Agriculture Reference
In-Depth Information
balanced by the energy costs involved in supporting the fungal partner. Up to 20 % of
current photosynthate may be used to support mycorrhizal structures in both arbuscular
and ectomycorrhizal plants (Marschner and Dell, 1994; Smith and Read, 1997).
Mycorrhizal fungi interact both positively and negatively with a wide range of other
soil organisms and thereby influence the success of their host plants (Fitter and Garbaye,
1994). Such interactions may occur within the root, in the rhizosphere or in the external
soil matrix and at all stages of mycorrhizal development. A range of herbivorous nema-
todes, arthropods and protists browse the hyphae and spores of mycorrhizal fungi
although Klironomous and Kendrick (1996) found that the mites and collembolans they
studied preferred the hyphae of non-mycorrhizal fungi which may represent some
selection for browse resistance in the mycorrhizal species.
Other organisms may stimulate mycorrhizal formation and function. Garbaye (1994)
discusses the positive effects of the 'mycorrhization helper bacteria' which are defined
as 'bacteria associated with mycorrhizal roots and mycorrhizal fungi which selectively
promote the establishment of mycorrhizal symbioses'. They are best known in ectomy-
corrhizae, poorly known in arbuscular associations and not at all in the minor types.
These bacteria selectively associate with particular fungal species
and may be antagonistic to non-compatible fungi. However, where they are compatible,
they may markedly increase mycorrhizal development, although the mechanisms by
which this is achieved remain uncertain.
Ecology of the association
Initiation of colonisation. Colonisation may derive from resting spores, previously
colonised roots or hyphae (Smith and Read, 1997). Once hyphae from these propagules
come into contact with the root, the probability of successful colonisation initially
depends on several factors (Hayman, 1983):
(i) The inoculum potential
i.e.,
the number of propagules present and the number of
germination tubes formed;
(ii) The host response, which is mediated through exudate production and which may
stimulate or inhibit colonisation;
(iii) The compatibility of the fungus with the tissues of the host plant;
(iv) Environmental factors, particularly light and temperature which influence host
plant vigour and hence the flux of exudates towards the roots. Due to their low oxygen
status, water-logged soils have few mycorrhizal associations. Extreme pH, salt or carbon
dioxide
concentrations,
fungicides and
high
inputs
of phosphorus or nitrogenous
fertilisers all adversely affect mycorrhizal development;
(v) The hormonal equilibrium at the root surface. The establishment and regulation
of mycorrhizae may be dependent on the production of fungal hormones, notably
auxins, cytokinins and giberellins although other factors have also been implicated,
including nutritional effects and the presence of other rhizosphere micro-organisms
(Gogala, 1991). These hormones have been considered to be responsible for mediating
morphogenetic changes in roots, including the suppression of root hairs and induction
of short roots (Slankis, 1973). Metabolic changes also occur within infected tissues:
ultrastructural and metabolic observations (Gianinazzi-Pearson and Gianinazzi, 1986)
indicate a greater metabolic activity in host tissues. These reactions differ substantially
