Vegetable and Fruit Products
131
and they include Pseudomonas spp. as well as Bacillus, Paenibacillus, and Clostridium. For fruits and
vegetables like tomatoes and potatoes, P. carotovorum subsp. carotovorum causes soft rot by entering
surface wounds. For root vegetables such as carrots, the roots are protected for some. Plant roots are
protected from invading microorganisms by their possession of hydrogen peroxide and superoxide,
and invading microorganisms produce catalase and superoxide dismutase to overcome this defense.
The Pseudomonas syringae group as well as erwiniae produce these enzymes.
The cementing substance of the vegetable body induces the formation of pectinases, which act
by hydrolyzing pectin, thereby producing the mushy consistency. In potatoes, tissue maceration has
been shown to be caused by an endopolygalacturonate transeliminase of Pectobacterium origin.
28 P.
chrysanthemi produces two pectin methyesterases, at least seven pectate lyases, a polygalacturonase,
and a pectin lyase.
19 Once pectin is destroyed, oligogalacturonidases are formed and utilized by the
varied microbiota. Because of the early and relatively rapid growth of bacteria, molds, which tend to
be crowded out, are of less consequence in the spoilage of vegetables that are susceptible to bacterial
agents.
Once the outer plant barrier has been destroyed by these pectinase producers, nonpectinase producers
no doubt enter the plant tissues and help bring about fermentation of the simple carbohydrates that
are present. The quantities of simple nitrogenous compounds present, the vitamins (especially the
B-complex group), and minerals are adequate to sustain the growth of the invading organisms until the
vegetables have been essentially consumed or destroyed. The malodors that are produced are the direct
result of volatile compounds (such as NH 3 , volatile acids, and the like) produced by the biota. When
growing in acid media, microorganisms tend to decarboxylate amino acids, leaving amines that cause
an elevation of pH toward the neutral range and beyond. Complex carbohydrates such as cellulose are
generally the last to be degraded, and a varied biota consisting of molds and other soil organisms is
usually responsible, as cellulose degradation by Erwinia spp. is doubtful. Aromatic constituents and
porphyrins are probably not attacked until late in the spoilage process, and again by a varied biota
of soil types. The genus Brenneria causes diseases of trees such as bark cankers, necrosis on walnut
trees, oozing of sap from acorns, etc.
18
The genes of P. carotovorum subsp. carotovorum that are involved in potato tuber maceration have
been cloned. Plasmids containing cloned DNA mediated the production of endopectate lyases, exopectate lyase, endopolygalacturonase, and cellulases.
39 The Escherichia coli strains that contained cloned
plasmids showed that endopectate lyases with endopolygalacturonase or exopectate lyase caused maceration of potato tuber slices. These enzymes, along with phosphatidase C and phospholipase A, are
involved in soft rot by this organism. Carrots infected with Agrobacterium tumefaciens undergo senescence at a faster rate because of increased ethylene synthesis. In normal uninfected plants, ethylene
synthesis is regulated by auxins, but A. tumefaciens increases the synthesis of indoleacetic acid, which
results in increased levels of ethylene.
The citrus canker pathogen, Xanthomonas anonopodis pv. (pathovar) citri wreaked havoc on the
Florida citrus industry in the late 1990s. The canker consists of external scabs and cork-like lesions
that occur on oranges, limes, grapefruits, and other citrus fruits. The organism enters fruit trees through
leaf stomata. Once inside, they employ a type III secretion system (see Chapter 22) to induce increased
cell division that leads to the brownish cankers. The infected fruits produce more ethylene, which leads
to senescence, increased ripening, and premature falling of fruits from trees. This infection does not
destroy trees. The bacterium is spread from tree to tree by wind, rain, insects, and other nonspecific
means.
The genus Xanthomonas is undergoing reclassification, thus some of the species and pathovars
noted in Table 6–6 are likely to be changed. Most form yellow mucoid and smooth colonies and
131
and they include Pseudomonas spp. as well as Bacillus, Paenibacillus, and Clostridium. For fruits and
vegetables like tomatoes and potatoes, P. carotovorum subsp. carotovorum causes soft rot by entering
surface wounds. For root vegetables such as carrots, the roots are protected for some. Plant roots are
protected from invading microorganisms by their possession of hydrogen peroxide and superoxide,
and invading microorganisms produce catalase and superoxide dismutase to overcome this defense.
The Pseudomonas syringae group as well as erwiniae produce these enzymes.
The cementing substance of the vegetable body induces the formation of pectinases, which act
by hydrolyzing pectin, thereby producing the mushy consistency. In potatoes, tissue maceration has
been shown to be caused by an endopolygalacturonate transeliminase of Pectobacterium origin.
28 P.
chrysanthemi produces two pectin methyesterases, at least seven pectate lyases, a polygalacturonase,
and a pectin lyase.
19 Once pectin is destroyed, oligogalacturonidases are formed and utilized by the
varied microbiota. Because of the early and relatively rapid growth of bacteria, molds, which tend to
be crowded out, are of less consequence in the spoilage of vegetables that are susceptible to bacterial
agents.
Once the outer plant barrier has been destroyed by these pectinase producers, nonpectinase producers
no doubt enter the plant tissues and help bring about fermentation of the simple carbohydrates that
are present. The quantities of simple nitrogenous compounds present, the vitamins (especially the
B-complex group), and minerals are adequate to sustain the growth of the invading organisms until the
vegetables have been essentially consumed or destroyed. The malodors that are produced are the direct
result of volatile compounds (such as NH 3 , volatile acids, and the like) produced by the biota. When
growing in acid media, microorganisms tend to decarboxylate amino acids, leaving amines that cause
an elevation of pH toward the neutral range and beyond. Complex carbohydrates such as cellulose are
generally the last to be degraded, and a varied biota consisting of molds and other soil organisms is
usually responsible, as cellulose degradation by Erwinia spp. is doubtful. Aromatic constituents and
porphyrins are probably not attacked until late in the spoilage process, and again by a varied biota
of soil types. The genus Brenneria causes diseases of trees such as bark cankers, necrosis on walnut
trees, oozing of sap from acorns, etc.
18
The genes of P. carotovorum subsp. carotovorum that are involved in potato tuber maceration have
been cloned. Plasmids containing cloned DNA mediated the production of endopectate lyases, exopectate lyase, endopolygalacturonase, and cellulases.
39 The Escherichia coli strains that contained cloned
plasmids showed that endopectate lyases with endopolygalacturonase or exopectate lyase caused maceration of potato tuber slices. These enzymes, along with phosphatidase C and phospholipase A, are
involved in soft rot by this organism. Carrots infected with Agrobacterium tumefaciens undergo senescence at a faster rate because of increased ethylene synthesis. In normal uninfected plants, ethylene
synthesis is regulated by auxins, but A. tumefaciens increases the synthesis of indoleacetic acid, which
results in increased levels of ethylene.
The citrus canker pathogen, Xanthomonas anonopodis pv. (pathovar) citri wreaked havoc on the
Florida citrus industry in the late 1990s. The canker consists of external scabs and cork-like lesions
that occur on oranges, limes, grapefruits, and other citrus fruits. The organism enters fruit trees through
leaf stomata. Once inside, they employ a type III secretion system (see Chapter 22) to induce increased
cell division that leads to the brownish cankers. The infected fruits produce more ethylene, which leads
to senescence, increased ripening, and premature falling of fruits from trees. This infection does not
destroy trees. The bacterium is spread from tree to tree by wind, rain, insects, and other nonspecific
means.
The genus Xanthomonas is undergoing reclassification, thus some of the species and pathovars
noted in Table 6–6 are likely to be changed. Most form yellow mucoid and smooth colonies and