ABSTRACT
Microorganisms embedded in plant tissues may be defined as ‘‘internalized,’’ derived from ‘‘internal,’’ meaning located inside the plant surface. Functionally, internalized microbes cannot be washed off the plant, they are protected from environmental stresses, and they cannot be inactivated by contact biocides or other surface disinfectants. Inside the plant, most microorganisms are located in spaces between cells called intercellular spaces, whereas plant viruses and certain other pathogens are inside host cells. Microbes in the intercellular spaces are bathed in nearly saturated relative humidity with a gas composition that enables aerobic metabolic activities [1]. The main threat to the survival of internalized microorganisms appears to be mechanisms that protect the plant against microbial attack [1]. As such, the microbe must either evade, counteract, or not induce its host’s defenses. Plant pathogenic microorganisms, which by nature harm plant tissues, have developed ways to cope with host defense
reactions. In contrast, nonplant pathogens usually do not harm living tissues
and, as a consequence, appear unlikely to stimulate plant defenses. Moreover,
the absence of tissue damage reduces the likelihood that nonpathogens will be
exposed to preformed antimicrobial chemicals, which would be compartmen-
talized in the cytoplasm or specialized cells. Microorganisms that are resistant to washing, surface disinfectants, or
environmental stresses are not always internalized. Romantschuk et al. [2]
noted that washing leaves, with or without sonication, does not remove all
bacteria that live entirely on the plant surface, perhaps because portions of
this population may embed in surface biofilms or other attached aggregates
(see Chapter 2). Additionally, bacteria have been observed partially buried
in surface waxes [3] and in cracks in the cuticle [4]. Microorganisms embedded
in aggregates, biofilms, surface waxes or ruptures in the cuticle are somewhat
protected against environmental stresses [5] and surface treatments. However,
truly internalized organisms, which are located beneath layers of plant cells,
would have much greater protection. Proof that microorganisms exist inside healthy, unblemished fruits and
vegetables was provided by Samish et al. [6]. Using special surface sterilization
procedures, her group isolated Gram-negative, motile, and rod-shaped bacteria
frequently from tomatoes, cucumbers, English peas, and green beans sampled
from farm fields. Populations were found less frequently in melons and
bananas, whereas successful isolations were infrequent in grapes, citrus fruits,
olives, and peaches. Internal populations of microorganisms would likely be
highest in root tissues [7] and lowest in the acidic environment within certain
fruit tissues [6]. Internalized microorganisms are part of a complex microbial ecosystem
associated with plants [8,9]. Epiphytic microorganisms survive and multiply
on the plant surface, whereas endophytes colonize the interior of plants
without causing noticeable damage [9]. Those that grow on or in plants and
cause damage are plant pathogens [10]. Epiphytes, endophytes, and plant
pathogens may be considered resident microorganisms because they compose
the plant-associated microbial ecosystem. Individual species that fail to
establish a presence in this ecosystem despite one or more introductions are
called casual microorganisms [11]. Casual microbes are usually ill-suited to
survive on the plant surface. Once inside the plant, however, casuals can
survive for prolonged periods of time depending on their ability to adapt to an
environment that is high in humidity but low in available nutrition. However,
under certain conditions, internalized casuals multiply. For example, Dong
et al. [12] observed endophytic growth of Escherichia coli and Salmonella
enterica (strains of Cubana, Typhimurium, and Infantis serovars) in alfalfa
and barrel medic seedlings grown in test tubes. Young [13] noted that water
congestion of leaf tissues enabled a wide range of bacteria to multiply. King
and Bolin [14] reported that severe tissue water congestion caused plant cell
membranes to leak minerals and metabolites, which supported the growth
of saprophytes. Furthermore, the development of large populations of
of and
microorganisms on fresh-cut vegetables could produce nonspecific spoilage, likely because plant defense mechanisms were compromised by anoxia.
