plant surface microbiology.pdf

20 Mycorrhizal Fungi and Plant Growth Promoting Rhizobacteria 359
8 Interactions Involved in Nutrient Cycling and Plant
Growth Promotion
It is well known that soil microorganisms are able to change the bioavailability
of mineral plant nutrients, and this ability has been shown by soil bacteria
probed to have PGPR activity and, in some cases, be used as plant inoculants
(Barea et al. 1997; Probanza et al. 2002). Several experiments have described
the improvement of plant growth and nutrition by means of synergistic interactions
between PGPR and AM fungi (Barea 2000). It has also been suggested
that a certain level of selectivity (“specificity”) is involved in these interactions
(Azcón 1989). From the perspective of sustainability, the re-establishment
of nutrient cycles after any process of soil degradation is of interest, as
is the understanding of the microbial interactions responsible for the subsequent
management of such natural resources, either for a low-input agricultural
technology (Bethlenfalvay and Linderman 1992; Gianinazzi and
Schüepp 1994; Jeffries and Barea 2001), or for the re-establishment of the natural
vegetation in a degraded area (Miller and Jastrow 1994, 2000; Requena et
al. 2001). Most of the information on this topic concerns N and P cycling
(Barea 2000).
In spite of this, Rhizobium sp. (general term) are not considered among the
PGPR types; due to the relevance of their interaction with AM fungi, it would
be interesting to make some comments on this. A great deal of work has been
carried out on the tripartite symbiosis legume (general term) – mycorrhiza-
Rhizobium (Azcón-Aguilar and Barea 1992; Barea et al. 1992; Barea 2000). The
inoculation of AM fungi has been shown to improve nodulation and N 2 fixation.
Using the isotope 15 N has made it possible to ascertain and quantify the
amount of N which is actually fixed in a particular situation, and measure the
contribution of the AM symbiosis to the process (Barea et al. 1992). The physiological
and biochemical mechanisms underlying the AM fungi x Rhizobium
interactions to improve legume productivity have also been discussed. In
spite of the main AM effect in enhancing Rhizobium activity mediated by a
generalized stimulation of host nutrition, more localized effects may occur at
the root or nodule level (Barea et al. 1992). Interactions can also take place at
either the pre-colonization stages, when both microorganisms interact as rhizosphere
inhabitants, or during the development of the tripartite symbiosis
(Azcón-Aguilar and Barea 1992). The influence of host and/or bacterial genotypes
in these interactions has also been discussed, suggesting a certain level
of specificity (Azcón et al. 1991; Ruiz-Lozano and Azcón 1993; Monzón and
Azcón 1996).
Multimicrobial interactions including AM fungi, Rhizobium sp. and PGPR
have also been tested (Requena et al. 1997). Target microorganisms were isolated
from a representative area of a desertification-threatened semi-arid
ecosystem in the south-east of Spain. Microbial isolates were characterized
and screened for effectiveness in soil microcosms. Anthyllis cytisoides L., an