Faculty
Jo Handelsman
(608) 263-8783
Department of Plant Pathology
University of Wisconsin - Madison
1630 Linden Dr.
Madison, WI 53706
rev. January 15, 2004
Biocontrol by Bacillus cereus to study
multichannel communication networks
We have chosen biocontrol of plant disease by Bacillus cereus as
a means of modeling a specific microbial communication
systems. The portrait of
biocontrol that has emerged over the last decade indicates that successful
disease suppression depends on carefully calibrated signal exchange among
the many partners of the system. We
have determined that B. cereus strains are ubiquitous on plant
roots and in soil and that many of these strains enhance plant health
when inoculated onto seeds at planting.
The bacterium suppresses disease through diverse mechanisms, including
the release of small, diffusible molecules.
We discovered a new antibiotic, zwittermicin A,
that is central to disease suppression by preventing normal development
of plant pathogenic protists. Zwittermicin
also inhibits growth of certain bacteria in the microbial community of
the soil. The mechanism of inhibition remains unsolved,
although a mutant analysis in E. coli points to an effect on transcription,
although studies of transcription in vivo and in vitro fail to support
this mechanism. Our working hypothesis is that zwittermicin inhibits growth
of bacteria by inhibiting transcription initiation in a promoter-specific
fashion.
To define the chemical signals exchanged between B. cereus and
its prokaryotic and eukaryotic associates, we developed a promoter trap
system to identify genes that are regulated by biotic signals.
We found a gene encoding a putative lipoprotein that is induced
by a consortium of amino acids released from seeds during germination.
The gene affects the ability of the bacterium to grow competitively on
germinating seeds. The promoter trap system has also identified
a suite of genes regulated by bacteria found associated with B. cereus
in the rhizosphere and most of these genes appear to be novel. The current thrust of this work is to determine
the function of genes regulated by chemical signals issued by other members
of the microbial communities in which B. cereus must function.
Our basic
research on microbial communication moves to application in the enhancement
of crop health by B. cereus. One strain of B. cereus, UW85, consistently suppressed
disease and increased soybean yields in five consecutive growing seasons
in Wisconsin while another, AS4-12, suppressed alfalfa diseases as effectively
as commonly used synthetic chemicals.
However, despite is reliability and efficacy, B. cereus has
not yet been registered for agricultural use because it contains human
enterotoxins that are associated with food poisoning.
We have attempted to remove the genes for enterotoxin production
to make a safer bacterium for agricultural use.
In our first attempt to remove the gene for one of the three subunits
of the human toxin from the B. cereus
genome, we demonstrated functional redundancy of one component of
the toxin and we are now in the final stages of constructing a triple
mutant that is deficient in all three enterotoxins.
We intend to complete the genetics soon so that the mutant can
be tested for biocontrol on soybeans in the ’02 field season.
In addition to bringing to fruition our longstanding goal of providing
effective biologically-based methods for disease control for agriculture,
this work will also provide the basis for studying the role of the enterotoxins
in the ecology of B. cereus in the soil microbial community.