An extended model of heartwood secondary metabolism informed by functional genomics

Abstract

The development of heartwood (HW) and the associated accumulation of secondary metabolites, which are also known as 'specialized metabolites' or 'extractives', is an important feature of tree biology. Heartwood development can affect tree health with broader implications for forest health. Heartwood development also defines a variety of wood quality traits that are important in the forest industry such as durability and colour of wood products. In the bioproducts industry, HW provides a source of high-value small molecules such as fragrances and antimicrobials. The HW properties of decay resistance in living trees, durability and colour of wood products, and small molecule bioproducts are largely defined by secondary metabolites, the biosynthesis of which appears to be activated during the onset of HW formation. Traditionally, it is thought that HW formation involves a spike in the activity of secondary metabolism in parenchyma cells in a transition zone between sapwood and HW, followed by programmed cell-death. The resulting HW tissue is thought to consist entirely of dead cells. Here, we discuss a variation of existing models of HW formation, based on the recent discovery of HW-specific transcriptome signatures of terpenoid biosynthesis in sandalwood (Santalum album L.) that invokes the activity of living cells in HW.

Differential Gene Profiling of the Heartwood Formation Process in Taiwania cryptomerioides Hayata Xylem Tissues

“…Overall, the ray parenchyma cells lose their vitality within 1-2 annual rings in the TZ, which would appear between [8][9][10][11][12][13] year rings counted from the cambium, depending on individual trees [17]. Similar results were also reported for other species, and the occurrence of this TZ is species-dependent, with the year ring numbers from cambium being 7-8 in Larix kaempferi [18], [12][13][14][15][16][17][18][19][20][21][22][23] in Cryptomeria japonica [19], and 14-22 and 28-33 in Pinus rigida and Pinus densiflora [20]. The extractives often start to accumulate from inner SW, increase in TZ, and deposit to a high level in the HW region [17,20,21].…”

“…HW formation is described as a form of programmed cell death [15], and it has been suggested that the longevity of the ray parenchyma cells and the depletion of reserve materials are closely related to the HW formation process [3,16]. Previous research has shown that the ray parenchyma cells of Taiwania gradually lose the starch grains from the inner SW and lose their nuclei in TZ, whereas the colored substances deposit into the cell lumens in the TZ [17].…”

Long-term dynamics of BVOC production, storage and emission in boreal Scots pine

“…Similarly, Moore and Hanover (1987) found large differences in monoterpene concentrations between different tissues of blue spruce (Picea pungens Engelm). Moreover, the conifer resin composition differs between sapwood and heartwood (Back 2002, Celedon andBohlmann 2018).…”

“…The resin duct (also called resin canal) network extends over the whole tree from the roots, trunks, branches and twigs to needles and cones. It consists of radial and axial ducts, which connect bark, sapwood and heartwood (Celedon and Bohlmann 2018). Resin duct dimensions in pine trunks vary from 50 µm (radial) to 200 µm (longitudinal).…”

“…Resin ducts are surrounded by epithelial cells, which are living in sapwood and dead in heartwood (e.g. Celedon and Bohlmann 2018). Monoterpene synthesis for resin production occurs in the plastids of these epithelial cells (Back 2002, Lewinsohn et al 1991 and references therein).…”