D. van Tuinen

Hydrolytic enzyme activity of Paenibacillus sp. strain B2 and effects of the antagonistic bacterium on cell integrity of two soil-borne pathogenic fungi.

Paenibacillus sp. strain B2, isolated from the mycorrhizosphere of Sorghum bicolor and having an antagonistic activity towards soil-borne fungal pathogens, possessed extracellular cellulolytic, proteolytic, chitinolytic and pectinolytic enzyme activities. The eventual role of these lytic enzymes in cellular interactions between Paenibacillus sp. strain B2 and Phytophthora parasitica and Fusariumoxysporum was investigated by electron microscopy and molecular cytology. Electron microscopic observations showed that the presence of Paenibacillus sp. strain B2 resulted in disorganisation of cell walls and/or cell contents of P. parasitica and F. oxysporum. However, when P. parasitica was treated with commercial purified cellulase, protease, chitinase and pectinase, only protease had an inhibitory effect on mycelial growth. It is proposed that the inhibitory effect of Paenibacillus sp. strain B2 on the growth of soil-borne fungal pathogens is probably derived from more than one mechanism.

Medicago truncatula gene responses specific to arbuscular mycorrhiza interactions with different species and genera of Glomeromycota

Plant genes exhibiting common responses to different arbuscular mycorrhizal (AM) fungi and not induced under other biological conditions have been sought for to identify specific markers for monitoring the AM symbiosis. A subset of 14 candidate Medicago truncatula genes was identified as being potentially mycorrhiza responsive in previous cDNA microarray analyses and exclusive to cDNA libraries derived from mycorrhizal root tissues. Transcriptional activity of the selected plant genes was compared during root interactions with seven AM fungi belonging to different species of Glomus, Acaulospora, Gigaspora, or Scutellospora, and under widely different biological conditions (mycorrhiza, phosphate fertilization, pathogenic/beneficial microbe interactions, incompatible plant genotype). Ten of the M. truncatula genes were commonly induced by all the tested AM fungal species, and all were activated by at least two fungi. Most of the plant genes were transcribed uniquely in mycorrhizal roots, and several were already active at the appressorium stage of fungal development. Novel data provide evidence that common recognition responses to phylogenetically different Glomeromycota exist in plants during events that are unique to mycorrhiza interactions. They indicate that plants should possess a mycorrhiza-specific genetic program which is comodulated by a broad spectrum of AM fungi.

Signalling between arbuscular mycorrhizal fungi and plants: Identification of a gene expressed during early interactions by differential RNA display analysis

Although there is evidence for an interplay of signalling/recognition events at different stages during plant/fungal interactions in arbuscular mycorrhiza, the nature of signalling molecules and signal perception/transduction processes are so far unknown. Virtually nothing is known of molecular interactions during initial contact between arbuscular mycorrhizal symbionts, but plant mutants which limit fungal development to these first steps (myc−1) provide proof of the involvement of plant genes. One such pea mutant (P2) has been used to investigate gene expression during early plant–fungal interactions by differential RNA display. Partial transcriptome analysis has indicated frequent similarities in up-regulated genes in Glomus mosseae-inoculated mutant and non-mutant pea genotypes. One of these has been putatively identified as a plant Clp serine protease and its eventual role in early events of arbuscular mycorrhiza interactions is discussed.

Structural and Functional Genomics of Symbiotic Arbuscular Mycorrhizal Fungi

The absorbing organs (roots, rhizomes) of nearly all terrestrial plant families host an intimate symbiotic association, called a mycorrhiza, with specialized functional groups of soil fungi. The most common type of root symbiosis is the arbuscular mycorrhiza where soil fungi interact with a tremendous diversity of plant species, including many forest trees and agricultural, horticultural, and fruit crops (Gianinazzi et al., 2002). The fungi involved are very ancient microorganisms compared to other true fungi. Fossil data and molecular phylogenetic analyses indicate that their origin dates back to the Ordovician-Devonian era some 460 to 400 million years ago (Remy et al., 1994; Redecker et al., 2000), coinciding with land colonization by early plants. Since then, arbuscular mycorrhizal (AM) fungi have become an integral part and key components of most terrestrial ecosystems (Smith and Read, 1997). Their ability to enhance plant resistance to biotic and abiotic stresses makes them potentially powerful biotools for low-input agriculture (Gianinazzi et al., 2002), and it is believed that their biodiversity can influence plant community structure in natural ecosystems (van der Heijden et al., 1998). More than 100 species of AM fungi have been described, and many of them are held in international culture collections (IBG/BEG, 1993; INVAM, 1996). Their taxonomical status has been a matter of debate because of their asexual nature and the difficulty to affiliate them closely to existing fungal groups. Until recently, they were organized into six genera, distributed in four families, and grouped into a unique order, Glomales (Zygomycota) which comprises Gigaspora and Scutellospora (Gigasporaceae), belonging to the suborder Gigasporineae, and Glomus, Sclerocystis (Glomaceae), Acaulospora, and Entrophospora (Acaulosporaceae), clustered in the Glomineae (Morton and Benny, 1990). However, a revised classification is presently being considered which places them in a new phylum, the Glomeromycota with four new orders (Glomerales, Archeaosporales, Paraglomales, and Diversisiporales) (Schusler et al., 2001). For clarification, the generic term “AM fungi” will therefore be used above the species level throughout this chapter.