Plants are covered by a protective layer made up of cutin polymers embedded in waxes [1,3,15-17]. This layer is relatively impervious to water penetration or loss, gas exchange, and penetration by particulates. Various structures in the plant surface enable the gas exchange required for vital metabolic and photosynthetic processes occurring in the underlying cells. Therefore, to internalize, a microorganism must either directly penetrate the surface layer or enter through a surface opening (aperture) or wound.
The surface coating of plants and structures beneath it may be categorized as either symplast or apoplast [16,17]. The symplast or living matter includes the cytoplasm of cells, whereas the apoplast includes the surface layer, cell walls, air spaces between cells and in the cell wall matrix, and the primary water-conducting tissues (xylem) . Sieve tube elements, which are primarily devoted to movement of the products of photosynthesis and other cellular processes, accompany the xylem vessels . However, the sieve tube elements are filled with a cytoplasm-like material. As such, whether they should be included in the apoplast is unclear. In certain parts of the plant, sieve tubes transport water, whereas in other parts, xylem vessels carry sugars, etc.
The xylem, composed of specialized vessels, tracheids, and associated parenchyma, connects the water-absorbing tissues in the root system with the rest of the plant . Vessels and tracheids are filled with water containing dissolved minerals and occasionally organic solutes. The general structure of these water-conducting elements tends to exclude microorganisms such that only a few specialized types are able to enter and move through the system. Individual vessel cells connect through perforation plates that would appear to allow passage of suspended particulates such as bacteria . However, Pao et al.  observed multiple, helical perforations in the walls and ends of the vessels in the stem scar of orange fruits that blocked movement of bacteria. Whether these were xylem vessels or tracheid cells, which do not possess perforation plates , is unclear. Both types of water-conducting cells attach to adjacent cells through pits in their secondary walls. The pits are paired with those in walls of an adjacent parenchyma or vessel cell. The base of each pit pair contains a membrane composed of the initial primary cell wall of the adjacent cells and the middle lamella. Pit membranes contain pores that are slightly larger than plasmodesmata. At a reported 0.3 ^ in diameter , such pores would not allow passage of bacteria. However, the pits are freely permeable to water and solutes. Microbes that can enzymatically digest the pits, such as the wilt pathogens, inhabit xylem vessels . Moreover, microbes that are able to weaken pit membranes that interface with adjacent parenchyma could egress from the vessel as well as obtain nutrition from the parenchyma cells.
A large portion of the apoplast of most plants consists of interconnected intercellular air spaces, which are linked with openings in the plant surface . Less than 1% of the volume of potato tubers is devoted to intercellular spaces, whereas up to 66% of certain leaves is air space .
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