Traumatic Resin Ducts and Polyphenolic Parenchyma Cells in Conifers

Abstract

Conifers integrate multiple constitutive and inducible defenses into a coordinated, multitiered defense strategy. Constitutive defenses, established before an attack, represent a fixed cost and function as an insurance against inevitable attacks. Inducible defenses, mobilized in response to an attack, represent a variable resistance that is turned on when it is needed. Polyphenolic parenchyma cells (PP cells) that are specialized for synthesis and storage of phenolic compounds are abundant in the phloem of all conifers. In addition to being a prominent constitutive defense component, PP cells are also involved in a range of inducible defense responses, including activation of existing PP cells, production of new PP cells, and wound periderm formation. Their abundance and varied defensive roles make the PP cells the single most important cell type in conifer defense. Another important defense are traumatic resin ducts which are induced in many conifers after various biotic or abiotic challenges. Traumatic resin ducts are primarily formed in the xylem where they appear in tangential rows, but inducible resin ducts are also formed in the phloem of some conifers. Activation of PP cells and formation of traumatic resin ducts take place through the octadecanoid pathway, involving jasmonate and ethylene signaling.