Cellular machinery of wood production: differentiation of secondary xylem in Pinus contorta var. latifolia

The objectives of this study were to define cell structure during pine secondary xylem development and to integrate this information with current knowledge of the biochemistry and physiology of secondary cell wall biosynthesis in gymnosperms. Lodgepole pine (Pinus contorta var. latifolia Englem.) cambium and secondary xylem were cryofixed using high pressure freezing and freeze-substitution which allowed excellent preservation of the cell structure of developing secondary xylem and enabled high-resolution transmission electron microscopic viewing of these cells for the first time. In contrast to their precursors in the adjacent cambial zone, developing tracheids were active in secondary wall deposition, with abundant cortical microtubules and developing bordered pits. These cells were also characterized by unusual Golgi structures: the trans-Golgi network was highly developed and the associated vesicles were large and darkly stained. These unusual Golgi structures persisted throughout the period of xylem maturation until programmed cell death occurred. Immuno-cytochemistry and enzyme-gold probes were used to investigate the distribution of key secretory products (mannans) and a lignification-associated enzyme (coniferin β-glucosidase) during xylogenesis. Mannans were localized to the secondary cell wall, the trans-Golgi cisternae and trans-Golgi network vesicles of developing xylem. Coniferin β-glucosidase was found only in the secondary cell wall. The cell wall localization of coniferin β-glucosidase, the enzyme responsible for cleaving glucose from coniferin to generate free coniferyl alcohol, provides a mechanism to de-glucosylate monolignols in muro. A two-step model of lignification of conifer tracheids is proposed. First, Golgi-mediated secretion deposits monolignols into the cell wall, where they polymerize in cell corners and middle lamella. Secondly, cell lysis releases stored, vacuolar monolignol glucosides into the wall where they are deglucosylated and their polymerization is influenced by the wall environment including the lignin deposited earlier.