plant surface microbiology.pdf

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Bernard R. Glick and Donna M. Penrose
gous recombination, with a version of the same gene that contained a tetracycline
resistance gene inserted within the coding region (Li et al. 2000). Results
of an earlier study showed that ACC deaminase mutants of Pseudomonas
putida GR12–2 did not promote the elongation of canola roots (Glick et al.
1994). However, in those experiments, the mutants were created by chemical
mutagenesis, and as a result, one could never be certain that the mutations
were within the ACC deaminase structural gene per se. In the experiments by
Li et al. (2000),ACC deaminase function was specifically eliminated by replacing
the functional gene with an inactive version in order to demonstrate that
there is no ambiguity as to the nature of the ACC deaminase minus mutants.
It has been observed that both Escherichia coli and two different nonplant
growth-promoting pseudomonads acquired the ability to significantly promote
root elongation after they were transformed with a broad-host-range
plasmid carrying the Enterobacter cloacae UW4 ACC deaminase gene and its
upstream transcriptional regulatory region (Shah et al. 1998). Moreover, elongation
of canola roots following treatment of seeds with an ACC deaminasecontaining
bacterium is invariably accompanied by a decrease in the level of
ACC found inside the root (Penrose et al. 2001). These observations confirm
the effectiveness of ACC deaminase in lowering ACC levels.
As mentioned earlier, many plants respond to biotic and abiotic stresses by
synthesizing ethylene.Among these stresses is the presence of heavy metals in
the environment. It has been reasoned that at least some of the inhibitory
effect of heavy metals on plant growth is the consequence of the plant synthesizing
excessive amounts of stress ethylene in response to the presence of the
metal, especially during early seedling development. Prior to being planted in
metal-contaminated soil, canola and tomato seeds were treated with a heavy
metal-resistant bacterium that also contained ACC deaminase. Seeds inoculated
with the bacterium, Kluyvera ascorbata, and then grown in the presence
of high concentrations of nickel chloride were partially protected against
nickel toxicity (Burd et al. 1998). The presence of this bacterium had no measurable
influence on the amount of nickel accumulated per mg dry weight in
either roots or shoots of canola plants. Therefore, the bacterial plant growthpromoting
effect in the presence of nickel was not attributable to a reduction
of nickel uptake by seedlings. Rather, it reflects the ability of the bacterium to
lower the level of stress ethylene caused by the nickel.
Transgenic canola plants that express Enterobacter cloacae UW4 ACC
deaminase were tested for the ability to proliferate and accumulate metal in
the presence of high levels of arsenate in the soil. In both the presence and
absence of the plant growth-promoting bacterium, Enterobacter cloacae
CAL2, the transgenic plants grew significantly larger than nontransformed
plants (Nie et al. 2002).
Flooding is a common biotic stress that affects many plants, often several
times during the same growing season. Plant roots suffer a lack of oxygen as
a consequence of flooding, and this in turn causes deleterious effects such as