What are Tyloses?

Sourav PanOctober 9, 2024

Answered

Tylosis is a significant physiological process observed in woody plants, characterized by the formation of bladder-like outgrowths in parenchyma cells. This condition occurs within the xylem vessels of trees, particularly in response to stressors such as drought or infection. Tylosis plays a crucial role in plant health by helping to protect against decay and facilitate the transition from sapwood to heartwood. Below is an organized overview of its anatomical features, functions, and roles in compartmentalization and heartwood formation.

• Anatomy of Tylosis:
  • Tyloses are defined as balloon-like protrusions that extend from axial paratracheal parenchyma cells into adjacent xylem vessels through pits that connect the two.
  • Under microscopic examination, these protrusions can vary in distinctiveness; some exhibit clear barriers, while in other cases, they may be less distinguishable.
  • The formation of tyloses can occur in response to environmental stress, effectively sealing the vascular tissue and preventing further damage to the plant.
• Mechanism of Formation:
  • When woody plants experience stress, such as drought conditions or fungal infections, tyloses detach from the sides of parenchyma cells and occlude the xylem vessels.
  • This blockage serves to “dam” the vascular tissues, limiting water flow and pathogen spread, thereby contributing to the plant’s defense mechanisms.
• Role in Compartmentalization of Decay:
  • Tylosis acts as a barrier against the axial spread of fungal hyphae and other pathogens, effectively slowing their vertical movement through the plant’s vascular system.
  • In gymnosperms, a similar process occurs, where tracheids are sealed off to block pathogen access.
  • The effectiveness of this protective barrier is influenced by the speed at which tyloses form, with healthier plants typically establishing these defenses more rapidly.
• Defense Against Pathogens:
  • The blocked vessels not only hinder vertical pathogen spread but also limit horizontal movement within the plant stem.
  • Protection is notably enhanced at the interfaces of annual rings, where the tyloses provide a robust defense against decay.
• Role in Heartwood Formation:
  • As trees mature, the cambium layer produces new wood annually, resulting in older wood becoming less critical for the plant’s physiological functions. This wood transitions into heartwood.
  • While heartwood does not possess active defenses against infection, it is safeguarded by tyloses and other substances, including gums and waxes rich in volatile organic compounds, which deter insects and pathogens.
  • These protective substances are produced by the cambium and transported to the center of the stem via medullary rays, ultimately entering living parenchyma cells.
  • As the wood ages, the contents of the parenchyma cells are released into the empty xylem vessels, facilitating the formation of effective decay inhibitors, notably tannins, which are absorbed by adjacent vessel walls.