Exploring the Seasonal Dynamics and Molecular Mechanism of Wood Formation in Gymnosperm Trees

Abstract

Forests, comprising 31% of the Earth's surface, play pivotal roles in regulating the carbon, water, and energy cycles. Despite being far less diverse than angiosperms, gymnosperms account for over 50% of the global woody biomass production. To sustain growth and development, gymnosperms have evolved the capacity to sense and respond to cyclical environmental signals, such as changes in photoperiod and seasonal temperature, which initiate growth (spring and summer) and dormancy (fall and winter). Cambium, the lateral meristem responsible for wood formation, is reactivated through a complex interplay among hormonal, genetic, and epigenetic factors. Temperature signals perceived in early spring induce the synthesis of several phytohormones, including auxins, cytokinins, and gibberellins, which in turn reactivate cambium cells. Additionally, microRNA-mediated genetic and epigenetic pathways modulate cambial function. As a result, the cambium becomes active during the summer, resulting in active secondary xylem (i.e., wood) production, and starts to become inactive in autumn. This review summarizes and discusses recent findings regarding the climatic, hormonal, genetic, and epigenetic regulation of wood formation in gymnosperm trees (i.e., conifers) in response to seasonal changes.

Keywords: conifer; environment; epigenetic; genetic; gymnosperm; season; wood formation.

Figure 1

The seasonal expression of genes and miRNAs involved in wood formation of gymnosperm trees. The relative expression of each gene or miRNA is shown as a color bar according to reactive, active, and dormancy: red indicates upregulation and purple indicates downregulation (bottom left). * Monolignol biosynthesis genes are CAD, COMT, CCoAOMT, CCR, C3H, C4H, 4CL, CSE, PAL, and HCT. Descriptions of each gene and miRNA are shown in Table 1. References are to the right of each color bar.

Figure 2

Summary of seasonal dynamics of wood formation in gymnosperm trees. Changes of cambium activity (i.e., quiescence, reactivation, activation, and rest) and xylem formation (i.e., cell expansion, SCW formation and PCD) with earlywood and latewood under the seasonal signaling were summarized. Seasonal changes of hormone concentrations (e.g., Auxins, cytokinins, ABA, and GAs) were shown with temperature fluctuation above. Representative genes and miRNAs responsible for each process were shown. Full gene names and references are in the main text.