Tylosis (plural: Tyloses) is the physiological process and the resulting occlusion in the xylem of woody plants as response to injury or as protection from decay in heartwood. It is an essential process in the Compartmentalization Of Decay In Trees (CODIT) and other woody plants, forming walls one and two of CODIT.
As a tree grows, its cambium expands, leaving heartwood behind its sapwood. This heartwood is the result of protection from decay as the plant responds by blocking the xylem vessels with gums, resins and waxes. These contain high levels of volatile organic compounds called terpenes that are toxic to tree pathogens such as bacteria, fungi and insect larvae. The heartwood or injured portions of the plant therefore receive protection from decay as long as the levels of terpenes are maintained high enough to deter or control pathogens. These products are produced by the cambium and transported to the centre of the stem by cellular structures called medullary rays radiating from the center of the stem. These then enter axial paratracheal parenchyma cells which are still alive after the xylem vessel has died. As the wood ages, the contents of the parenchyma cell burst into the dead vessel through a pit connecting the two. The parenchyma cell then dies as its contents are disgorged into the empty space of the dry vessel.
Observed in section under a microscope, tyloses appear as balloon-like protrusions emanating from axial paratracheal parenchyma cells into xylem vessels. In some types, there may be a distinct barrier between the tyloses emanating from the pits into the vessels, while they may be barely distinguishable in other cases.
Role in compartmentalization
Tylosis of broadleaf plant vessels counteracts the axial spread of fungal hyphae and other pathogens by slowing down their vertical spread with a physical barrier. A similar process occurs in gymnosperms, which block access to tracheids by closing the pits that join them to each other.
The blocked vessels provide a defense against the radial spread of pathogens, limiting their horizontal spread through the plant stem. Protection is stronger at the boundaries where annual rings meet.
The effectiveness of both vertical and horizontal barriers is affected by the speed at which they are established by tylosis, being typically faster in healthier plants.
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