Morphological, physiological, biochemical and molecular analyses reveal wounding-induced agarwood formation mechanism in two types of Aquilaria sinensis (Lour.) Spreng

Agarwood, a resinous portion of injured Aquilaria trees, has become scarce and precious because of its low yields in ordinary Aquilaria sinensis. A new type of A. sinensis, namely easier induced agarwood A. sinensis germplasm (EIAA), has recently been selected and cultivated. However, no information has been reported about its agarwood formation mechanism yet. Multiple approaches, including anatomical, physiological, biochemical and molecular profiling, were employed in this study to investigate wounding-induced agarwood formation in two types of A sinensis. After mechanical damage, the related physiological and biochemical parameters regulating defense response were increased significantly, while the related parameters regulating growth were decreased significantly in two types of A sinensis. The strength of defense response was decreased and the growth performance was recovered over time. Compared to ordinary A. sinensis, EIAA showed higher hormones levels of salicylic acid, jasmonic acid and ethylene, higher activities of antioxidant enzymes, more total phenols and terpenes, higher net photosynthetic rate and photochemical efficiency of PSII, more consumption of non-structural carbohydrates, higher agarwood yield and oil content. There were marked differences in the components of agarwood essential oils between two types of A. sinensis. In particularly for 2-(2-phenylethyl) chromone and 2-[2-(4′-methoxybenzene) ethyl] chromone, their richness in agarwood essential oil of EIAA was significantly higher in contrast with the ordinary A. sinensis agarwood. Differentially expressed genes further ensured the differences in defense responses to wounding stress and secondary metabolism patterns between two types of A. sinensis. These data collectively demonstrate that there is a tradeoff between growth and defense during wounding-induced agarwood formation and EIAA had stronger defense ability in physiological and biochemical levels compared with ordinary A. sinensis. This is the physiological mechanism by which EIAA has higher agarwood yield and essential oil content. Moreover, the differential expression of flavonoid biosynthesis-related genes explained the different chromone synthesis pathways between two types of A. sinensis. These findings provide new insights into wounding-induced agarwood formation mechanism in A. sinensis, especially for EIAA.