Agriculture Reference
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host by rhizobia, the host produces ROS, namely hydrogen
peroxide and superoxide radical, at the site of infection
(Santos
et al.,
2001). This resembles the hypersensitive
response of plants in incompatible plant-pathogen inter-
actions. However, in the development of the compatible
symbiotic interaction, the rhizobia are able to deal with
the host defence response, and the plant adopts a different
defence strategy (Mithofer, 2002). Indeed, the plant con-
trols the invading bacteria by confining them in an organ
formed for this purpose, the nodule. Failure of effective
nodule formation, due to defective surface polysaccha-
rides of rhizobia, often results in pathogenic responses
(Oldroyd & Downie, 2008). Notably, the ROS level has to
be controlled not only by the invading rhizobia but also by
the host plant to diminish the oxidative stress effects.
Both, the host plant and the rhizobia possess an arsenal of
antioxidant tools, among which are the enzymes catalase,
superoxide dismutase and various peroxidases (Becana
et al.,
2000; Matamoros
et al.,
2003). For example,
Sinorhizobium meliloti
has several antioxidant enzymes
including two superoxide dismutases, SodA and SodC,
and three catalases, KatA, KatB and KatC (Jamet
et al.,
2003; Minchin
et al.,
2008; Becana
et al.,
2010). In mutants
that lack expression of any of the three catalases or one of
the two dismutases, an increased sensitivity of the
bacterium to ROS is observed but not the prevention of
nodulation (Jamet
et al.,
2007; Minchin
et al.,
2008).
However, in double
katB
/
katC
and
katA
/
katC
mutants, the
rhizobia lose their ability to colonize root hairs of the host
plant
Medicago truncatula
(Jones
et al.,
2007).
In addition to antioxidant defence, the efflux of ROS
from the host roots is inhibited by specific rhizobial sig-
nalling molecules commonly known as Nod factors
(NFs) (Shaw & Long, 2003). It is suggested that NFs sup-
press the activity of the ROS-generating plant system
and thus allow a compatible interaction between plant
and rhizobia to occur (Chang
et al.,
2009). In this respect,
host defence against oxidative stress seems also to be a
crucial component for successful nodulation, since cata-
lase deficiency induced transiently by RNAi in
Medicago
truncatula
roots, resulted in reduced nodule formation,
especially under osmotic stress conditions. However,
the efflux of ROS from the host roots is indispensable
for nodule formation and the development of a compat-
ible plant-microbe interaction. ROS, such as hydrogen
peroxide, are crucial for inter-cross-linking and strength-
ening the cell wall formation of infection threads.
Studies have shown that inhibition of ROS production
prevents root hair curling and formation of infection
threads (Chang
et al.,
2009, and references therein).
Conclusively, the disparity of ROS accumulation discerned
at different time points of the symbiotic interaction high-
lights the essential role of ROS at multiple spatiotemporal
steps in the nodule formation process.
8.3 Legume-rhizobia symbiosis:
A vulnerable association under
osmotic stresses
Biological nitrogen fixation (BNF) by symbiotic
association is a highly desirable option for restoration of
marginal lands damaged by environmental stresses.
However, it is well documented that BNF is vulnerable
to abiotic constraints (Zahran, 1999). In addition, plant
responses to abiotic stresses, such as salinity and
drought, involve complex mechanisms and different
pathways that are not entirely characterized. Further,
understanding the legume-rhizobium symbiotic inter-
action under abiotic stress is even more challenging. It is
generally accepted that under salinity and drought con-
ditions, the reduced water supply is the most significant
limitation on growth and crop yield (Zhu, 2001; Schleiff,
2008). The osmotic constraint induces the plant and the
bacteria to decrease their internal water potential to
avoid desiccation (Zahran, 1999; Tonon
et al.,
2004).
Water is a prerequisite for all biochemical activities in
known life forms (Xiong & Zhu, 2002), thus any distur-
bance of water potential seriously affects cell metabolic
activities. In nodules, responses to reduced water avail-
ability are marked by adaptive changes including growth
limitation, cortical structure modification and decline of
metabolic activities. Undoubtedly these changes lead to
alteration of nitrogen-fixing machinery, and conse-
quently to decline of symbiosis effectiveness.
8.4 Nodulation process and symbiotic
performance variability
Nodule formation depends largely on effective
development of the root hair system (Oldroyd &
Downie, 2008). Water deficit adversely affects root
architecture, reducing the formation of hairy roots.
Additionally, disturbance of ionic balance due to water
limitation impinges on the signalling interactions of host
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