Bacteria contribute to plant secondary compound degradation in a generalist herbivore system

Abstract

Herbivores must overcome a variety of plant defenses, including coping with plant secondary compounds (PSCs). To help detoxify these defensive chemicals, several insect herbivores are known to harbor gut microbiota with the metabolic capacity to degrade PSCs. Leaf-cutter ants are generalist herbivores, obtaining sustenance from specialized fungus gardens that act as external digestive systems, degrading the diverse collection of plants foraged by the ants. There is in vitro evidence that certain PSCs harm Leucoagaricus gongylophorus, the fungal cultivar of leaf-cutter ants, suggesting a role for the Proteobacteria-dominant bacterial community present within fungus gardens. Here, we investigate the ability of symbiotic bacteria present within fungus gardens of leaf-cutter ants to degrade PSCs. We cultured fungus garden bacteria, sequenced the genomes of 42 isolates, and identified genes involved in PSC degradation, including genes encoding cytochrome p450s and genes in geraniol, cumate, cinnamate, and α-pinene/limonene degradation pathways. Using metatranscriptomic analysis, we show that some of these degradation genes are expressed in situ. Most of the bacterial isolates grew unhindered in the presence of PSCs and, using GC-MS, we determined that isolates from the genera Bacillus, Burkholderia, Enterobacter, Klebsiella, and Pseudomonas degrade α-pinene, β-caryophyllene, or linalool. Using a headspace sampler, we show that sub-colonies of fungus gardens reduced α-pinene and linalool over a 36-hour period, while L. gongylophorus strains alone only reduced linalool. Overall, our results reveal that the bacterial community in fungus gardens play a pivotal role in alleviating the effect of PSCs on the leaf-cutter ant system.ImportanceLeaf-cutter ants are dominant neotropical herbivores capable of deriving energy from a wide range of plant substrates. The success of leaf-cutter ants is largely due to their external gut composed of key microbial symbionts, specifically, the fungal mutualist L. gongylophorus and a consistent bacterial community. Both symbionts are known to have critical roles in extracting energy from plant material, yet comparatively little is known about their role in the detoxification of plant secondary compounds. Here, we assess if the bacterial community associated with leaf-cutter ant fungus gardens can degrade harmful plant chemicals. We identify plant secondary compound detoxification in leaf-cutter ant gardens as a process that depends on the degradative potential of both the bacterial community and L. gongylophorus. Our findings suggest the fungus garden and its associated microbial community influences the generalist foraging abilities of the ants, underscoring the importance of microbial symbionts in plant substrate suitability for herbivores.

The ant fungus garden acts as an external digestive system

“…The existence of chemical gradients in the fungus garden resembles a digestion process, with substrates being modified to facilitate their consumption and generating residues that need to be removed or discarded, as shown here by plant constituents passing through an ant-fungus garden ecosystem. The metabolic transformations observed here are consistent with the modification of lipids by fungus gardens, as well as plant volatile compounds by fungus-garden-associated bacteria, as recently reported from leaf-cutter ant fungus gardens, 23,24 adding further support to the model describing fungus gardens as external digestive systems for ants. The chemical modifications and the types of (bio)transformations observed in our study might not vary based on changes in the available plant material, although environmental factors such as temperature or humidity, and the composition of microbiome that are associated with ants and their fungus gardens, 23 will likely influence these modifications.…”

Evidence for Succession and Putative Metabolic Roles of Fungi and Bacteria in the Farming Mutualism of the Ambrosia Beetle Xyleborus affinis

“…Farming involves the selection of beneficial fungi over less beneficial (or antagonistic) fungi, a task that is more easily managed by groups of individuals exhibiting division of labor ( 3 ). For ants and termites, it has been shown that bacteria play a prominent role in the farming practices ( 4 ), in particular by defending the fungal crops against pathogens ( 5 , 6 ) but also by nitrogen fertilization of fungus cultivars ( 7 , 8 ) as well as the enzymatic degradation of plant biomass ( 9 , 10 ) and plant defenses ( 11 ).…”

“…Nevertheless, no study has monitored the fungal and bacterial communities associated with all the offspring's life stages and throughout the development of a beetle gallery using metabarcoding. This kind of study has revealed roles of specific and apparently essential symbionts in leaf-cutter ants, for example (11). In ambrosia beetles, dynamics of symbionts are poorly known, and therefore, we can currently only make guesses about the roles of specific symbionts and their interactions in the development of the beetles.…”

The multidimensional nutritional niche of fungus-cultivar provisioning in free-ranging colonies of a neotropical leafcutter ant

“…The nutritional contributions of bacteria within the farming symbiosis are also increasingly coming into focus, as they fix nitrogen (70), metabolize lipids (e.g. lipids, (71)), and can detoxify plant secondary metabolites (72). Within individual leafcutter ants, bacteria can also fix nitrogen (70) and metabolize citrate (33).…”