plant surface microbiology.pdf

19 Functional Diversity of Arbuscular Mycorrhizal Fungi on Root Surfaces 335
ciency in P uptake and transfer. Circumstantial evidence of seasonal variation
in the function of different species of AM fungi present in a woodland environment
has been clearly demonstrated (Merryweather and Fitter 1998b).
There have been few direct measurements of P uptake by communities of AM
fungi (Jakobsen et al. 2001), but it is difficult to attribute P transferred to the
activity of a particular fungus within a community.
The relative abundance of AM fungi in field-grown plants can be manipulated
by environmental and management processes that occur in the field,
where dual AM fungal occupancy of roots is the norm (Merryweather and Fitter
1998a). With manipulation of P supply, the important observations of
Thomson et al. (1986) can be used to identify the P status of the plant at which
there is a point of transition from growth enhancement to growth depression
in plants colonised by S. calospora. In contrast, G. invermaium did not display
such a physiological transition in this study.
3.1 Metabolic Activity During Mycorrhiza Formation
Changes in metabolic activities during mycorrhiza formation provide evidence
for hypotheses related to biochemical mechanisms of C and P exchange
between symbionts (Saito 2000). These have mostly been examined by comparing
mycorrhizal and non-mycorrhizal roots, but without identifying the
mechanisms of nutrient exchange. Changes in quantity and concentration of
soluble carbohydrates in roots have not shown consistent trends with mycorrhizal
colonisation (Pakovsky 1989; McArthur and Knowels 1993; Pearson et
al. 1994; Solaiman and Saito 1997).
The localisation of alkaline phosphatase (ALPase) in arbuscular hyphae
was observed by histochemical study (Saito 1995). ALPase has also been
located in vacuoles in intraradical hyphae of AM fungi (Gianinazzi et al.
1979), and its activity varied with environmental conditions (Jabaji-Hare et al.
1990). A correlation between the number of ALPase-active arbuscules and P
uptake by mycorrhizal plants indicates that arbuscular ALPase plays a significant
role in phosphorus transformation from the AM fungus to the host plant
(Tijssen et al. 1983). Histochemical observation of intraradical hyphae has
located lipid and polyphosphate (polyP) granules in hyphae. Although polyP
molecules are likely to play an important role in P translocation, their existence
has been contradicted. Recently, a successive extraction method showed
that a granular fraction of polyP was contained in hyphae of Gigaspora margarita
(Solaiman et al. 1999). The contribution of polyP was calculated from
these data and it has been concluded that the contribution is not significant
(Smith et al. 2001).