Enzyme Activities at Different Stages of Plant Biomass Decomposition in Three Species of Fungus-Growing Termites.

Rafael R Da Costa,Jesper Harholt,Haofu Hu,Julia Schückel,Panagiotis Sapountzis,Peter K Busk,Stjepan K Kračun,Sabine M E Vreeburg,Bo Pilgaard,Lene Lange,Duur K Aanen,Michael Poulsen,Kristine S K Pedersen

doi:10.1128/aem.01815-17

Abstract

ABSTRACTFungus-growing termites rely on mutualistic fungi of the genus Termitomyces and gut microbes for plant biomass degradation. Due to a certain degree of symbiont complementarity, this tripartite symbiosis has evolved as a complex bioreactor, enabling decomposition of nearly any plant polymer, likely contributing to the success of the termites as one of the main plant decomposers in the Old World. In this study, we evaluated which plant polymers are decomposed and which enzymes are active during the decomposition process in two major genera of fungus-growing termites. We found a diversity of active enzymes at different stages of decomposition and a consistent decrease in plant components during the decomposition process. Furthermore, our findings are consistent with the hypothesis that termites transport enzymes from the older mature parts of the fungus comb through young worker guts to freshly inoculated plant substrate. However, preliminary fungal RNA sequencing (RNA-seq) analyses suggest that this likely transport is supplemented with enzymes produced in situ. Our findings support that the maintenance of an external fungus comb, inoculated with an optimal mixture of plant material, fungal spores, and enzymes, is likely the key to the extraordinarily efficient plant decomposition in fungus-growing termites.IMPORTANCE Fungus-growing termites have a substantial ecological footprint in the Old World (sub)tropics due to their ability to decompose dead plant material. Through the establishment of an elaborate plant biomass inoculation strategy and through fungal and bacterial enzyme contributions, this farming symbiosis has become an efficient and versatile aerobic bioreactor for plant substrate conversion. Since little is known about what enzymes are expressed and where they are active at different stages of the decomposition process, we used enzyme assays, transcriptomics, and plant content measurements to shed light on how this decomposition of plant substrate is so effectively accomplished.

Highlights

Fungus-growing termites rely on mutualistic fungi of the genus Termitomyces and gut microbes for plant biomass degradation
Already identified as Odontotermes spp. by Otani et al [22], we successfully amplified the cytochrome oxidase II (COII) gene from termites derived from four Odontotermes nests sampled in 2015, one nest of which was Odontotermes sp. (Od159) and three of which were Odontotermes cf. badius (Od145, Od150, and Od151), determined by their phylogenetic placement
The presence of a diverse assembly of plant-degrading enzymes, involving key glycoside hydrolases (GHs), AAs, CEs, and PLs, coded for by Termitomyces, underlines previous suggestions that the termite fungus can produce the enzymes necessary to decompose complex substrates [24]. Their differential presence at different stages of the decomposition process highlights how the integrated combination of gut passage and an external fungal symbiont serves as an effective means to obtain very efficient plant biomass degradation

Summary

Introduction

Fungus-growing termites rely on mutualistic fungi of the genus Termitomyces and gut microbes for plant biomass degradation. Due to a certain degree of symbiont complementarity, this tripartite symbiosis has evolved as a complex bioreactor, enabling decomposition of nearly any plant polymer, likely contributing to the success of the termites as one of the main plant decomposers in the Old World. We evaluated which plant polymers are decomposed and which enzymes are active during the decomposition process in two major genera of fungus-growing termites. Our findings support that the maintenance of an external fungus comb, inoculated with an optimal mixture of plant material, fungal spores, and enzymes, is likely the key to the extraordinarily efficient plant decomposition in fungus-growing termites. The termites have become major biomass decomposers in the Old World [7, 8], where they play an important role in the turnover of dead plant material [9,10,11]. In addition to Termitomyces, fungus-growing termites harbor a complex and codiversified gut microbiota [20,21,22] that complements the plant decomposition properties of Termitomyces [23, 24]

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Conclusion