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

ameliorate drought stress effects on wetland rice, as grain yield increased, together
with increased nutrient uptake and proline content [196]. Proline is an important
osmoregulator, accumulated as a consequence of drought stress. Creus et al. [197]
studied the effects of A. brasilense Sp245 inoculation on water relations in two wheat
cultivars. They found that Azospirillum stimulated growth of wheat seedlings grown
in darkness under osmotic stress, together with a significant decrease in osmotic
potential and relative water content at zero turgor, in volumetric cell wall modulus of
elasticity and in absolute symplastic water volume and a significant rise in apoplastic
water fraction parameters. These are known physiological mechanisms of adaptation
that give plants the ability to tolerate a restricted water supply [198]. As in this
hydroponic test system no nutrients were present, therefore, the improved water
status of the wheat seedlings cannot be attributed to enhanced mineral uptake and
consequently growth promotion. Similarly, in a hydroponic system without nutrients,
A. brasilense Sp245 was found to partially reverse the negative effects that drought
stress had on wheat seedlings, as it was observed in the growth rate of coleoptiles
[199]. Apart from alleviating osmotic stress in plants, inoculation with diazotrophs
can also enhance oxidative stress tolerance. Oxidative stress is defined as the oxidative
damage caused by reactive oxygen species (ROS) such as the superoxide anion radical,
hydrogen peroxide, the hydroxyl radical and singlet oxygen [200,205]. These highly
reactive oxygen species can be generated by the oxidative metabolism of normal cells
and by different stress situations. Although ROS contribute to plant defense against
pathogens, they are potentially harmful to plant viability [201]. With the production of
antioxidant enzymes such as superoxide dismutase (SOD), peroxidase and catalase,
the cell can neutralize and thus control free radical formation. Also, pigments such as
carotenoids could be involved in scavenging singlet oxygen and thus decrease oxidative
stress [202]. Inoculation with Azotobacter chroococcum was reported to improve
oxidative stress defense ability in sugar beet leaves since inoculated plants showed
increased activities of superoxide dismutase, peroxidase and catalase and increased
chlorophyll and carotenoid content [203]. High activities of antioxidant enzymes
(especially SOD) are linked with oxidative stress tolerance [204]. However, the
observed effects have not been linked yet to certain traits of diazotrophic bacteria.
Therefore, it is not clear whether this increase in oxidative stress tolerance is a direct
result of inoculation or rather an indirect consequence of an overall increase in plant
health because of inoculation with Azotobacter.
5.6.4
Quorum Sensing
5.6 Other Dimensions of Plant Growth Promoting Activitiesj99
It has been recognized that bacteria not only can behave as individual cells but
under appropriate conditions, when their number reaches a critical level, can also
modify their behavior to act as multicellular entities. This phenomenon is based
on the dynamics of a natural ecosystem, since bacteria do not exist as solitary cells
but are typically colonial organisms that live as consortia to exploit the elaborate
system of intracellular communication that facilitates adaptations to changing
environmental conditions. When the bacterial population reaches a threshold,