The internalization process whereby microorganisms enter the plant apoplast is either active or passive. During active internalization, microbes grow through the plant surface into intercellular spaces, which is consistent with the activity of various plant pathogens . During active internalization, plant pathogens penetrate directly through the cuticle or indirectly through stomata, lenticels, hydathodes, or wounds. Passive internalization implies that microbes are carried into the apoplast due to contact with an object causing injury or by a penetration of apertures by water, aerosol, or particulate that contains microbes. Plant viruses may internalize in plant tissues that are being fed upon by insects . Aerosols may enter open stomata during a mass flow of gases into leaves . Aqueous suspensions of microorganisms may infiltrate surface apertures or wounds either spontaneously  or because of pressure differentials between the apoplast and the external environment [30-32]. Suspensions also may diffuse or be drawn into plants through water channels, which are a direct liquid connection between a plant's intercellular spaces and its exterior environment .
Most surface apertures of plants are large enough to allow passage of bacteria and smaller particulates, whereas fungal spores would likely be excluded. The stem scar of tomato fruit may allow the passage of spores of the sour rot fungus, Geotrichum candidum , although the evidence was not conclusive. Lesions of Rhizopus stolonifer and Geotrichum candidum developed around and beneath the stem scar of tomato fruit that had been previously treated to cause an internalization of the spores of these fungi . Vigneault et al.  reported that tomatoes cooled with water containing spores of R. stolonifer usually decayed during subsequent storage. However, whether the spores in these examples internalized through the stem scar is unclear. In contrast, wounds involving tissues with large intercellular spaces appear likely to internalize fungal spores.
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