with the notion that ACC deaminase-containing PGPB probably also provide an
advantage to uncultivated plants growing in the natural environment. In fact, in one
experiment, researchers isolated PGPB from the rhizosphere (the area around the
plant roots) of wild barley plants growing in a region of northern Israel termed
“Evolution Canyon” where the two slopes of this canyon are around 250 m apart at
their bases (Timmusk et al. 2011). The south-facing slope of this canyon is quite
arid, receives an excessive amount of sunlight and has only very sparse plant growth
compared to the north-facing slope where the plant growth is relatively lush. Both
slopes contained similar genera of bacteria in the barley rhizospheres. However,
nearly all of the bacteria that were examined from the much more highly stressed
(water and light stress) south-facing slope included ACC deaminase activity that
allowed the plants on this slope to withstand the harsher conditions encountered.
Thus, in this natural environment, the drought conditions on the south-facing slope
selects for bacteria that contain traits, such as the presence of ACC deaminase, that
allow both the PGPB and the host plants to better survive these harsh conditions.
Moreover, under the more moderate conditions on the north-facing slope, the same
bacteria are largely devoid of those traits that facilitate bacterial and plant survival
under harsh conditions.
Enzyme ACC deaminase is a common component of a large number of PGPB,
enabling these bacterial strains to be highly efficacious in facilitating plant growth
under a wide range of stressful conditions. ACC deaminase (EC: 3.5.99.7) belongs
to the enzyme superfamily entitled tryptophan synthase beta subunit-like pyridoxal
phosphate (PLP)-dependent enzymes and is quite ubiquitous in nature (Singh et al.
2015) occurring in all three domains of life, i.e., Archaea, Bacteria, and Eukarya,
having been observed in bacteria, fungi, Stramenopiles, Archaea and plants. In the
material that follows, the biochemistry and molecular biology of this key microbial
enzyme are examined in some detail.
19.2 Prevalence of ACC Deaminase in Biota
The ACC deaminase was initially purified and characterized from members of the
γ-Proteobacteria, mainly Pseudomonas (Honma and Shimomura 1978; Klee et al.
1991). Subsequently, ACC deaminase and its homologues were purified and characterized from other microbial sources (Fujino et al. 2004; Jacobson et al. 1994;
Hontzeas et al. 2004; Minami et al. 1998). To date, the active enzyme has been
reported in α, β, and γ Proteobacteria, Actinobacteria, Firmicutes, Bacteroidetes,
Archaea, various fungi, and yeast (Ekimova et al. 2018; Fujino et al. 2004; Hontzeas
et al. 2004; Marques et al. 2010; Minami et al. 1998; Nascimento et al. 2014; Singh
et al. 2015). Moreover, ACC deaminase has also been reported in plants including
Arabidopsis, poplar, tomato, and corn (McDonnell et al. 2009; Singh et al. 2015).
Despite the fact that bacterial genera Chlorobi, Bacteroidetes, and Firmicutes have
been documented to contain ACC deaminase activity, the genes (acdS) responsible
for such activity have yet to be reported (Nascimento et al. 2014). This apparent
19 Biochemistry and Molecular Biology of the Enzyme ACC Deaminase
367
advantage to uncultivated plants growing in the natural environment. In fact, in one
experiment, researchers isolated PGPB from the rhizosphere (the area around the
plant roots) of wild barley plants growing in a region of northern Israel termed
“Evolution Canyon” where the two slopes of this canyon are around 250 m apart at
their bases (Timmusk et al. 2011). The south-facing slope of this canyon is quite
arid, receives an excessive amount of sunlight and has only very sparse plant growth
compared to the north-facing slope where the plant growth is relatively lush. Both
slopes contained similar genera of bacteria in the barley rhizospheres. However,
nearly all of the bacteria that were examined from the much more highly stressed
(water and light stress) south-facing slope included ACC deaminase activity that
allowed the plants on this slope to withstand the harsher conditions encountered.
Thus, in this natural environment, the drought conditions on the south-facing slope
selects for bacteria that contain traits, such as the presence of ACC deaminase, that
allow both the PGPB and the host plants to better survive these harsh conditions.
Moreover, under the more moderate conditions on the north-facing slope, the same
bacteria are largely devoid of those traits that facilitate bacterial and plant survival
under harsh conditions.
Enzyme ACC deaminase is a common component of a large number of PGPB,
enabling these bacterial strains to be highly efficacious in facilitating plant growth
under a wide range of stressful conditions. ACC deaminase (EC: 3.5.99.7) belongs
to the enzyme superfamily entitled tryptophan synthase beta subunit-like pyridoxal
phosphate (PLP)-dependent enzymes and is quite ubiquitous in nature (Singh et al.
2015) occurring in all three domains of life, i.e., Archaea, Bacteria, and Eukarya,
having been observed in bacteria, fungi, Stramenopiles, Archaea and plants. In the
material that follows, the biochemistry and molecular biology of this key microbial
enzyme are examined in some detail.
19.2 Prevalence of ACC Deaminase in Biota
The ACC deaminase was initially purified and characterized from members of the
γ-Proteobacteria, mainly Pseudomonas (Honma and Shimomura 1978; Klee et al.
1991). Subsequently, ACC deaminase and its homologues were purified and characterized from other microbial sources (Fujino et al. 2004; Jacobson et al. 1994;
Hontzeas et al. 2004; Minami et al. 1998). To date, the active enzyme has been
reported in α, β, and γ Proteobacteria, Actinobacteria, Firmicutes, Bacteroidetes,
Archaea, various fungi, and yeast (Ekimova et al. 2018; Fujino et al. 2004; Hontzeas
et al. 2004; Marques et al. 2010; Minami et al. 1998; Nascimento et al. 2014; Singh
et al. 2015). Moreover, ACC deaminase has also been reported in plants including
Arabidopsis, poplar, tomato, and corn (McDonnell et al. 2009; Singh et al. 2015).
Despite the fact that bacterial genera Chlorobi, Bacteroidetes, and Firmicutes have
been documented to contain ACC deaminase activity, the genes (acdS) responsible
for such activity have yet to be reported (Nascimento et al. 2014). This apparent
19 Biochemistry and Molecular Biology of the Enzyme ACC Deaminase
367