Plant Bacteria Function

1. Bacteria
More than 200 different plant bacteria cause plant disease
in Australia. However, there’s also a host of bacteria that
are beneficial to the plants we grow. Some bacteria have
been harnessed for commercial products, to control plant
pests and diseases, and some live in mutually beneficial,
symbiotic relationship with plants.
Agrobacterium sp
Scientists had been battling the ‘Baddie’ Agrobacterium that causes Crown Gall Disease for decades.
It causes cancer-like growths on lower stems and roots. It was a particularly devastating disease of
the fruit industry worldwide.
In 1969, a student at the Waite Agricultural Research Institute in Adelaide isolated a strain of the
Agrobacterium that infected plants but didn’t cause the cancer-like growth. In fact, this benign
strain somehow protected young fruit trees against the virulent form of the same bacterium.
Professor Allen Kerr and his team dubbed the strain K84 and later discovered why it worked. K84
produces an antibiotic that is deadly to the pathogenic strain but it is unaffected itself because it
possesses a resistance gene that makes it immune to its own antibiotic (and this antibiotic was also
found to be harmless to most other bacteria).
Unfortunately, the close relationship also meant that eventually there was gene swapping and the
pathogenic strain also developed resistance. Undeterred, the team then worked with overseas
scientists, and identified the region of DNA that allowed the transfer to other bacteria. They deleted
this region. The bacterium retained its ability to synthesise the antibiotic, but could not pass the
resistance gene to other bacteria.
This genetically modified bacterium was then called K1026. In 1988, the NSW Department of
Agriculture approved K1026 for commercial use as a pesticide. It was subsequently given the name
No Gall and this product is now used all over Australia. It’s not been approved for release overseas,
as K84 is still an effective control.
In 1990 Professor Allen Kerr, Professor Eugene Nestor (USA), and Professor Jeff Schell (Germany)
were awarded the Prime Minister’s Australia Prize for Science for this work.

2. Bacillus thuringiensis (Bt)
Various forms of this bacterium have been harnessed to control some species of leaf-eating
caterpillars (like the Cabbage White caterpillar pictured right), mosquitoes and varieties of beetles.
It can be found in products called Dipel, Xen Tari, Cybate, Vectobac, and Novodor.
Bt is a naturally occurring soil bacterium and several strains are known to kill insects. Bt‘s ability
was discovered in 1911, but it wasn’t commercially available until the 1950s.
Unlike typical nerve-poison insecticides, Bt acts by producing proteins that react with the gut lining.
The proteins paralyse the digestive system and the insect stops feeding. The Bt-affected insects die
from starvation, which can take several days.
Rhizobia
Nitrogren-fixing bacteria. Legumes (members of the Fabaceae family, which include peas and beans)
have a symbiotic relationship with this bacterium. The bacteria are seen as nodules on the roots of
legume plants, as can be seen above, and should not be confused with nematode damage (see our
information sheet on Nematodes).
Rhizobia provides the plant with additional nitrogen that would otherwise be unavailable to it by
converting nitrogen from the atmosphere into ammonia that plants can use.
Legume crops therefore require less nitrogen than other crops because of this relationship.

3. Acacias have the same relationship with bacteria, and because they are one of the first plants to
germinate and grow after fire, they help the plants that follow. Acacias provide nearby plants with
nitrogen all year round.
When using pea straw as mulch, often pea plants will germinate. This is to be encouraged. Chop the
plants down before they flower, and you also have additional nutritious mulch. Win, win!
Biodegradation – various species
Various bacteria species have been found to break down pesticides, pollutants (such as
hydrocarbons in oil spills, as seen above), sulphur and nutrient residues into non-toxic compounds.
They are being used in bioremediation and as natural biodegradation agents.
Other
Plant growth promoting bacteria (PGPR) are various species of bacteria that exist in the root zones
and enhance plant growth.
Ice nucleation active (INA) bacteria, such as Erwinia herbicola, are epiphytes, that is, they are
grown upon or attached to plants. These bacteria actually promote frost damage to sensitive plants
by initiating the formation of ice crystals.
Information sources:
Colorado State University Extension – Horticulture: www.ext.colostate.edu
Handreck, K., 2004, Gardening Down-Under, LandLinks.
Kerruish, R.M. & Unger, P.W., 2003, 3rd Edition, Plant Protection 1, published by RootRot Press
ACT.
Prime Minister’s Science Awards