Structural, defense enzyme activity and chemical composition changes in the xylem of Aquilaria sinensis during fungus induction

Abstract

Agarwood is a dark resinous wood, of high quality and in a short period, can be formed after inoculation with the efficient fungus in Aquilaria sinensis. However, the action site and mechanism of the fungus inducing the agarwood formation remain unclear. In this work, the efficient fungal infection was used to induce agarwood formation on A. sinensis, and samples were collected according to inoculation time. Subsequently, the xylem structure, histochemistry, defense enzyme activities, and the accumulation and in-situ distribution of chemical composition were investigated. The transition layer (TL) was the critical layer for the continuous accumulation of agarwood, and began to appear in the injection of fungal inducers for 15 days (15D). The parenchyma cells of the xylem rays and interxylary phloem were the main action site of fungus in the formation and accumulation of agarwood. The sesquiterpenes and 2-(2-phenylethyl) chromones (PECs) were mainly accumulated in the agarwood layer (AL) and decay layer (DL), while their relative contents were highest in the injection of fungal inducers for 30 days and 180 days (30D and 180D), respectively. Among them, PECs appeared earlier than sesquiterpenes, and both were flindersia-type PECs. They were mainly concentrated in the cell corners and intercellular layer of the cell wall. In addition, Redundancy analysis (RDA) and Mantel test revealed that the different layer percentage (DLP), phenylalanine ammonia lyase (PAL), and starch grains (SG) were the drivers of PECs, and catalase (CAT) was the driver of sesquiterpenes. Therefore, this study will provide a theoretical foundation for revealing the mechanism of fungus-induced agarwood formation and inducing high-quality and high-yielding agarwood in the future.