Cell death of long-lived ray parenchyma cells during heartwood formation in trees

Abstract

Cell death plays an important role in the determination of secondary xylem cell functions. Tracheary elements (TEs), such as vessel elements and tracheids, lose their organelles due to rapid autolysis after the completion of secondary wall thickening and lignification, and play an important role in water movement along the stem. In contrast, xylem axial and ray parenchyma cells (xylem parenchyma cells) remain alive for several years or longer and retain their organelles even after maturation. As a result, xylem parenchyma cells play important roles in nutrient storage, axial and radial transportation of materials, and defense responses in the stem. In addition, they are involved in the formation of heartwood, which contributes to increases in the resistance of the tree trunk to decay, as they synthesize heartwood components such as polyphenols prior to their death. The present review focuses on changes in long-lived ray parenchyma cells during heartwood formation, such as morphology and contents of organelles, gene expression, and survival rate in sapwood. This review also summarizes the differences in cell death characteristics between TEs and ray parenchyma cells. The elucidation of the cell death mechanism of ray parenchyma cells is expected to provide useful information for controlling the properties of heartwood.