Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

contrast to the in planta results, population enhancements were not observed
when R. etli and P. polymyxa were cocultured in vitro using minimal media in
the absence of the seedling. The addition of seed exudates to the growth media
also failed to stimulate the population increases. Mutants of A. brasilense and
R. leguminosarum altered in the production of extracellular polysaccharides.
Binaciotto et al. [180] showed the involvement of these polysaccharides in the
attachment of these bacteria to the structures of AM fungi. In soil, an extensive
network of AM fungi develops and PGPR are usually associated with fungal
surfaces [181]. Azotobacter chroococcum and Pseudomonas fluorescens were attracted
toward tomato roots colonized by Glomus fasciculatum compared to nonvesiculararbuscular
mycorrhizal tomato roots [182]. The presence of G. clarum decreased
or did not significantly affect plant growth under the different culture conditions.
The presence of AM fungi stimulated the nitrogen-fixing bacterial population of
upland rice. Bacterial species had different effects, under both culture conditions,
and some genera of nitrogen-fixing bacteria increased root and shoot growth at
different plant growth stages. The level of mycorrhiza colonization had no influence
on plant growth [183].
5.6
Other Dimensions of Plant Growth Promoting Activities
5.6.1
ACC Deaminase Activity
5.6 Other Dimensions of Plant Growth Promoting Activitiesj97
Ethylene is the only gaseous hormone produced by plants. It is also known as the
wounding hormone because its production in the plant can be induced by physical
or chemical perturbation of plant tissues. Among its myriad effects on plant growth
and development, ethylene production can inhibit root growth. In some cases, the
growth promotion effects of ACC deaminase producing PGPR appear to be best
expressed in stressful situations such as in heavy metal contaminated soils [184].
The enzyme ACC deaminase plays a key role in degrading ACC. The products of
this hydrolysis, ammonia and a-ketobutyrate, can be used by the bacterium as a
source of nitrogen and carbon for growth [185]. In this way, the bacterium acts as a
sink for ACC and as such is lowering the ethylene level in plants, preventing some
of the potentially deleterious consequences of high ethylene concentrations. In
nature, ACC deaminase has been commonly found in soil bacteria that colonize
plant roots [186]. Many of these microorganisms were identified by their ability to
grow on minimal media containing ACC as its sole nitrogen source. In this
way, Azospirillum spp., Herbaspirillum spp., Azoarcus, Azorhizobium caulinodans,
Gluconoacetobacter diazotrophicus, Burkholderia vietnamiensis, Azotobacter spp.,
Azorhizophilus and Pseudomonas spp. were all found to be able to use ACC as the
sole nitrogen source for growth. An example of such an ACC deaminase containing
bacterium is the PGPR Pseudomonas putida GR12-2 [187] that stimulates
root growth of a number of different plants (canola, lettuce and tomato) under