Grass-endophyte interactions: a note on the role of monosaccharide transport in the Neotyphodium lolii-Lolium perenne symbiosis.

Symbiotic mutualistic associations of plants with ectoand endomycorrhizal and endophytic fungi are very common in natural and agricultural ecosystems; these fungi improve nutrient acquisition and ⁄ or tolerance to biotic and abiotic stresses of their hosts (Smith & Gianinazzi-Pearson, 1988; Nehls et al., 2007; Schardl et al., 2007). A major feature of associations between the heterotrophic fungi and autotrophic plants is an exchange of fungus-derived benefits (e.g. phosphorous in ectoand endomycorrhizal associations, or antiherbivorous alkaloids in endophytic Neotyphodium–grass associations) for plant-derived carbohydrates as carbon (C) and energy source for fungal growth and maintenance (Bago et al., 2000; Nehls et al., 2007). Neotyphodium ⁄Epichloë spp. endophytic fungi occur in 20– 30% of cool-season grass species and are of widespread interest to ecological and agricultural research (Schardl, 2001). These obligate symbiotic fungi are an additional C sink and affect host C metabolism and mobilization (Pan & Clay, 2004; Hunt et al., 2005; Rasmussen et al., 2008, 2009). Under stressful conditions, in particular, such as drought and high temperatures, endophyte symbionts have been shown to enhance host photosynthesis and potentially increase total C reserves (Richardson et al., 1993; Marks & Clay, 1996). It has also been shown that endophyte concentrations are reduced in Lolium perenne cultivars accumulating high amounts of water-soluble carbohydrates compared with control cultivars (Rasmussen et al., 2007; Liu et al., 2011). These studies indicate that endophytic fungi are important regulators of host carbohydrate metabolism; and that carbohydrate supply by hosts and C utilization by endophytes interact with each other. Carbon transfer from host plants to fungal symbionts is catalysed by transporter proteins, and several sugar transporters from mutualistic ectoand endomycorrhizal fungi have been functionally characterized (Schübler et al., 2006; Nehls et al., 2007; Helber et al., 2011). In general, it has been assumed that simple soluble sugars such as glucose and fructose are the major source of C for symbiotic fungi (Bago et al., 2000); however, some of the isolated sugar transporters have been shown to be able to catalyse the uptake of cell wall sugars such as mannose and xylose as well (Schübler et al., 2006; Fajardo López et al., 2008; Helber et al., 2011; Doidy et al., 2012). To provide more information about sugar uptake by endophytic Neotyphodium ⁄Epichloë fungi from their hosts, we report here the isolation and functional characterization of a monosaccharide transporter from Neotyphodium lolii (MSTN). Initial studies indicated that MSTN preferentially catalyses the uptake of mannose, a monosaccharide mainly found in polymeric cell wall carbohydrates. We therefore hypothesized that N. lolii might be able to hydrolyse cell wall carbohydrates. We tested this hypothesis by quantifying the expression of cell wall carbohydrate hydrolysing fungal mannosidase, cellulase and glucanase, as well as sucrose hydrolysing fungal and plant invertases in cultured N. lolii mycelia and in ryegrass cultivars and tissues differing in their endogenous sugar content.