Agriculture Reference
In-Depth Information
transcription. However, in this case, a large portion of the additional ACC which is
synthesized is hydrolysed by the bacterial ACC deaminase. Therefore, the use of
such plant growth-promoting bacteria containing ACC deaminase may prove useful
in developing strategies to facilitate plant growth in stressed soil environments.
2.3.3.2 Role of ACC Deaminase in Nodulation
ACC deaminase-containing bacteria are relatively common in soil and have been
found in a wide range of environments across the world. Indeed, the ability of
bacteria to hydrolyse ACC has a competitive advantage over other soil inhabitants
because it can use ACC as an N source (Jacobson et al.
1994
). This hypothesis
suggests that ACC may act as a unique/novel source of N for some soil bacteria.
While searching for ACC deaminase positive rhizobial strains, it was found that
amongst 13 different rhizobial strains, five strains displayed enzyme activity while
seven strains had the
acdS
gene (Ma et al.
2003
). Conclusively, it was reported that
the
Mesorhizobium
strain only expressed this activity when the bacterium was
present within a root nodule. In other investigation conducted in southern Saskatch-
ewan, Canada, of the total 233 rhizobial strains isolated from soil samples collected
from 30 different sites, nearly 12 % (27 strains) displayed the ACC deaminase
activity (Duan et al.
2009
). Similarly, ACC deaminase genes have been reported in
chickpea
Mesorhizobium
isolates (Nascimento et al.
2012
),
B. japonicum
E109,
USDA110 and SEMIA5080 (Boiero et al.
2007
). Rhizobial strains that express
ACC deaminase are up to 40 % more efficient at forming nitrogen-fixing nodules
than strains that lack this activity (Ma et al.
2003
,
2004
). However, strains of
rhizobia that express ACC deaminase have only a low level of enzyme activity
compared with free-living plant growth-promoting bacteria, i.e. typically around 2-
10 %. Thus, free-living bacteria bind relatively non-specifically to plant tissues
(mainly roots) and have a high level of ACC deaminase activity that can protect
plants from different abiotic and biotic stresses by lowering ethylene levels
throughout the plant. On the other hand, (symbiotic) rhizobia that generally bind
tightly only to the roots of specific plants have a low level of enzyme activity which
facilitates nodulation by locally lowering ethylene levels. It is not known whether
the large differences in enzyme activity that are observed when comparing free-
living bacteria with rhizobia are a consequence of differences in the amount of
enzyme synthesized by one type of bacteria versus the other or of differences in the
specific catalytic activity of the enzymes from the different types of bacteria. It has
also been observed that some rhizobia reduces the plant ethylene levels mediated by
ACC deaminase activity and enhances nodulation in host legumes (Zahir
et al.
2008
; Belimov et al.
2009
) or modifies root system of non-legumes. For
instance, strains of
R. leguminosarum
bv.
viciae
and
Mesorhizobium loti
increased
the number of lateral roots in
Arabidopsis thaliana
because of this plant growth-
promoting mechanism (Contesto et al.
2008
). In addition to the more common
mode of
acdS
transcriptional regulation,
acdS
genes from various strains of
M. loti
have been found to be under the transcriptional control of the
nifA
promoter that is
