b Leaf-cutter ants use plant matter to culture the obligate mutualistic basidiomycete Leucoagaricus gongylophorus. This fungus mediates ant nutrition on plant resources. Furthermore, other microbes living in the fungus garden might also contribute to plant digestion. The fungus garden comprises a young sector with recently incorporated leaf fragments and an old sector with partially digested plant matter. Here, we show that the young and old sectors of the grass-cutter Atta bisphaerica fungus garden operate as a biphasic solid-state mixed fermenting system. An initial plant digestion phase occurred in the young sector in the fungus garden periphery, with prevailing hemicellulose and starch degradation into arabinose, mannose, xylose, and glucose. These products support fast microbial growth but were mostly converted into four polyols. Three polyols, mannitol, arabitol, and inositol, were secreted by L. gongylophorus, and a fourth polyol, sorbitol, was likely secreted by another, unidentified, microbe. A second plant digestion phase occurred in the old sector, located in the fungus garden core, comprising stocks of microbial biomass growing slowly on monosaccharides and polyols. This biphasic operation was efficient in mediating symbiotic nutrition on plant matter: the microbes, accounting for 4% of the fungus garden biomass, converted plant matter biomass into monosaccharides and polyols, which were completely consumed by the resident ants and microbes. However, when consumption was inhibited through laboratory manipulation, most of the plant polysaccharides were degraded, products rapidly accumulated, and yields could be preferentially switched between polyols and monosaccharides. This feature might be useful in biotechnology.
In the nests of leaf-cutter ants of the genera Atta and Acromyrmex, gardens of the obligate mutualistic basidiomycete fungus Leucoagaricus gongylophorus (1, 2) are cultivated on fresh vegetal materials (3-5). Although few leaf-cutter species prefer monocots, most leaf-cutter ants collect plant materials from both monocots and dicots (6). The ants collect and transport these plants to their nests and then fractionate and extensively clean the leaf fragments (7,8). Subsequently, the ants deposit a drop of fecal fluid onto the leaf fragment surface and inoculate the leaf fragment with L. gongylophorus hyphae. The plant-digesting enzymes in the fecal fluid facilitate fungal development (9). This fungus produces swollen hyphal tips, called gongylidia, that cluster together to form macroscopic structures known as staphylae, which are food sources for the ants (3-5). L. gongylophorus also secretes enzymes that act synergistically with ant enzymes to degrade plant matter, generating soluble compounds that are subsequently ingested by the ants (10-13). These enzymes attack plant polysaccharides, including starch, hemicellulose, pectin, and, to a lesser extent, cellulose (10,(14)(15)(16)(17)(18)(19)(20)(21)(22)(23). In addition, other microbes living in the fungus garden might mediate ant nutrition...
