Agriculture Reference
In-Depth Information
Fig. 2.4
A schematic model of how plant growth-promoting bacteria that both produce ACC
deaminase and synthesize IAA may facilitate plant growth. The only enzyme shown in this scheme
is ACC deaminase. SAM is converted to ACC by the enzyme ACC synthase; ACC is converted to
ethylene by ACC oxidase. IAA biosynthesis, both in bacteria and in plants, is a complex multi-
enzyme/protein process as is IAA signal transduction.
ACC
1-aminocyclopropane-1-carboxylate,
IAA
indole-3-acetic acid,
SAM
S-adenosyl methionine [Adapted from Glick (
2014
)]
increasing amounts of ACC that ensue from the induction of ACC synthase in the
plant so that the magnitude of the second, deleterious, ethylene peak is decreased
significantly (typically by 50-90 %). Because ACC oxidase has a greater affinity for
ACC than does ACC deaminase, when ACC deaminase-producing bacteria are
present, plant ethylene levels are dependent upon the ratio of ACC oxidase to ACC
deaminase. That is, to effectively reduce plant ethylene levels, ACC deaminase
must function before any significant amount of ACC oxidase is induced. Thus, in
the absence of some other mechanism, IAA-producing bacteria might all be
expected to ultimately be inhibitory to plant growth. However, this is in fact not
the case because as plant ethylene levels increase, the ethylene that is produced
through feedback mechanism inhibits IAA signal transduction thereby limiting the
extent that IAA can activate ACC synthase transcription (Pierik et al.
2006
;
Prayitno et al.
2006
; Czarny et al.
2007
; Stearns et al.
2012
). In plants inoculated
with PGPR that secrete both IAA and ACC deaminase, the level of ethylene does
not increase compared to the plants inoculated only with IAA-secreting bacteria. In
the presence of ACC deaminase, there is much less ethylene and subsequent
ethylene feedback inhibition of IAA signals transduction so that the bacterial IAA
can continue to promote both plant growth and increase ACC synthase
