The results of analyses of the distribution, structure and function of ericoid, ecto and vesicular-arbuscular mycorrhizas are used to challenge the conventional view, which was based largely upon studies of isolated plants and excised plant roots under controlled conditions, that the symbiosis is primarily involved in the capture of phosphate ions. In nature, each mycorrhizal type is associated with an ecosystem and soil environment with distinctive characteristics in which selection has favoured the development of a particular range of attributes. These attributes are evaluated and their importance for the individual plant and for the ecosystems in which they occur is assessed. It is concluded that knowledge of the full range of functions of each mycorrhizal type is essential for an understanding of the distribution and dynamics of the ecosystem in which it predominates.

Mycorrhizas in ecosystems

The loss of carbon from roots (rhizodeposition) and the consequent proliferation of microorganisms in the surrounding soil, coupled with the physical presence of a root and processes associated with nutrient uptake, gives rise to a unique zone of soil called the rhizosphere. In this review, we bring together evidence to show that roots can directly regulate most aspects of rhizosphere C flow either by regulating the exudation process itself or by directly regulating the recapture of exudates from soil. Root exudates have been hypothesized to be involved in the enhanced mobilization and acquisition of many nutrients from soil or the external detoxification of metals. With few exceptions, there is little mechanistic evidence from soil-based systems to support these propositions. We conclude that much more integrated work in realistic systems is required to quantify the functional significance of these processes in the field. We need to further unravel the complexities of the rhizosphere in order to fully engage with key scientific ideas such as the development of sustainable agricultural systems and the response of ecosystems to climate change. Contents I. Introduction 460 II. What is rhizodeposition? 460 III. Regulation of rhizodeposition 460 IV. How large is the root exudation C flux? 463 V. How responsive is the root exudation C flux? 463 VI. How responsive is the microbial community to root exudation? 464 VII. The role of root exudates in nutrient acquisition 464 VIII. Mycorrhizal fungi and rhizodeposition 471 IX. Future thoughts 474 Acknowledgements 474 References 474.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.2004.01130.x

Plant and mycorrhizal regulation of rhizodeposition

Plant growth in the boreal forest, the largest terrestrial biome, is generally limited by the availability of nitrogen. The presumed cause of this limitation is slow mineralization of soil organic nitrogen1,2. Here we demonstrate, to our knowledge for the first time, the uptake of organic nitrogen in the field by the trees Pinus sylvestris and Picea abies, the dwarf shrub Vaccinium myrtillus and the grass Deschampsia flexuosa. These results show that these plants, irrespective of their different types of root–fungal associations (mycorrhiza), bypass nitrogen mineralization. A trace of the amino acid glycine, labelled with the stable isotopes 13C and 15N, was injected into the organic (mor) layer of an old successional boreal coniferous forest. Ratios of 13C:15N in the roots showed that at least 91, 64 and 42% of the nitrogen from the absorbed glycine was taken up in intact glycine by the dwarf shrub, the grass and the trees, respectively. Rates of glycine uptake were similar to those of 15N-ammonium. Our data indicate that organic nitrogen is important for these different plants, even when they are competing with each other and with non-symbiotic microorganisms. This has major implications for our understanding of the effects of nitrogen deposition, global warming and intensified forestry.

Boreal forest plants take up organic nitrogen

Molecular tools have now been applied for the past 5 years to dissect ectomycorrhizal (EM) community structure, and they have propelled a resurgence in interest in the field. Results from these studies have revealed that: (i) EM communities are impressively diverse and are patchily distributed at a fine scale below ground; (ii) there is a poor correspondence between fungi that appear dominant as sporocarps vs. those that appear dominant on roots; (iii) members of Russulaceae, Thelephoraceae, and/or non-thelephoroid resupinates are among the most abundant EM taxa in ecosystems sampled to date; (iv) dissimilar plants are associated with many of the same EM species when their roots intermingle — this occurs on a small enough spatial scale that fungal individuals are likely to be shared by dissimilar plants; and (v) mycoheterotrophic plants have highly specific fungal associations. Although, these results have been impressive, they have been tempered by sampling difficulties and limited by the taxonomic resolution of restriction fragment length polymorphism methods. Minor modifications of the sampling schemes, and more use of direct sequencing, has the potential to solve these problems. Use of additional methods, such as in situ hybridization to ribosomal RNA or hybridization coupled to microarrays, are necessary to open up the analysis of the mycelial component of community structure.

The molecular revolution in ectomycorrhizal ecology: peeking into the black-box.

A check-list of mycorrhiza in the british flora*

IN a previous paper from this Institute1 it was shown, by using the isotope technique, that hyphae of Boletus variegatus (Sw.) Fr. in mycorrhizal connexion with pine seedlings transfer nitrogen from a solution of an ammonium salt to the roots. The labelled nitrogen was added as ammonium nitrate containing the stable isotope nitrogen-15 in the ammonium group. By a special arrangement, only the mycelium was exposed to the isotope, and then the amounts of nitrogen-15 were measured in different parts of the seedlings.

/pdf/transfer-of-labelled-nitrogen-from-glutamic-acid-to-pine-220yvwaxny.pdf

Transfer of labelled nitrogen from glutamic acid to pine seedlings through the mycelium of Boletus variegatus (Sw.) Fr.

Transfer of Radioactive Phosphorus to Pine Seedlings by Means of Mycorrhizal Hyphae

Influence of Root‐Metabolites on the Growth of Tree Mycorrhizal Fungi

The effects of several monoterpenes and the sesquiterpene longifolene on the vegetative growth of two mycorrhizal fungi, Boletus variegatus and Rhizopogon roseolus, were studied in petri plate experiments on an agar medium. The test substances are natural constituents of Pinus silvestris (Scots pine) roots. Vapors from 20 μl of all the test substances, upon a 5 day exposure inhibited the growth of both the mycorrhizal fungi 55% to 86%. The two fungi, however, differed in their tolerance to individual compounds. It is suggested that the terpenes and the sesquiterpenes play an important role in the development of the symbiotic condition in ectomycorrhizae by restricting the growth of the mycorrhizal fungi within the host root.

Studies on Ectomycorrhizae of Pine II. Growth Inhibition of Mycorrhizal Fungi by Volatile Organic Constituents of Pinus silvestris (Scots Pine) Roots

1. Mycorrhizal seedlings of Pinus virginiana Mill. were cultured aseptically with mycorrhizae produced by Rhizopogon roseolus (Corda) Th. Fr. The fungus was exposed to Na22, and the movement of the isotope into the seedlings was observed. 2. The experiments showed that relatively large amounts of sodium were absorbed from the external supply and transferred to the mycorrhizae through the fungus in 48 hours. From them the ions were readily translocated into the root and shoot tissues.

Elias Melin

Papers

Transfer of labelled nitrogen from glutamic acid to pine seedlings through the mycelium of Boletus variegatus (Sw.) Fr.

Transfer of Radioactive Phosphorus to Pine Seedlings by Means of Mycorrhizal Hyphae

Influence of Root‐Metabolites on the Growth of Tree Mycorrhizal Fungi

Studies on Ectomycorrhizae of Pine II. Growth Inhibition of Mycorrhizal Fungi by Volatile Organic Constituents of Pinus silvestris (Scots Pine) Roots

Translocation of Cations to Seedlings of Pinus virginiana Through Mycorrhizal Mycelium