Interxylary phloem and xylem rays are the structural foundation of agarwood resin formation in the stems of Aquilaria sinensis

Abstract

The formation of resin in the wood of A. sinensis was observed, and the results showed that the interxylary phloem, together with xylem rays, provided a structural foundation for the formation of agarwood resin. Agarwood is a costly resinous wood harvested from wounded Aquilaria trees and has been widely used in medicine, incense, and perfumery. A defensive response of Aquilaria to various wounds has been shown to be the key reason for agarwood formation; however, our understanding of the anatomical basis of agarwood formation is still fragmentary. In this study, we examined the structural characteristics of A. sinensis wood and its relationship with agarwood formation. The results showed that interxylary phloem together with xylem rays is the main tissue that contains living parenchyma cells in the wood of healthy A. sinensis and that the main reserve substance in these parenchyma cells is in the form of starch grains. After Agar-Wit treatment, these starch grains undergo a series of changes and are eventually converted into agarwood resin; the non-starch polysaccharides and phenols are some of the intermediate products in the process of agarwood resin formation. The resin initially forms and mainly accumulates in the parenchyma cells of the interxylary phloem and xylem rays. The results indicated that the interxylary phloem and xylem rays in the wood are not only the primary location of agarwood resin formation but also the main accumulation site. The stored starch grains might be the material underlying the formation of the resin.