BIOFERTILIZERS AS COMPLEMENTS TO SYNTHETIC AND ORGANIC FERTILIZATION - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 166

The Gram-negative bacterium Rahnella aquatilis is ubiquitous and is characterized by its
beneficial metabolism leading to mineral phosphate solubilization, antimicrobial activity,
nitrogen fixation and plant disease suppression (Calvo et al., 2007). This bacterium produces
PQQ and its mineral phosphate solubilizing capacity is contributed by mechanisms similar to
other phosphate solubilizing microbes. Strain HX2 R of R. aquatilis has been used as a
biocontrol agent for grapevine crown gall caused by Agrobacterium vitis . PQQ mutants of
this bacterium become ineffective in its biocontrol activity. Different reports suggest that the
GDH-PQQ holoenzyme is involved in production of antimicrobial compounds in P.
fluorescens (Schnider et al., 1995; de Werra et al., 2009) and Enterobacter intermedium 60-
2G (Han et al., 2008). Particularly, E. intermedium , a phosphate-solubilizing bacterium, has
the ability to induce systemic resistance in plants against the soft rot bacterial pathogen
Erwinia carotovora and the fungus Magnaporthe grisea . Mutations in PQQ cause E.
intermedium lose their biocontrol ability and its capacity for enhancing the systemic
resistance of plants (Han et al., 2008).
I NDIRECT M ECHANISMS
The indirect promotion of plant growth occurs when biofertilizers prevent, reduce or
eliminate one or more pathogenic organisms (Glick et al. , 1999; Hernández & Charlloux,
2001) through the following mechanisms:
Competition for Space and Nutrients
To exert their beneficial effect on plant growth, microorganisms must be rhizosphere-
competent, i.e. , should be capable of compete with other microorganisms present in the
rhizosphere for nutrients secreted by the root and the physical space available inside or over
the root. Only a small part of the root surface is covered by bacteria. Favorite sites for
bacterial growth are the junctions between epidermal cells and origin points of the lateral
roots. Once soil microorganisms colonize plant roots, they colonize space and consume
nutrients that otherwise could be used by plant pathogens (Kloepper et al. , 1988, O'Sullivan &
O'Gara, 1992).
Siderophore Production
Siderophore synthesis is a dual mechanism for promoting plant growth, since it increases
iron availability to plants but also contributes to biological control of phytopathogenic agents.
Sequestering iron from the soil and making it available for themselves and for plant cells
capable of assimilating the bacterial siderophore-iron complexes, plant growth promoting
microorganisms can limit the pool of this element for other microorganisms that are unable to
access the sequestered iron (Castignetti & Smarrelli, 1986; O'Sullivan & O'Gara, 1992;
Dowling et al. , 1996).