ABSTRACT
Max-Planck-Institut für Terrestrische Mikrobiologie, Marburg/Lahn, Germany
Gerrit Kuhn University of Lausanne, Lausanne, Switzerland
Vivienne Gianinazzi-Pearson UMR INRA/Université de Bourgogne, Dijon, France
1INTRODUCTION
Mycorrhiza, which literally means “fungus root,” was introduced into the literature by Frank [1] and describes the phenomenon of an intimate symbiotic association of certain groups of soilborne fungi with absorbing organs (roots and rhizoids) of higher plants. Different types of mycorrhiza are distinguished depending on the plants and the fungi that are involved [2]. Ectomycorrhiza are formed by the roots of woody plants and a great variety of fungi, predominantly belonging to the Basidiomycota. Arbutoid and monotropoid ectendomycorrhiza are restricted to a few plant species. Among the endomycorrhiza, the ericoid and orchidoid mycorrhiza can only be found in the ericaceae and orchidaceae. Arbuscular mycorrhizas (AM) contrast with all these in that they represent a very widespread type of endomycorrhiza and some 80% of vascular land plant families as hosts for the fungal endosymbionts [3]. In contrast, only ~130 species of AM fungi are described. Six genera are distributed in four families and grouped in the order Glomales, Zygomycota [4]. Gigaspora and Scutellospora (Gigasporaceae) belong to the suborder Gigasporineae, while Glomus, Sclerocystis (Glomaceae), and Acaulospora and Entrophospora (Acaulosporaceae) are clustered in the Glomineae. The Glomales are very ancient microorganisms compared to other true fungi. Fossil data and molecular phylogenetic analyses indicate that their origin dates back to the OrdovicionDevonion era some 460 to 400 Myr ago [5-7]. This period coincides with the colonization of the land by the plants, and AM fungi might have been essential for this process [8]. They became an integral part of most terrestrial ecosystems [9], and it was recently shown in microcosms how their biodiversity can influence aboveground biomass and plant species distribution [10]. Three basic mechanisms are discussed to be functional in influencing plant growth and development. AM fungi are able as “biofertilizers” to take up nutrients like phosphate, nitrate, or trace elements from the soil, to transport them into the roots, and to exchange these nutrients against carbohydrates, which are delivered by the plant [11]. As “bioprotectors,” they can induce plant resistance against root pathogens [12] or raise the plant’s tolerance to abiotic stress like
drought [13] or heavy metal contamination of the soil [14]. In addition, AM fungi influence plant hormone levels in the plant acting as “bioregulators” [15]. These activities result in most cases in a positive growth response of the plant. This phenomenon, the socalled mycorrhiza effect, has led to the use of AM fungi in plant production systems, in particular in horticulture [16] and in acclimatization of plantlets produced by micropropagation [17].
