Functional analysis of liverworts in dual symbiosis with Glomeromycota and Mucoromycotina fungi under a simulated Palaeozoic CO2 decline.

Most land plants form mutualistic associations with arbuscular mycorrhizal fungi of the Glomeromycota, but recent studies have found that ancient plant lineages form mutualisms with Mucoromycotina fungi. Simultaneous associations with both fungal lineages have now been found in some plants, necessitating studies to understand the functional and evolutionary significance of these tripartite associations for the first time. We investigate the physiology and cytology of dual fungal symbioses in the early-diverging liverworts Allisonia and Neohodgsonia at modern and Palaeozoic-like elevated atmospheric CO2 concentrations under which they are thought to have evolved. We found enhanced carbon cost to liverworts with simultaneous Mucoromycotina and Glomeromycota associations, greater nutrient gain compared with those symbiotic with only one fungal group in previous experiments and contrasting responses to atmospheric CO2 among liverwort-fungal symbioses. In liverwort-Mucoromycotina symbioses, there is increased P-for-C and N-for-C exchange efficiency at 440 p.p.m. compared with 1500 p.p.m. CO2. In liverwort-Glomeromycota symbioses, P-for-C exchange is lower at ambient CO2 compared with elevated CO2. No characteristic cytologies of dual symbiosis were identified. We provide evidence of a distinct physiological niche for plant symbioses with Mucoromycotina fungi, giving novel insight into why dual symbioses with Mucoromycotina and Glomeromycota fungi persist to the present day.The ISME Journal advance online publication, 27 November 2015; doi:10.1038/ismej.2015.204

Role of plant–fungal nutrient trading and host control in determining the competitive success of ectomycorrhizal fungi

Multiple ectomycorrhizal fungi (EMF) compete to colonise the roots of a host plant, but it is not known whether their success is under plant or fungal control, or a combination of both. We assessed whether plants control EMF colonisation by preferentially allocating more carbon to more beneficial partners in terms of nitrogen supply or if other factors drive competitive success. We combined stable isotope labelling and RNA-sequencing approaches to characterise nutrient exchange between the plant host Eucalyptus grandis and three Pisolithus isolates when growing alone and when competing either indirectly (with a physical barrier) or directly. Overall, we found that nitrogen provision to the plant does not explain the amount of carbon that an isolate receives nor the number of roots that it colonises. Differences in nutrient exchange among isolates were related to differences in expression of key fungal and plant nitrogen and carbon transporter genes. When given a choice of partners, the plant was able to limit colonisation by the least cooperative isolate. This was not explained by a reduction in allocated carbon. Instead, our results suggest that partner choice in EMF could operate through the upregulation of defence-related genes against those fungi providing fewer nutrients