plant surface microbiology.pdf

8 ACC Deaminase-Containing Plant Growth-Promoting Bacteria 137
promoting bacteria, Pseudomonas putida ATCC 17399 and Pseudomonas fluorescens
ATCC 17400, by conjugational transfer, the transconjugants acquired
the ability to grow on minimal media using ACC as the sole source of nitrogen,
and to promote the elongation of canola roots (Shah et al. 1998).
In 1998, Glick et al. proposed a model in which plant growth-promoting
bacteria can lower plant ethylene levels and in turn stimulate plant growth. In
this model, the plant growth-promoting bacteria bind to the surface of either
the seed or root of a developing plant; in response to tryptophan and other
small molecules in the seed or root exudates (Whipps 1990), the plant growthpromoting
bacteria synthesize and secrete indole acetic acid (IAA; Fallik et al.
1994; Patten and Glick 1996), some of which is taken up by the plant. This IAA
together with endogenous plant IAA, can stimulate plant cell proliferation,
plant cell elongation or induce the activity of ACC synthase to convert SAM to
ACC (Kende 1993).
Much of the ACC produced by this latter reaction is exuded from seeds or
plant roots along with other small molecules normally present in seed or root
exudates (Penrose and Glick 2001). The ACC in the exudates may be taken up
by the bacteria and subsequently hydrolyzed by the enzyme, ACC deaminase,
to ammonia and a-ketobutyrate. The uptake and cleavage of ACC by plant
growth-promoting bacteria decreases the amount of ACC outside the plant.
Increasing amounts of ACC are exuded by the plant in order to maintain the
equilibrium between internal and external ACC levels.As a result of the activity
of ACC deaminase, the presence of the bacteria induces the plant to synthesize
more ACC than it would otherwise need and as well, stimulates the
exudation of ACC from the plant.
Thus, plant growth-promoting bacteria are supplied with a unique source
of nitrogen in the form of ACC that enables them to proliferate under conditions
in which other soil bacteria may not flourish. As a result of lowering the
ACC level within the plant, either the endogenous level or the IAA-stimulated
level, the amount of ethylene in the plant is also reduced.
Plant growth-promoting bacteria that possess the enzyme ACC deaminase
and are bound to seeds or roots of seedlings, can reduce the amount of plant
ethylene and the extent of its inhibition on root elongation. Thus, these plants
should have longer roots and possibly longer shoots as well, inasmuch as stem
elongation is also inhibited by ethylene, except in flooding-resistant plants
(Abeles et al. 1992).
3.1 Treatment of Plants with ACC Deaminase Containing Bacteria
Consistent with the above mentioned model, ACC deaminase activity was
completely lost and the ability to promote the elongation of canola roots
under gnotobiotic conditions was greatly diminished when the ACC deaminase
gene (acdS) from Enterobacter cloacae UW4 was replaced, by homolo-