in the culture medium, even at nitrate concentrations as high as 0.8 M, thus suggesting that nitrite reduction does not limit nitrate assimilation. The presence of NaCl or KCl did not affect nitrate or nitrite uptake, which were completely inhibited by ammonium or glutamine. Rhodococcus sp. RB1 nitrate reductase had an apparent molecular mass of 142 kDa and used NADH and reduced bromophenol blue or viologens as electron donors, independently of the presence of salt. The enzyme was associated with an NADH-diaphorase activity and was induced by nitrate and repressed by ammonium or glutamine, thus showing typical biochemical and regulatory properties of bacterial assimilatory NADH-nitrate reductases. The enzyme was active in vitro in the presence of 3 M NaCl or KCI, but the maximal activity was observed at 0.5 M salt. Addition of 2 M NaCl increased the optimal temperature of the enzyme from 12 to 32 degrees C, but the optimal pH (10.3) was unaffected.