Allelochemicals Biologica... - Name

ALLELOPATHIC BACTERIA IN WEED MANAGEMENT 147
Successful competition of bacteria living in the rhizosphere depends on several
factors, including rapid growth on multiple substrates, antibiotic production, and
downward growth with the root. A major factor contributing to successful competition
of rhizobacteria over other microorganisms is the growth stimulation by exuded organic
compounds and sloughed-off root hair and epidermal cell materials (De Weger et al.,
1995). The ability to efficiently compete for these available resources and to produce
siderophores for obtaining iron is important in establishment, colonization, and
persistence of rhizobacteria in the rhizosphere.
A characteristic of many AB is the high specificity toward their weed host(s)
with no detrimental effects on growth of nonweedy plant species (Cherrington and
Elliott, 1987; Elliott and Lynch, 1985; Kennedy et al., 1991; 2001). Although effects
on plants are subtle (Kremer and Kennedy, 1996), AB may be as significant as
traditional bacterial pathogens in affecting plant growth (Schroth and Hancock, 1982;
Suslow and Schroth, 1982). Because AB attack the seed and/or seedling rather than
the growing plant, weed seed or vegetative propagule production is suppressed, a key
to any weed management program, which reduces the need for repeated postemergence
herbicide applications and increases the chances of success for control of a growing,
competitive weed (Aldrich and Kremer, 1997).
4. MODES OF ACTION OF ALLELOPATHIC BACTERIA
Many AB strains produce secondary metabolites that are inhibitory to plants, including
phytotoxins and antibiotics, which can be considered allelopathic. Phytotoxins from
fluorescent Pseudomonas spp., a diverse group of plant pathogenic bacteria abundant
in the soil and rhizosphere, have been well studied (Mitchell, 1991). There are fewer
reports on phytotoxins from AB and many have not been extensively studied.
A phytotoxin from Pseudomonas fluorescens strain D7 was shown to be
responsible for root growth inhibition of downy brome (Bromus tectorum) (Tranel et
al., 1993). Further characterization revealed that the active fraction was a complex of
chromopeptides, other peptides and fatty acid esters in a lipopolysaccharide matrix
(Gurusiddaiah et al., 1994). Secondary metabolites isolated from Pseudomonas
syringae strain 3366 inhibitory to downy brome consisted of phenazine-1-carboxylic
acid, 2-aminophenoxazone and 2-aminophenol (Gealy et al., 1996). Gealy et al.
(1996) showed that phenazine-type antibiotics of Pseudomonas fluorescens also
inhibited downy brome root growth. Electron microscopy of AB colonizing the
rhizoplane and endorhizal cells of leafy spurge (Euphorbia esula) revealed disruption
of plant cell walls and membranes apparently due to production of phytotoxins and/or
enzymes by the bacteria, which consequently inhibited seedling growth (Souissi et
al., 1997). AB may also produce “phytotoxic antibiotics” that affect plant growth
such as the broad-spectrum antibiotic, 2,4-diacetylphloroglucinol, released by P.
fluorescens strain CHA0, which suppressed soilborne fungal plant pathogens but was
also highly phytotoxic to seedlings of several plant species (Keel et al., 1992).
Plant-inhibitory effects of some AB are auxin-mediated, illustrated by direct uptake
of bacterially produced indoleacetic acid (IAA). Plant response to microbially