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plant and rhizobia. Alterations of plant root architecture
lead to the formation of atypical nodulation sites due to
a rigid root cortex and deformed root hairs, causing a
decline in successful infections and nodulation (Zahaf
et al.,
2012; ZĂ©licourt
et al.,
2012). It is known that sym-
biosis establishment and functioning under stressful
conditions are related to the vigour of the host legume
(Zahran, 1999). The bacterial partners (i.e. rhizobia) are
more tolerant than their host plants because rhizobia
species are able to survive under conditions of high
salt concentrations and desiccation (Jebara
et al.,
2001; Vriezen
et al.,
2007; Mnasri
et al.,
2007b). Yet, the
adaptation of rhizobia to water deficit is accompanied by
morphological and physiological changes that decrease
their infection and nodulation capacity (Zahran &
Sprent, 1986; Predeepa & Ravindran, 2010).
Salinity and drought severely affect biomass produc-
tion and nitrogen-fixing capacity, which are considered
as essential indices of symbiosis effectiveness (Serraj
et al.,
1999; Mhadhbi
et al.,
2004, 2008, 2011a; Verdoy
et al.,
2004; Mhadhbi & Aouani, 2008). Studies have
shown that the nitrogenase activity and consequently
the nodule N-fixing mechanism are more sensitive than
plant growth to abiotic stress (Mhadhbi
et al.,
2004,
2011a). This particular sensitivity is explained by the
complexity of the symbiotic association and the high
energy demand for nodule function. Energy provided
by the host plant may be restricted because the salinity/
drought stress causes nutrient deprivation due to a
decline in water uptake and reduced function of the
photosynthetic apparatus (Zahran, 1999; Lopez
et al.,
2008). Recently, Mhadhbi
et al.
(2008, 2011a) and
Pimratch
et al.
(2008) reported that more efficient sym-
bioses were recorded in normal conditions than under
osmotic stress. These results suggest that high biomass
production and nitrogen-fixing capacity under osmotic
stress are decreased when compared with non-stressful
conditions. Nevertheless, other studies reported that
bacterial strains characterized by moderate efficiency at
normal conditions could enhance symbiosis tolerance
under stressful conditions and vice versa (Mnasri
et al.,
2007a; Ben Rhomdhane
et al.,
2007; Tajini
et al.,
2008;
Mhadhbi
et al.,
2011b). Which of the symbiotic partners
plays the most significant role for achieving better toler-
ance under diverse conditions is the subject of a
diachronic debate between microbiologists (defending
the input of the bacterial partner) and plant breeders
(insisting on the superiority of plant vigour). Aiming to
provide novel evidence on this long-standing debate,
we studied the modulation of the symbiotic effectiveness
of different legume-rhizobia associations, in relation to
plant genotype and rhizobial strain (Mhadhbi
et al.,
2004, 2005, 2008, 2011a,b,c; Jebara
et al.,
2005; Mnasri
et al.,
2007a,b; Ben Rhomdhane
et al.,
2007; Mhadhbi &
Aouani, 2008).
8.5 Variability of symbiotic partners'
input to symbiosis resilience
There is no doubt that rhizobia are more tolerant than
legume host plants to environmental perturbations.
Indeed, some rhizobia strains are able to survive on
media containing more than 40 g/L NaCl (Mnasri
et al.,
2007b), a salinity level higher than that of the
Mediterranean Sea (36 g/L). However, the question that
remains unresolved is how and through which mecha-
nisms the microsymbiont can enhance plant growth
under adverse or stressful conditions. Studies indicate
that a competitive and persistent rhizobial strain is not
expected to express its full capacity for nitrogen fixation
if limitations constrain the host legume's vigour (Zahran,
1999). On the other hand, results showed that
inoculation with tolerant and efficient rhizobial strains
enhanced legume production under abiotic stress condi-
tions (Mhadhbi
et al.,
2004, 2008, 2011b; Mnasri
et al.,
2007a; Ben Rhomdhane
et al.,
2007). This provides sub-
stantial potential means of enhancing legume cultivation
in marginal lands of arid and semi-arid climates.
Statistical approaches have estimated the contribution
of each partner as well as their interaction on the vari-
ability of symbiotic effectiveness under salt and drought
stress conditions (Mhadhbi
et al.,
2005, 2008, 2011a;
Mhadhbi & Aouani, 2008). Results under controlled
conditions showed that in wild legumes such as
Medicago
truncatula,
the host plant genotype is the major factor
contributing to the total variance of symbiotic effective-
ness indices. Similar results were reported under field
conditions (Fesenko
et al.,
1994; Robinson
et al.,
2000).
The tolerance of the symbiotic association is primarily
determined by the degree of host plant tolerance. The
input of the bacterial partner is mostly related to its
potential efficiency under stress. The prominent role of
the plant genotype could be attributed to the contrasting
behaviour of different
M. truncatula
lines, which imply
different metabolic responses involving variations in
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