Tansley Review No. 82. Strategies of arbuscular mycorrhizal fungi when infecting host plants

Abstract

summary Arbuscular mycorrhizal (AM) fungi are ancient Zygomycetes, thought to have colonized the first land plants; today, they are associated with the roots of about 80% of plant species. The symbiosis they form is potentially valuable not only for developmental programmes based on low‐input agriculture, but also as a complex experimental model, where both fungal and host plant growth are regulated. Here we review some recent progress m the area of cell and molecular biology of arbuscular mycorrhizas. Particular attention is given to strategies followed by AM fungi when, as obligate biotrophs, they establish a successful symbiosis with their host plants. Four topics are analysed: (i) parameters controlling fungal growth in the absence and presence of the host root, Le. events of DNA replication and timing of the cell cycle; (ii) mechanical and enzymatic mechanisms which allow the fungus to colonize root tissues, maintaining host viability; (iii) morphological changes induced by AM fungus host cells and, in particular, the formation de novo of a subcellular compartment termed the interface, and (iv) modifications of plant gene expression during fungal colonization, including those involving structural, symbiotic and defence‐related genes.

Plant defence genes are induced in the pathogenic interaction between bean roots and Fusarium solani, but not in the symbiotic interaction with the arbuscular mycorrhizal fungus Glomus mosseae

“…In some cases, an induction of defence responses was detected at early stages of root colonization followed by suppression at later stages of the symbiosis (Spanu et al, 1989 ;Lambais & Mehdy, 1993 ;Volpin et al, 1994Volpin et al, , 1995 ; in other cases, there was no major change in the expression of plant defence-related genes in response to mycorrhizal root colonization (Franken & Gna$ dinger, 1994 ;Blee & Anderson, 1996). The overall impression from these studies was that plant defencerelated genes are only weakly or transiently expressed in response to infection by AM fungi (for reviews, see Bonfante & Perotto (1995), Gianinazzi-Pearson et al (1996)). By contrast, numerous studies have demonstrated that, in pathogenic interactions, distinct changes in plant gene expression take place, many of which have presumed protective functions.…”

“…In other mycorrhizal systems, both stimulation and suppression of plant defence responses have been reported (for reviews, see Bonfante & Perotto (1995), Gianinazzi-Pearson et al (1996)). Early induction followed by suppression of plant defencegene expression has been demonstrated for mycorrhizal beans (Lambais & Mehdy, 1993).…”

Branched absorbing structures (BAS): a feature of the extraradical mycelium of symbiotic arbuscular mycorrhizal fungi

“…Mosse & Hepper (1975) considered it ' interesting (…) that finely branched, thin-walled structures very similar to arbuscules can develop outside the root '. However, to the best of our knowledge, there has been no further investigation into these structures which seem to be, paraphrasing Bonfante & Perotto (1995), a dead-end in the growth of AM fungi outside the plant.…”

“…They are greatly extended apices of the fungal colony within root tissues (Gianinazzi-Pearson et al, 1995), and considered to be a dead-end in the growth of AM fungi inside the plant (Bonfante & Perotto, 1995). Owing to the increased contact surface area, and the fact that almost no physical barrier remains between plant and fungal plasmamembranes at the arbuscular branch level, arbuscules have been proposed as the preferential site for plant-fungal nutrient exchange (Bonfante-Fasolo, 1984 ;Smith & Smith, 1990).…”

Spatial distribution of chitinases and β‐1,3‐glucanase transcripts in bean arbuscular mycorrhizal roots under low and high soil phosphate conditions

“…Genotypic and environmental factors influence whether an interaction results in extensive infection of cortical cells or, alternatively, in aborted or limited infection (Bonfante & Perotto, 1995). These phenotypic differences in infection might be related to differences in host defence-related gene expression.…”