Carbohydrate uptake and catabolism in fungi. Importance of sugar trafficking during host invasion

VARGAS WA

Fungi: Types, Environmental Impact and Role in Disease

Nova Science Publishers Inc

Año: 2011;

Fungal species have evolved different strategies to obtain and degrade resources available in the natural environments and/or their hosts. In general, these compounds can act as a carbon source or signaling molecules indicating the presence of the host in the surroundings. These molecules have been extensively studied as they contribute to the host specificity and also to determine regulatory aspects of the host-fungus interaction. The knowledge and understanding of the metabolic traits and regulatory aspects of host-derived metabolites represent an interesting subject for research since it would lead to identifying novel targets for fungal disease control and management. According to the host and the colonizing strategies, the different fungal species were forced to develop alternative metabolic pathways and enzymatic machinery for the optimal use of metabolic resources. Plants represent a very rich environment to be colonized by pathogenic, symbiotic and saprophytic fungal species. Plants represent an endless source of carbohydrates and other compounds to support microbial growth. Carbohydrate metabolism in plants has been widely studied and plays crucial roles in plant physiology, including important roles in the establishment and regulation of microbial colonization in host plants. In the last decades, many reports have revealed physiological aspects of fungal species that made them successful in plant colonization. Mycorrhizal species rely on enzymes from the plant cells to degrade sucrose and use the resulting monosaccharides as a source of carbon and energy. A completely different scenario happens in other symbiotic, pathogenic or saprophytic fungal species. These later species have evolved host-independent mechanisms that enable them to obtain and degrade sugars from host tissues. In general, the different species have evolved specific sucrose transporters and intracellular sucrose-hydrolyzing enzymes, or extracellular hydrolytic enzymes and hexose transporters to uptake the resulting monosaccharides. For instance, the non-pathogenic species Trichoderma virens has acquired an intracellular degrading enzyme and a sucrose transporter, which are induced when colonizing plants (Vargas et al, 2009; Vargas et al., 2011). It was also demonstrated that the ability of T. virens to use plant-derived sucrose is an important event to control fungal growth inside the plant. In contrast, a high-affinity sucrose carrier in the pathogenic fungus Ustilago maydis was demonstrated as a virulence factor during maize plants colonization (Wahl et al., 2010). The comparison of these systems brings to light that the metabolism of the same metabolite can determine the establishment of a beneficial or a pathogenic interaction. It is likely that both species evolved different regulatory mechanisms of the same metabolism that lead to determine the outcome of the interaction. In this chapter, the most outstanding findings on filamentous fungi carbohydrate metabolism are reviewed. This revision also provides a comparative analysis of carbohydrate metabolism in symbiotic, pathogenic and saprophytic species including sugar uptake and further degradation through glycolysis and tricarboxylic acid pathway. Even though fungal species interacting with plants are the main subject of discussion, some human pathogens, such as Candida albicans, Aspergillus fumigatus, are also included in the analysis