Cellular and molecular defence‐related root responses to invasion by arbuscular mycorrhizal fungi

Abstract

SUMMARYArbuscular mycorrhizal (AM) fungi, which date from the Devonian era, must have dealt ver\' earlj' on with problems of plant defence, a tactic which has enabled them to colonize roots of most extant plant taxa. Conversely, plants forming arbuscular mycorrhiza must exert some sort of control over the fungi during symbiotic interactions since mycelial proliferation within roots is confined to the cortical parenchyma tissue. Plants possess a panoply of defence mechanisms which are triggered by microbial attack. Here we review present-day knowledge of defencerelated root responses to colonization by AM fungi, and assess their possible implications in the symbiosis. Weak, transient, unco-ordinated or extremely localized activation of inducible defence responses occurs during compatible interactions in AM which differs from that in plant-pathogen interactions. When symbiosis-related plant genes are mutated, typical defence responses are associated with resulting root resistance to arbuscular mycorrhizal fungi whilst, contrastingly, plants constitutively expressing defence genes are unaffected in their symbiotic capacities. The mechanisms by which plants can modulate defence responses during symbiotic interactions and the \vay in which AM fungi might contend with these are discussed.

Expression of chitin synthase genes in the arbuscular mycorrhizal fungus Gigaspora margarita

“…During the later steps of fungal colonization, chitin is ' naked ' and amorphous in the thin branches of arbuscules and can act as a potential source of elicitors. This hypothesis is supported by in situ hybridization experiments which have located transcripts of defence-related genes mainly in arbuscule-containing cells (Harrison & Dixon, 1994 ;Blee & Anderson, 1996 ;Gianinazzi-Pearson et al, 1996 ;Balestrini et al, 1997).…”

Regulation of Arbuscular Mycorrhization by Carbon. The Symbiotic Interaction Cannot Be Improved by Increased Carbon Availability Accomplished by Root-Specifically Enhanced Invertase Activity

“…Intracellular fungal structures are separated from the plant cytoplasm by an extension of the plasma membrane, forming the periarbuscular membrane surrounding the arbuscule. The greatly increased surfaces of host and arbuscule plasma membranes offers optimized conditions for effective nutrient exchange via the established symbiotic interface (for review, see Gianinazzi-Pearson et al, 1996;Harrison, 1999).The exchange of phosphate via the periarbuscular interface and the induction of AM-specific phosphate transporters are already well characterized (Karandashov and Bucher, 2005;Requena, 2005). The same holds true for the regulation of AM by the phosphate availability for the plant.…”

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).…”