Agriculture Reference
In-Depth Information
2.3.3.1 How the Bacterial ACC Deaminase Works
Mechanistically, the ACC deaminase-producing plant growth-promoting bacteria
first bind to the surface of a plant (usually seeds or roots), although these bacteria
may also be found on leaves and flowers or within a plant's internal tissues, i.e. as
an endophyte (Glick et al.
1998
). Along with other small molecular components of
root exudates, some of the plant ACC (a non-ribosomal amino acid) is exuded from
seeds, roots or leaves (Penrose et al.
2001
) and may be taken up by the bacteria
associated with these tissues and subsequently cleaved by ACC deaminase (Penrose
and Glick
2003
). The ACC, the immediate precursor of C
2
H
4
, when hydrolysed by
ACC deaminase results in NH
3
and
α
-ketobutyrate formation (Glick et al.
1998
;
Penrose and Glick
2003
; Reed et al.
2005
; Safronova et al.
2006
), and hence, it
strongly alleviates the stress induced by ethylene-mediated impact on plants by
lowering the C
2
H
4
levels in plants (Glick et al.
2007
; Sessitsch et al.
2005
; Sun
et al.
2009
). The bacteria utilize the NH
3
so evolved from ACC as a source of N and
thereby restrict the accumulation of C
2
H
4
within the plant, which otherwise inhibits
plant growth (Belimov et al.
2002
). Thus, the decreased levels of C
2
H
4
in turn allow
the plants to grow better (Zahir et al.
2008
). A model to explain how ACC
deaminase promotes plant growth is depicted in Fig.
2.4
. It has been observed
that plants inoculated with PGPR containing ACC deaminase were dramatically
more resistant to the deleterious effects of stress ethylene, synthesized under
stressful conditions such as flooding (Grichko and Glick
2001
), heavy metals
(Burd et al.
1998
; Grichko et al.
2000
), presence of phytopathogens (Wang
et al.
2000
), drought and high saline conditions (Mayak et al.
2004
). The net result
of the cleavage of exuded ACC by bacterial ACC deaminase is that the bacterium is
de facto acting as a sink for ACC. Additionally, plants growing in association with
ACC deaminase-containing plant growth-promoting bacteria generally have longer
roots and shoots and are more resistant to growth inhibition by a variety of ethylene-
inducing stresses. Furthermore, the reduction of ethylene levels in plant tissues
following ACC deaminase activity can cause significant morphological changes in
root tissue, such as changes in root hair length and increases in root mass, accom-
panied by the consequent improvement in nutrient uptake. The morphological
changes in plants are further increased when ACC deaminase action is coupled
with the production of auxins by PGPR. The question arises, how bacterial ACC
deaminase selectively reduces the deleterious ethylene levels (the second ethylene
peak) without affecting the small first peak of ethylene that is thought to activate
plant defence responses. In this regard, ACC deaminase is generally present in
bacteria at a relatively low level until it is induced, and the induction of enzyme
activity is a rather slow and complex process. Immediately following an abiotic or
biotic stress, the pool of ACC in the plant is low as is the level of ACC deaminase in
the associated bacterium. Stress induces the induction of ACC oxidase in the plant
so that there is an increased flux through ACC oxidase resulting in the first (small)
peak of ethylene that in turn induces the transcription of protective/defensive genes
in the plant. At the same time, bacterial ACC deaminase is induced by the
