19.6 Levels of ACC Deaminase Activity Among Diverse
Microbes
Generally, ACC deaminase activity of microbes is detected and quantified in vitro
and the results of these assays vary from method to method. Table 19.4 presents the
levels of enzyme activity quantified among various bacteria by different research
groups; this table is representative of the data in the literature but is by no means
exhaustive. At the outset, it should be noted that, perhaps surprisingly, ACC
deaminase activity within one bacterial genus is not constant. For example, the
enzyme activity for the bacterial genus Pseudomonas varies from 0.001 to
21.23 μmol mg
–1 h
–1 , and the same pattern of activity variation is true for the
other genera (Table 19.4). Moreover, the level of ACC deaminase activity produced
by nodule forming Rhizobia was observed to be considerably lower than for freeliving bacteria when quantified under the same experimental conditions (Ma et al.
2003; Duan et al. 2009), (Table 19.4). Nodule forming Rhizobia produce only
2–10% of the amount of ACC deaminase activity compared to free-living bacteria
but are up to 40% more efficient at functional nodule formation than ACC deaminase
negative Rhizobia strains (Glick 2014). ACC deaminase produced by noduleresiding Rhizobia facilitates the process of nodulation, by decreasing the localized
level of ethylene by breaking down the localized concentration of ACC, but does not
lower the ethylene level throughout the plant and therefore does not protect the plant
against various forms of environmental stress (Nascimento et al. 2012).
The other prominent microbial group that can produce ACC deaminase is fungi.
For example, the levels of ACC deaminase activity were found to be as high as
9 μmol mg
–1 h
–1 in the fungal strain Trichoderma estonicum SKS1ACC
(Saravanakumar et al. 2018) and 12.16 μmol mg
–1 h
–1 in the fungal strain
Trichoderma asperellum T203 (Viterbo et al. 2010). Thereby, according to the
levels of ACC deaminase produced in in vitro conditions, these three phylogenetic
groups can be arranged as follows: free-living bacteria > fungi >Rhizobia (from the
highest to the lowest levels of enzyme activity). These differences in activity may be
a consequence of differences in enzyme catalytic activities among these three groups
or may be due to the differences in the amounts of enzyme produced by one specific
type of microbe. Interestingly, in the phylogenetic study carried out on the acdS
gene, it was found that the various phylogenetic groups (fungi cluster with other
fungi while Rhizobia cluster with other Rhizobia, very similar to the 16S/18S rRNA
gene phylogram) cluster together based on the gene sequence that they carry
(Nascimento et al. 2014). Despite the fact that microbes vary with respect to the
levels of ACC deaminase that they produce, a low (minimal) level of ACC deaminase activity (20 nmol mg
–1 h
–1 ) is required to allow them to grow on ACC as a sole
nitrogen source and to lower stress ethylene levels. Interestingly, as mentioned
earlier, microbes with higher levels of ACC deaminase enzymatic activity do not
necessarily perform any better under laboratory conditions than do organisms that
produce a moderate level (>20 nmol mg
–1 h
–1 ) of enzyme activity (Penrose and
Glick 2003).
19 Biochemistry and Molecular Biology of the Enzyme ACC Deaminase
377
Microbes
Generally, ACC deaminase activity of microbes is detected and quantified in vitro
and the results of these assays vary from method to method. Table 19.4 presents the
levels of enzyme activity quantified among various bacteria by different research
groups; this table is representative of the data in the literature but is by no means
exhaustive. At the outset, it should be noted that, perhaps surprisingly, ACC
deaminase activity within one bacterial genus is not constant. For example, the
enzyme activity for the bacterial genus Pseudomonas varies from 0.001 to
21.23 μmol mg
–1 h
–1 , and the same pattern of activity variation is true for the
other genera (Table 19.4). Moreover, the level of ACC deaminase activity produced
by nodule forming Rhizobia was observed to be considerably lower than for freeliving bacteria when quantified under the same experimental conditions (Ma et al.
2003; Duan et al. 2009), (Table 19.4). Nodule forming Rhizobia produce only
2–10% of the amount of ACC deaminase activity compared to free-living bacteria
but are up to 40% more efficient at functional nodule formation than ACC deaminase
negative Rhizobia strains (Glick 2014). ACC deaminase produced by noduleresiding Rhizobia facilitates the process of nodulation, by decreasing the localized
level of ethylene by breaking down the localized concentration of ACC, but does not
lower the ethylene level throughout the plant and therefore does not protect the plant
against various forms of environmental stress (Nascimento et al. 2012).
The other prominent microbial group that can produce ACC deaminase is fungi.
For example, the levels of ACC deaminase activity were found to be as high as
9 μmol mg
–1 h
–1 in the fungal strain Trichoderma estonicum SKS1ACC
(Saravanakumar et al. 2018) and 12.16 μmol mg
–1 h
–1 in the fungal strain
Trichoderma asperellum T203 (Viterbo et al. 2010). Thereby, according to the
levels of ACC deaminase produced in in vitro conditions, these three phylogenetic
groups can be arranged as follows: free-living bacteria > fungi >Rhizobia (from the
highest to the lowest levels of enzyme activity). These differences in activity may be
a consequence of differences in enzyme catalytic activities among these three groups
or may be due to the differences in the amounts of enzyme produced by one specific
type of microbe. Interestingly, in the phylogenetic study carried out on the acdS
gene, it was found that the various phylogenetic groups (fungi cluster with other
fungi while Rhizobia cluster with other Rhizobia, very similar to the 16S/18S rRNA
gene phylogram) cluster together based on the gene sequence that they carry
(Nascimento et al. 2014). Despite the fact that microbes vary with respect to the
levels of ACC deaminase that they produce, a low (minimal) level of ACC deaminase activity (20 nmol mg
–1 h
–1 ) is required to allow them to grow on ACC as a sole
nitrogen source and to lower stress ethylene levels. Interestingly, as mentioned
earlier, microbes with higher levels of ACC deaminase enzymatic activity do not
necessarily perform any better under laboratory conditions than do organisms that
produce a moderate level (>20 nmol mg
–1 h
–1 ) of enzyme activity (Penrose and
Glick 2003).
19 Biochemistry and Molecular Biology of the Enzyme ACC Deaminase
377