Publisher Summary This chapter discusses the role of mycorrhizas in the growth and nutrition of nitrogen-fixing nodulated plants. It reviews some general, well-established principles on mycorrhizal types, morphology, physiology, and function. The interactions between nodular and mycorrhizal endophytes related to the formation and effects of these dual symbioses that greatly enhance the development of the common host plant are discussed. It presents the ecological significance of plants bearing the two types of symbioses, with emphasis on the possibilities of harnessing them to increase crop yield. Mycorrhizal associations play an important role in the growth and nutrition of higher plants. It is recognized that this symbiosis can be harnessed to improve nutrient cycling and crop productivity by reducing industrial fertilizer inputs, thereby conserving and reducing environmental costs. Mycorrhizal infection can help plants to become reestablished in eroded or degraded habitats, to thrive in arid conditions, to deter pathogens, and to cope with various stress situations. The common mycorrhizal types, the vesicular-arbuscular mycorrhizas (VAM), are nearly omnipresent and are studied intensively throughout the world. VAM enhance growth, nodulation, and nitrogen fixation in grain and forage legumes, crops of the greatest interest for food production in the biosphere.

Mycorrhizas and their Significance in Nodulating Nitrogen-Fixing Plants

summary
More than 200 angiosperms, distributed in 25 genera, develop root nodule symbioses (actinorhizas) with soil bacteria of the actinomycetous genus Frankia. Although most soils studied contain infective Frankia, cultured strains are available only after isolation from root nodules. Frankia infects roots via root hairs in some hosts or via intercellular penetration in others. The nodule originates in the pericycle. The number of nodules in Alnus is determined by the plant in an autoregulated process that, in turn, is modulated by nutrients such as nitrogen and phosphate. Except in the genera Allocausarina and Casuarina, Frankia in nodules develops so-called vesicles where nitrogenase is localized. Sporulation of Frankia occurs in some symbioses. As a group, actinorhizal plants show a large range of anatomical and biochemical adaptations in order to balance the oxygen tension near nitrogenase. In symbioses with well aerated nodule tissue like Alnus, the vesicles have a multilayered envelope composed mainly of lipids, bacterio-hopanetetrol and their derivatives. This envelope is assumed to retard the diffusion of oxygen into the nitrogenase-containing vesicle. In symbioses like Casuarina, the infected plant cells themselves, rather than Frankia, appear to retard oxygen diffusion, and high concentrations of haemoglobin indicate an infected region with a low oxygen tension. At least in Alnus spp., ammonia resulting from N2 fixation is assimilated by glutamine synthetase in the plant. The carbon compound(s) used by Frankia in nodules is not yet known. Nitrogenase activity decreases in response to a number of environmental factors but recovers upon return to normal conditions. This dynamism in nitrogenase activity is often explained by loss and recovery of active nitrogenase and has been traced to loss and recovery of the nitrogenase proteins themselves. Recovery is partly due to growth of Frankia and to development of new vesicles in the Alnus nodules. In the field, varying conditions continuously affect the plants and the measured rate of N2 fixation is a result not only of the conditions prevailing at the moment but also of the conditions experienced over preceding days. N2 fixed by actinorhizal plants is substantial and actinorhizal plants have great potential in soil reclamation and in various types of forestry. Several species are also useful in horticulture.

Actinorhizal symbioses and their N2 fixation

CHAPTER 5 – Mineral Nutrition of Ectomycorrhizae

Transfer of radioactive phosphorus to pine seedling by means of mycorrhizal hyphae.

Actinorhizal plants, including those of the genus Alnus (alders; Betulaceae), and their nitrogen-fixing bacterial symbionts rely on mycorrhizal fungi for phosphorus and other mineral nutrients. To date, alders are known to associate with only 20-30 species of ectomycorrhizal fungi which are highly host-specific. This study aimed to determine the species richness and the relative importance of host species, soil and site variables on the community composition of Alnus-associated ectomycorrhizal fungi on root tips. Using rDNA internal transcribed spacer (ITS) and large subunit (LSU) sequence analysis, 40 species of putatively ectomycorrhizal fungi were identified from seven sites dominated by Alnus incana or Alnus glutinosa. Alnicola spp. and Tomentella aff. sublilacina were most prevalent in all sites. Species of the /pseudotomentella, /inocybe, /peziza michelii-peziza succosa, /genea-humaria, /pachyphloeus-amylascus, /helvella-tuber and /tarzetta-geopyxis lineages were recorded as natural symbionts of alders for the first time. All basidiomycetes were specific to Alnus, whereas four out of seven Pezizales spp. (ascomycetes) were nonspecific. The complex of soil variables and geographical (site) effect drives the community composition of ectomycorrhizal fungi in alder forests. Alder-associated fungi have independently evolved and subsequently radiated in several ectomycorrhizal lineages, indicating frequent and persistent host shifts after the divergence of Alnus and Betula.

Revisiting ectomycorrhizal fungi of the genus Alnus: differential host specificity, diversity and determinants of the fungal community

Summary
Three subtypes of mycorrhizas, sensu Dominik (1955), were found on roots of Alnus viridis growing in New Zealand. Roots with 60% of their rootlets mycorrhizal absorbed phosphate five times as rapidly as non-mycorrhizal roots. Root nodules appear unable to absorb labelled phosphorus actively from externally supplied solution.

V. Mejstrik

Papers

The ectotrophic mycorrhizas of alnus viridis (chaix) d.c. and their significance in respect to phosphorus uptake