Rhizosphere Microorganisms and Their Effect on Fruit Quality - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 363

Research on mechanisms of growth promotion has shown the complexity of PGPR-plant
association. Currently, the role of active volatiles other than ethylene produced by PGPR has
been added to that complexity. Ryu et al. (2003) reported that the volatile compounds 2,3-
butanediol and acetoin, produced by B. subtilis and Bacillus amyloliquefaciens , promoted
growth of Arabidopsisthaliana . In this sense, most of our work has been done with systems
based on Bacillus spp. showing that growth promotion is also related with the production
of peptides (Jimenez-Delgadillo, 2004).
17.3 Arbuscular mycorrhizal fungi
As reported in the literature, a variety of mechanisms have been proposed for how AMF
may exert positive effects on plant physiology. The colonization of plant roots by AMF
can greatly affect the plant uptake of mineral nutrients. The contribution of AMF to plant
nutrient uptake is mainly due to acquisition of nutrients by the extraradical hyphae (George,
2000). The plant makes use of the tremendous surface area of mycelium to absorb min-
eral nutrients from the soil. It is also believed that the mycelia of mycorrhizal networks
have chemical absorption capabilities that are able to access some plant nutrients at lower
concentrations that nonmycorrhizal plant roots are not capable of absorbing. Many arbus-
cular mycorrhizal fungi can transport nitrogen, phosphorous, zinc, and copper to the host
plant. It has been shown that phosphate in soil solution is absorbed by a phosphate trans-
porter in the extraradical hyphae, condensed into polyphosphate and translocated through
the intraradical hyphae, being released across the fungal membrane in the arbuscule (Saito,
2000). The plant water balance could also be affected by AMF. The mycorrhizal influence
most often examined within this field has been the alteration of stomatal behavior in host
plants. Moreover, mycorrhizal symbiosis affects photosynthesis, a physiological process
intimately linked with stomatal behavior (Auge, 2001).
It has been suggested that phytohormones released by the infecting fungi may con-
tribute to the enhancement of plant growth, since elevated hormonal levels in mycorrhizal
plants have been reported (Ludwig-Muller, 2000). Although it has been shown that mycor-
rhizae play a significant role somehow protecting plants against phytopathogenic organisms,
the experimental demonstration of this phenomenon has been difficult. However, different
mechanisms have been suggested: (a) enhanced nutrition, (b) competition for nutrients and
infection sites, (c) morphological changes, (d) changes in chemical constituents in plant
tissues, (e) alleviation of abiotic stress, and (f) interactions with other rhizosphere microor-
ganisms, occurring in the zone of soil surrounding the roots and fungal hyphae commonly
referred to as the “mycorrhizosphere” (Rambelli, 1973). It is a fact that the success of root
colonization and the mycorrhizal effectiveness depend on a variety of biotic and abiotic
factors including the species of arbuscular mycorrhizal fungi forming the symbiosis, plant
cultivar, availability of P in soil, and type of soil, among others.
The complex cellular relationship between host roots and AMF requires a continuous ex-
change of signals, which in return affects the regulation of genes whose products participate
in metabolic and structural changes that lead to the symbiosis. AMF colonization induced
genes related to defense or stress responses, and they showed an increase in transcript level
during the initial period of contact between the symbionts with a subsequent decrease as
the symbiosis developed (Liu et al., 2003). Transcription of new genes that are similar
to the components of signal transduction pathways had also shown a sustained increase,