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

10
Principles, Applications and Future Aspects of Cold-Adapted
PGPR
Mahejibin Khan and Reeta Goel
10.1
Introduction
The concept of optimal temperature for an organism is a fundamental principle in
biology; however, organisms are unable to control temperature variations. Therefore,
they either avoid stress or tolerate it. Since plants are not able to move from one place
to another to handle stress, they, therefore, use different strategies to deal with
temperature fluctuations within their habitat. Chilling may occur at temperature
below 15 C in the absence of ice nucleation in plant cells. The symptoms of chilling
may gradually appear afterwards, especially when the plants return to optimal
growth temperature, and include loss of vigor, reduction in growth rate, autolysis
of cell and loss of chlorophyll [1]. Moreover, disruption of cell membranes at low
temperature allows the leakage of solutes and nutrients, providing an excellent
growth medium for opportunistic pathogens such as bacteria and fungi. The rates
of CO2 and ethylene production usually increase before vital symptoms such as seed
germination [2] appear. If the soil temperature is very low at the time of planting of
seeds, the initial uptake of water disrupts membrane integrity and increases electrolyte
leakage and blocks seed germination. Nevertheless, in vegetative stages,
seedlings are generally more sensitive to chilling than mature plants.
To cope with chilling injuries and to protect plants such injuries, various methods
have been investigated, such as prevention of exposure of plants to chilling and use
of tolerant cultivars. Genetic variability and transfer of chilling tolerance into commercially
well-adapted cultivars is a complex and time consuming process; therefore,
a solution for the protection of plants from chilling and for their growth
enhancement involves the application of cold-adapted plant growth promoting rhizobacteria
(PGPR). This term was initially used to describe strains of naturally
occurring nonsymbiotic soil bacteria having the ability to colonize the plant rhizosphere
and stimulate plant growth. PGPR activity has been reported in strains
belonging to several genera such as Azotobacter, Arthrobacter, Bacillus, Clostridium,
Plant-Bacteria Interactions. Strategies and Techniques to Promote Plant Growth
Edited by Iqbal Ahmad, John Pichtel, and Shamsul Hayat
Copyright Ó 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-31901-5
j195