Agriculture Reference
In-Depth Information
are therefore more prone to invasion by both plant and human pathogens. Some of
the best examples come from the fi eld of plant mycology with stressed corn, wheat,
and other crops being most susceptible to infections from a wide range of mycotoxi-
genic molds including
Aspergillus
,
Fusarium
,
Penicillium
, and
Claviceps
(Papendic
and others 1971; Payne and others 1982). When Wilson and others (1999) assessed
the population dynamics of plant pathogenic and nonpathogenic strains of
Pseudomonas
syringae
as well as the nonpathogenic species
Pantoea agglomerans
,
Stenotrophomonas
maltophilia
, and
Methylobacterium organophilum
in the bean phyllosphere, the plant
pathogens survived better than the nonpathogens on leaves as well as inside these
plants during environmentally stressful conditions. Once invaded, various enzymes
such as cellulase, amylase, and pectinase commonly produced by plant pathogens can
degrade the cell wall and other tissues of plants. Syringomycin, one such enzyme
produced by
Pseudomonas syringae
, forms pores in the plant cell membrane leading
to the release of cell contents (Alfano and Collmer 1996). During the degradation
process, new carbon sources are released that can foster bacterial growth (Aruscavage
and others 2006 ).
Portals of Entry
Bacteria can become internalized throughout the farm-to-fork continuum (Fig. 3.1).
The most common portals of entry include the stomata, lenticels, lateral roots, germi-
nating radicles, trichomes, wounds, areas of decay, and stem scars (Aruscavage and
others 2006; Hallmann and others 1997b; Sturz and others 1999, 2000), with similar
invasive routes shown for human pathogens (Kenny and others 2001). Stomata, len-
ticels, and stem scars are among the most important portals of entry for both plant
endophytic bacteria (Hallman and others 1997b; Lamb and others 1996) and food-
borne pathogens, including
Salmonella
(Aruscavage and others 2006) and
E. coli
O157:H7 (Itoh and others 1998; Seo and Frank 1999; Takeuchi and Frank 2000) with
the calyx, stem end, and fl oral tubes of fruits being most vulnerable to infi ltration
(Buchanan and others 1999; Burnett and others 2000; Kenney and others 2001).
Preharvest
Endophytic bacteria frequently penetrate plant tissue via root hair cells or junctions
between the root hairs and adjacent epidermal cells (Hallmann and others 1997b;
Sturz and others 2000) followed by transport through the vascular system (Lamb and
others 1996 ).
Enterobacter asburiae
(a plant endophytic bacterium) reportedly
entered and spread in cotton through active uptake (Quadt-Hallmann and others
1997). In this case, penetration was not dependent on any wounds. The authors sug-
gested that this organism may have degraded cellulose and then migrated through the
plant via intercellular spaces.
Wax cutin, which fi lls the lenticels and seals any cracks, minimizes bacterial attach-
ment to the phyllosphere and provides another natural barrier to bacterial penetration
(Aruscavage and others 2006; Burnett and others 2000; Kenney and others 2001).
However, wounds resulting from insect, fungal, or nematode infestations, cuts, bruises,
or other types of physical damage to the waxy cuticle can easily lead to bacterial
harborage sites along with infi ltration of the underlying tissue.
