Abstract
Plant pathogenic fungi use a wide range of different strategies to gain access to the carbon sources of their host plants. The hemibiotrophic maize pathogen Colletotrichum graminicola (teleomorph Glomerella graminicola) colonizes its host plants, and, after a short biotrophic phase, switches to destructive, necrotrophic development. Here we present the identification of five hexose transporter genes from C. graminicola, CgHXT1 to CgHXT5, the functional characterization of the encoded proteins, and detailed expression studies for these genes during vegetative and pathogenic development. Whereas CgHXT4 is expressed under all conditions analyzed, transcript abundances of CgHXT1 and CgHXT3 are transiently up-regulated during the biotrophic phase, and CgHXT2 and CgHXT5 are expressed exclusively during necrotrophic development. Analyses of the encoded proteins characterized CgHXT5 as a low-affinity/high-capacity hexose transporter with a narrow substrate specificity for glucose and mannose. In contrast, CgHXT1 to CgHXT3 are high affinity/low capacity transporters that also accept other substrates, including fructose, galactose, or xylose. CgHXT4, the largest of the identified proteins, has only little transport activity and may function as a sugar sensor. Phylogenetic studies revealed hexose transporters closely related to the five CgHXT proteins also in other pathogenic fungi suggesting conserved functions of these proteins during fungal pathogenesis.
Highlights
The ascomycete Colletotrichum graminicola (Cesati) Wilson [teleomorph Glomerella graminicola (Politis)] is the causal agent of the worldwide occurring stem rot and leaf anthracnose of maize (Zea mays)
Nothing is known about the carbon sources used by C. graminicola or other hemibiotrophic fungi at these different infection stages or about the transport proteins used for carbon acquisition in infected plant material
Over the last years plasma membrane-localized carbohydrate transporters were identified from the biotrophic basidiomycetes Uromyces fabae [5] and Ustilago maydis [6], the symbiotic, mycorrhizaforming glomeromycete Geosiphon pyriformis [7], and from the root-colonizing ascomycete Metarhizium robertsii that is beneficial for plant health as it antagonizes pathogens and herbivores [8]
Summary
The hemibiotrophic maize pathogen Colletotrichum graminicola (teleomorph Glomerella graminicola) colonizes its host plants, and, after a short biotrophic phase, switches to destructive, necrotrophic development. During the necrotrophic phase which is initiated 48 to 72 h postinoculation, fast growing thin secondary hyphae breach the plant plasma membrane, kill the cell, and start to ramify within the host tissue. Over the last years plasma membrane-localized carbohydrate transporters were identified from the biotrophic basidiomycetes Uromyces fabae [5] and Ustilago maydis [6], the symbiotic, mycorrhizaforming glomeromycete Geosiphon pyriformis [7], and from the root-colonizing ascomycete Metarhizium robertsii that is beneficial for plant health as it antagonizes pathogens and herbivores [8]. Virulence was restored by expressing the cDNA of the Arabidopsis thaliana SUC9 sucrose transporter [10] from the SRT1 promoter
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