plant surface microbiology.pdf

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Ramesh Chander Kuhad et al.
of b-glucosidase and cellobiohydrolase, where soil representing the highest
disease suppression had the highest activity. Furthermore, soil suppressiveness,
as well as the enzyme activity significantly correlated with the soil content
of total C and N.
4 Plant Growth Promoting Substances Produced by Soil
Microbes
The ability of soil microorganisms to produce various metabolites stimulating
plant growth is considered to be one of the most important factors in soil
fertility (Frankenberger and Arshad 1995; Paul and Clark 1998; Subba Rao
1999). Some PGPR control the damage to plants from plant pathogens by a
number of different mechanisms including physical displacement and outcompeting
the phytopathogen, secretion of siderophores to prevent
pathogens in the immediate vicinity from proliferating, production of
enzymes, antibiotics and a variety of small molecules that inhibit the phytopathogen
and stimulation of systemic resistance in plants (Glick and
Bashan 1997). Microbially produced antibiotics have a potential role in indirectly
promoting plant growth by controlling plant diseases (Kumari and Srivastava
1999). Two prominent antifungal antibiotics are griseofulvin, a metabolic
product of Penicillium griseofulvum and aureofungin, a metabolic
product of Streptoverticillium cinnamomeum.
Soil microorganisms produce a variety of phytohormones such as auxins,
gibberellins, cytokinins, ethylene and abscisic acid.Auxin production is widespread
among many soil and rhizosphere microorganisms (fungi,bacteria and
actinomycetes) and algae (Martens and Frankenberger 1993). Tryptophan is
considered the physiological precursor of auxin for both plant and soil
microbes. A number of indole compounds and phenylacetic derivatives have
been reported with auxin activity. Indole-3-acetic acid (IAA) is considered the
most physiologically active auxin in plants. Auxins are known to affect cell
enlargement involving cell wall extensibility. Plant growth responses also
include root and shoot dry weights, root/stem elongation and root/shoot
ratios.Species of Agrobacterium,Azospirillum,Pseudomonas,Rhizobium,Ustilago,
Gymnosporangium, Rhizopus and Synchytrium produce IAA in pure cultures
or in association with higher plants (Subba Rao 1999).
Gibberellins (GA) are an important group of plant hormones that are
diterpenoid acids. The involvement of GA in almost all phases of plant growth
and development, starting from germination to senescence is well known.
However, the most prominent physiological effect of GA is in shoot elongation.
Some other plant growth related functions of GA include overcoming
dormancy and dwarfism in plants, inducing flowering in some photoperiodically
sensitive and other low temperature-dependent plants, and contributing
to fruit setting. Several soil microbes are known to produce gibberellins or