Interaction Between AM Fungi and Soil Bacteria

When the first in vitro growth of AM fungi was achieved more than 40 years ago (Mosse 1962), it was reported that a Pseudomonas isolate was essential for AM root colonization and growth of the AM fungus under aseptic conditions. Various types of growing medium filtrates showed a similar effect. Later, many other soil bacteria were shown to promote AM fungi spore germination and hyphal growth, either with or without direct contact between the organisms (Table 1). For example, unidentified bacteria isolated from soil, aswellasspore-associatedbacteriaincluding Corynebacterium sp. and Pseudomonas sp. enhanced Glomus mosseae and G. versiforme spore germination, hyphal growth and sporulation in vitro (Mayo et al. 1986; Azcon

Table 1. Interaction between AM fungi and bacteria in vitro

Bacteria speciesa

AMF species3

Interaction type

Reference

Azospirillum brasilense

Glomus intraradices

Neutral

Hildebrandt et al.

(2002)

Bacillus chitinosporus;

G. clarum

Positive,

Xavier and Germida

B. pabuli and other

neutral or

(2003)

spore-associated

negative

bacteria

Clavibacter

G. intraradices

Neutral

Filion et al. (1999)

michiganensis ssp.

michiganensis

Corynebacterium sp.

G. versiforme

Positive

Mayo et al. (1986)

Escherichia coli

G. intraradices

Neutral

Hildebrandt et al.

(2002)

Paenibacillus validus

G. intraradices

Positive

Hildebrandt et al.

(2002)

Pseudomonas sp.

Endogone sp.

Positive

Mosse (1962)

Pseudomonas sp.

G. versiforme

Positive

Mayo et al. (1986)

P. aeruginosa

G. intraradices

Positive

Villegas and Fortin

(2001,2002)

P. chlororaphis

G. intraradices

Positive

Filion et al. (1999)

P. fluorescens

Gigaspora margarita

Positive

Bianciotto et al.

(1996b)

P. putida

G. intraradices

Positive or

Villegas and Fortin

neutral

(2001,2002)

Rhizobium

Gi. margarita

Positive

Bianciotto et al.

leguminosarum

(1996b)

Serratia plymutica

G. intraradices

Neutral

Villegas and Fortin

(2001,2002)

Streptomyces

G. mosseae

Positive

Tylkaet al. (1991)

avermitilis

S. avermitilis

Scutellospora

Negative

Tylkaet al. (1991)

heterogama

S. griseus

G. mosseae

Positive

Tylkaet al. (1991)

S. orientalis

Gi. margarita

Positive

Mugnier and Mosse

(1987); Tylka et al. (1991)

S. orientalis

G. mosseae

Positive

Tylkaet al. (1991)

S. orientalis

S. heterogama

Negative or

Tylkaet al. (1991)

positive

Spore-associated

G. versiforme

Positive

Mayo et al. (1986)

bacteria

Unidentified soil

G. mosseae

Positive

Azcon (1987, 1989)

bacteria bacteria a Species names are those used in the cited references

1987, 1989). Streptomyces avermitilis, S. griseus and S. orientalis were also shown to increase spore germination of Gigaspora margarita or G. mosseae (Mugnier and Mosse 1987; Tylka et al. 1991). Azcon (1987) further reported that cell-free fractions from rhizosphere bacteria cultures have the same stimulatory effect as living bacteria. These results were used to support the involvement of volatile or diffusible factors excreted by bacteria (Azcon-Aguilar et al. 1986; Azcon 1989). On the other hand, S. avermitilis and S. orientalis were shown to suppress Scutellospora heterogama spore germination when grown in the same growth compartment, but to increase germination in different compartments (Tylka et al. 1991). In this case, the inhibitory effect was attributed to a pH increase in the growth medium, caused by bacterial growth, while the stimulatory effect in a different compartment was postulated to result from a volatile, but without further direct evidences.

Somebacteria wereshowntoliveincloseassociationwithAMfungi, and even as obligatory intracellular endophytes of Gigaspora margarita spores (Bianciotto et al. 1996a). Other species, such as Pseudomonas fluorescens and Rhizobium leguminosarum, adhere to and colonize the surface of germinating spores and growing hyphae (Bianciotto et al. 1996b). While these results support the observation that interactions between AM fungi and rhizosphere bacteria maybe mediated by either soluble factors or physical contact, these authors hypothesized that AM fungi may be a vehicle for the colonization of plant roots by soil rhizobacteria. Recently, Hildebrandt et al. (2002) have reported that Paenibacillus validus frequently develops on surface-sterilized spores of Glomus intraradices. The bacteria supported the growth of the fungus on the agar Petri plates, and induced hyphal branching, development of coiled structures and production of new spores. Under the same conditions, Escherichia coli and Azospirillum brasilense did not display any similar effect. Among various bacteria isolated from spores of Glomus clarum, Xavier and Germida (2003) found that most bacteria did not alter AM fungus spore function, while some bacteria inhibited or stimulated spore germination. Moreover, they reported that stimulation of spore germination occurred only when bacteria were in contact with spores, and inhibition of spore germination was the result of volatile bacterial metabolites.

Using the split-plate approach (following St-Arnaud et al. 1995, 1996) to grow G. intraradices extraradical mycelia separated from the colonized roots, Filion et al. (1999) concentrated crude extracts from the growing medium of the extraradical mycelial compartment, presumably containing soluble biologically active substances, and tested these against various soil bacteria and fungi. Growth of Pseudomonas chlororaphis was stimulated while Clavibacter michiganensis was unaffected. Two fungal species were also differentially affected (see below). The measured effects were generally in direct correlation with extract concentrations. In this case, volatiles were not involved, since the extracts were previously lyophilized before dilution to the desired concentration. Differences in pH were noted between the extracts from the AM fungus mycelium colonized media and the non-AM control. However, no significant pH influence was noted on bacterial growth, which strongly suggested that non-volatile substances, released by the AM fungus in the growth medium, were the main factor explaining differential growth of the micro-organisms tested.

The AM in vitro systems were also helpful to study the capacity of the extraradical mycelium of AM fungi to interact with soil bacteria to take up insoluble forms of phosphate. Using the two-compartment Petri plate approach with NH+ or NO- as N sources, both G. intraradices colonized transformed carrot roots or AM fungus extraradical mycelium alone significantly altered the pH of the growth medium, but the magnitude and direction of this change were dependent on the N source available. While in the presence of NH+ the pH was reduced from 5.5 to around 4.5, when NO-was used, pH was increased from 5.5 to 8.0 after 13 weeks of growth. However, these pH changes were not sufficient to solubilize Ca-P in the presence of the AM fungus alone. On the other hand, species-specific interactions were obtained when G. intraradices was grown along with Pseudomonas aeruginosa, P. putida or Serratia plymutica. While the inherent ability of the fungus and the bacteria to solubilize a recalcitrant form of Ca-P was low, P. aeruginosa and P. putida interacting with the extraradical mycelium markedly increased P availability in the growth medium, and this increase was dependent on the N source (Villegas and Fortin 2001, 2002).

Was this article helpful?

0 0

Post a comment