plant surface microbiology.pdf

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Ramesh Chander Kuhad et al.
from Myrica. This is indicative of the importance of actinorhizal plants in the
ecosystem development. Actinorhizal plants are also used as intercrops for
other tree species (Dommergues 1997).
3.3 Plant Growth-Promoting Rhizobacteria
The rhizosphere is the region of soil surrounding the roots that is subject to
influence by the root and rhizobacteria are plant-associated bacteria that are
able to colonize and persist on roots (Subba Rao 1999). Several genera of bacteria
such as Arthrobacter, Agrobacterium, Azotobacter, Burkholderia, Cellulomonas,
Micrococcus, Flavobacterium, Mycobacterium, Pseudomonas and
others have been reported to be present in the rhizosphere (see chap. 12, this
vol.). It has been demonstrated that the metabolic activities of bacteria associated
with the rhizosphere are different from those of the nonrhizosphere
soils. Electron and direct microscopy has revealed that up to 10 % of the root
surface is colonized by microorganisms in a random fashion depending on
the presence of soil organic matter. Some strains of plant growth-promoting
rhizobacteria (PGPR) can effectively colonize plant roots and protect plants
from diseases caused by a variety of root pathogens and growth promotion of
plants through formation of plant growth hormones. Considerable progress
has been made using molecular techniques to elucidate the important microbial
factors or genetic traits involved in the PGPR-stimulated plant growth
and in the suppression of fungal root diseases (Glick and Bashan 1997;
Kumari and Srivastava 1999; Bloemberg and Lugtenberg 2001; Zehnder et al.
2001). Several genera of allelopathic nonpathogenic bacteria have been identified
and characterized which produce plant growth-inhibiting allelochemicals
(Barazani and Friedman 2001). Allelochemicals like phytoxins,
geldanamycin, nigericin and hydanthocidin have been isolated from Streptomyces
viridochromogenes.
PGPR can affect plant growth either directly or indirectly. The direct
effect of PGPR include providing the host plants with fixed nitrogen, P and
Fe solubilized from the soil and phytohormones that are synthesized by the
bacteria (Glick 1995). The indirect effect on plant growth occurs when PGPR
reduces or prevents the harmful effects of one or more phytopathogenic
organisms. PGPR effective in biocontrol produce a variety of substances
including antibiotics, siderophores and a variety of enzymes (chitinase, protease,
lipase, b-1,3-glucanase etc.) to limit the damage to plants by phytopathogens.
PGPR have also been reported to reduce heavy metal toxicity
in plants (Burd et al. 2000).
Symbiotic nitrogen fixation has long been considered to be an excellent
replacement of N fertilization. The most efficient nitrogen fixers are strains of
Rhizobium, Sinorhizobium, Mesorhizobium, Bradirhizobium and Azorhizobium,
which form a host-specific symbiosis with leguminous plants (Paul and