Ethylene signaling induces gelatinous layers with typical features of tension wood in hybrid aspen

The phytohormone ethylene has the potential to regulate secondary stem growth in plants. Application of exogenous ethylene, or of its in planta precursor 1-aminocyclopropane-1-carboxylic acid (ACC), to wild type and ethylene insensitive trees, has previously been shown to stimulate typical attributes of tension wood such as xylem growth and vessel frequency in stems of hybrid aspen (Populus tremula x tremuloides) in an ethylene-signaling dependent way. The present study demonstrates that ACC and ethylene also induced typical gelatinous-layers (G-layers) and altered the cellulose microfibril angle in secondary cell wall layers in hybrid aspen fibers through ethylene-signaling. G-layers are tertiary wall layers rich in cellulose, typically found in tension wood of aspen trees. This observation further strengthens the concept that the increase in ethylene biosynthesis observed in association with tension wood formation is a key signaling component that triggers tension wood formation. RNA-Seq data in combination with motif-analysis in promoters of regulated genes downstream of ethylene-signaling highlights potential molecular connections between ethylene signaling and tension wood formation.

植物激素乙烯（ethylene）具备调控植物茎次生生长的潜力。既往研究显示，向野生型及乙烯不敏感型树木施加外源乙烯或其植物体内前体1-氨基环丙烷-1-羧酸（ACC），可通过依赖乙烯信号通路的方式，诱导杂交杨（美洲山杨×欧洲山杨，*Populus tremula* × *tremuloides*）茎部产生应力木的典型特征，包括木质部增生与导管频率提升。本研究证实，ACC与乙烯还可通过乙烯信号通路，诱导杂交杨纤维的次生细胞壁层形成典型的胶质层（G-layers），并改变其中的纤维素微纤丝角。此类胶质层为富含纤维素的第三层细胞壁，通常存在于杨树的应力木中。这一观察结果进一步佐证了“与应力木形成相伴的乙烯生物合成上调，是触发应力木形成的关键信号组分”这一学术论断。结合RNA测序（RNA-Seq）数据与对乙烯信号通路下游受调控基因启动子的基序分析，本研究揭示了乙烯信号通路与应力木形成之间的潜在分子关联。