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• W2071569502 abstract "‘This finding very nicely explains how the biotrophic fungus succeeds in competing against the plant for nutrients …’ Ustilago maydis is a facultative biotroph that can be cultivated in liquid medium and on nutrient agar where it lives saprotrophically and multiplies by budding of rod-shaped cells. In this stage of its life cycle, cells of U. maydis are very similar to cells of the yeast Saccharomyces cerevisiae, and the yeast is successfully used as host for functional complementation of disrupted genes through heterologous expression of the analogous U. maydis genes. Sugar uptake and sensing have been well studied in S. cerevisiae (see reviews by Özcan & Johnston, 1999; Santangelo, 2006), and this knowledge has been used to unravel the mystery of nutrient uptake in U. maydis – which revealed several surprises. In S. cerevisiae, sugar uptake is mediated by 17 closely related hexose transporters (Hxt1–17), with low or high affinity to their hexose substrates (Özcan & Johnston, 1999). The transporters are specific to hexoses and cannot transport sucrose, a disaccharide composed of the two hexoses, glucose and fructose. To use sucrose, the yeast secretes an invertase (Suc2) that enzymatically cleaves sucrose to fructose and glucose, and the hexoses are then taken up by one of the 17 hexose transporters (Fig. 1). Expression of the low and high affinity hexose transporters is under a complex regulation that involves the Snf1p kinase and the hexokinase 2 (Hxk2p). Hxk2p converts glucose to glucose-6-phosphate after phosphorylation and activation by the Snf1 kinase (Fernández-García et al., 2012). In addition to its enzymatic role, it has a regulatory role, and a small fraction of the enzyme locates to the nucleus (Fernández-García et al., 2012). In the absence of glucose, the Snf1p kinase phosphorylates the transcriptional repressor Mig1p that is subsequently exported from the nucleus (Papamichos-Chronakis et al., 2004). Export of Mig1p de-represses expression of the glucose-repressed genes mth1 and snf3, among others (Fig. 1). Mth1p and its homolog Std1p are co-regulators of Rgt1p, a negative regulator of hexose transporter gene expression in the absence of glucose. In the presence of glucose, Mth1p and Std1p are phosphorylated, which leads to their degradation via the proteasome and to de-repression of hexose transporter gene expression in the nucleus (Fig. 1). Phosphorylation is achieved through the membrane-anchored yeast casein kinases 1 and 2 (Yck1/2p) that are activated by Snf3p in the presence of low concentrations of glucose, or by Rgt2p in the presence of high concentrations of glucose (Fig. 1). Interestingly, Snf3p and its homolog Rgt2p have similarity to, but do not function as, hexose transporters. Instead, they serve as glucose concentration sensors that bind glucose with different affinity likely leading to a conformational change that is sensed by the Yck1/2p kinases (Özcan et al., 1998). Like S. cerevisiae, U. maydis has 19 genes that possibly encode hexose transporters (Wahl et al., 2010). In addition, the U. maydis genome encodes at least two invertases (inv) that are predicted to convert extracellular and intracellular sucrose to fructose and glucose (two different versions of um01945), or to hydrolyze intracellular sucrose-6-phosphate to glucose-6-phosphate and fructose (um03605) (Fig. 2). Ustilago maydis has also a homolog to Snf1p that can functionally replace the role of Snf1p in sucrose utilization of yeast (Nadal et al., 2010). A protein with sequence identity to Mig1p exists in U. maydis (um04909), as well as a protein with homology to the casein kinases Yck1/2p (um00274). By contrast, proteins with sequence similarity to Rgt1p, Mth1p or Std1p are lacking from the genome of U. maydis (Schuler et al.). This suggests that glucose signal transfer functions via a different mechanism (Fig. 2), possibly involving conversion of an intracellular glucose signal to a cAMP signal that would be further transferred via a protein kinase A leading to activation of transcription factors by phosphorylation. Using a systematic gene deletion approach, Kämper and his group have shown that two of the 19 putative hexose transporters (Srt1 and Hxt1) are necessary for full virulence of U. maydis on maize (Wahl et al., 2010). Both identified proteins are remarkable. The first one characterized had the strongest effect on virulence and turned out not to be a hexose transporter after all. Unexpectedly, the Srt1 protein was shown to function as an extremely efficient sucrose transporter with an affinity for sucrose that is several orders of magnitude higher than that of the maize sucrose transporters (Wahl et al., 2010). This finding very nicely explains how the biotrophic fungus succeeds in competing against the plant for nutrients, since sucrose is the most abundant sugar in maize and serves as a sugar storage and transport vehicle in the plant. The second transporter characterized (Hxt1) also had a surprise in store. The protein was shown to transport a variety of sugars including glucose, fructose and mannose with very high affinity, and to serve as the main hexose uptake protein for saprophytic growth of U. maydis (Schuler et al.). Complementation of hxt1 deletion strains with high-affinity hexose transporters of other organisms restored the growth defect but not the virulence defect of the mutant, suggesting an additional function of Hxt1 that does not involve sugar transport. Interestingly, the closest homologs to Hxt1 in S. cerevisiae are the glucose sensor proteins Rgt2p and Snf3p. The glucose sensor proteins can be converted into constitutively active signaling proteins by exchange of a conserved arginine with a lysine residue leading to constitutive expression of glucose-induced genes in S. cerevisiae (Özcan et al., 1996). Expression in hxt1 deletion strains of an Hxt1 protein carrying the R164K mutation did not result in complementation but in total loss of virulence. This suggests that Hxt1 has a glucose sensing function and that glucose sensing is important for plant colonization by U. maydis (Schuler et al.). With these exciting findings we now begin to understand how U. maydis succeeds in thriving in the plant environment. Having established the general importance of sugar sensing for virulence, more difficult-to-answer questions involving intracellular growth behavior of biotrophic smut fungi that colonize different plant tissues at different stages of their development can now be tackled. Hexose transporters have been shown to be induced during symbiotic growth of ectomycorrhiza (Nehls et al., 1998; Schüssler et al., 2006; Polidori et al., 2007), during growth of hemibiotrophic fungi (Lingner et al., 2011), and in haustoria of rust fungi (Voegele et al., 2001). It will be exciting to find out whether in these systems glucose signaling is similarly important during plant colonization. Clearly, there is still a lot to uncover in order to completely understand how fungi can invade, survive, and grow directionally within living plant tissues: I anticipate future discoveries that are both pivotal and sweet." @default.
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• W2071569502 date "2015-04-10" @default.
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• W2071569502 title "Invasion is sweet" @default.
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