In this issue of New Phytologist, the Forum section is devoted to mycorrhizal research. In his Editorial (pp. 225–228), Francis Martin (Interaction Section Editor) opens the discussion with a brief ‘look back’ at the history of mycorrhizal research published in New Phytologist and brings us up to the present day with a focus on the impacts of genomics and modern molecular tools on the ecology and evolution of mycorrhizas. Commentary authors Verena Wiemken and Ian Dickie take a closer look at individual research papers published in this issue, dealing with trehalose biosynthesis (pp. 228–230) and niche differentiation (pp. 230–233), respectively. In this final section of the Forum, we further encourage the exchange of ideas and open debate by featuring a series of Letters inspired by the Fifth International Conference on Mycorrhiza (July 2006, Granada, Spain; Selosse & Duplessis, 2006). These Letters focus on some of the important and intriguing issues currently facing mycorrhizal research, provide discussion in a wider context, and suggest future directions and perspectives.
Flow rates and pathways involved in the exchange of matter between sources and sinks are core issues for understanding mycorrhiza function in quantitative terms. Examples are the early data for phosphate flux in arbuscular mycorrhiza fungi provided by Sanders & Tinker (1971) and the recent model for nitrogen (N) uptake (Govindarajulu et al., 2005). Tools are now emerging for quantifying all metabolic fluxes in cells or organisms, and the prospects for such fluxomics studies in mycorrhizas are discussed by Yair Shachar-Hill (pp. 235–240). Two powerful approaches are described: (1) dynamic analysis of time-course data for the distribution of isotopic label, and (2) steady-state analysis of metabolic labeling patterns under conditions of isotopic steady state. Examples are given of what fluxomics could teach us about the exchange of carbon for mineral nutrients, the importance of which was emphasized in a recent Letter (Fitter, 2006). Fluxomics studies may eventually provide the information required to understand the background of the observed functional diversity in mycorrhizas.
Suggestions for future work to elucidate the variation in functional aspects of mycorrhizas are provided by Koide et al. (pp. 240–243) for ectomycorrhiza fungi and by van der Heijden & Scheublin (pp. 244–250) for arbuscular mycorrhizal fungi. The prediction of impacts of a given community of mycorrhizal fungi on nutrient cycling and productivity of ecosystems will require that we know which functional traits are variable across species and isolates and which are more robust. Functional grouping of fungi would be helpful and may well become possible; hence phosphorus (P) transport on a length-specific basis was robust at the intraspecific level of mycorrhizal fungi, but differed between species (Munkvold et al., 2004). Methods required to identify the crucial traits must be carefully chosen, as exemplified by the work of Smith et al. (2004), who measured a major contribution of arbuscular mycorrhizal fungi to plant P uptake by means of radiotracer isotopes, even when no difference could be detected in total plant P content. Both Koide et al. and van der Heijden & Scheublin emphasize that studies of function and diversity of mycorrhizas will also need to measure how perturbations in the environment (soil and host plant) influence the function of individual fungi. Having identified the crucial functional traits of mycorrhizal fungi and the variation in these traits across species and isolates, the next step would be to use this information to predict the impact of a given community of mycorrhizal fungi at the ecosystem level.
Molecular tools have provided many new insights into the composition and structure of communities of mycorrhizal fungi. Recent examples of this are studies on effects of environmental factors on communities of ectomycorrhizal fungi (Parrent et al., 2006) and the contribution of temporal and spatial variation to the structure of an arbuscular mycorrhizal fungal community in undisturbed vegetation (Rosendahl & Stukenbrock, 2004). The need for more work in this area is highlighted by Lilleskov & Parrent (pp. 250–256) with the aim of generating models suitable for predicting how communities of mycorrhizal fungi are affected by the environment. Their ambition originates in the presumed impact of human-accelerated environmental change on communities of mycorrhizal fungi on a global scale and they provide a solid framework for sampling strategies and experimental designs, methods for identification of the fungi and selection of the most appropriate environmental data. The authors appreciate the complex nature of the required data collection and modeling efforts and emphasize the need for increased collaboration and resource allocation. Achieving this goal is important in the context of climate change, and the incorporation of the functional traits of the fungi will further allow prediction of the role of mycorrhizas in the flow of matter in ecosystems.
The final contribution by Rubini et al. (pp. 256–259) provides exciting insights into the life cycle of truffles and discusses possible implications for management of these precious ascocarps. Molecular tools revealed that the haploid phase prevails in the truffle life cycle, with the dikaryotic phase being confined to the initial stages of ascocarp development. It appears that outcrossing may be more common in truffles than previously assumed and fruiting of these fungi may accordingly turn out to depend on the presence of strains that are genetically distinct or of opposite sexuality. The potential impact of this on choice of procedures for truffle cultivation is highlighted.
Although this forum special does not intend to cover all aspects of mycorrhiza research, it will hopefully be a source of inspiration for digging deeper into the biology and function of these symbioses in their shaping of ecosystems. We do need a continued research effort to determine the role of mycorrhizas in nutrient exchanges between above- and below-ground compartments, which appear to have high potential impacts on the productivity of ecosystem components and on the sequestration of atmospheric carbon.