The cellulose--lignin balance affects the twisted growth of Yunnan pine trunk

It is a debate whether trunk twisting belongs to environmental alteration or genetic variation. Through a diallel cross experiment, we first determined that trunk twisting of Yunnan pines was controlled by recessive genes. Anatomical analysis identified that straight and twisty types differed in xylem and phloem. RNA-seq of materials enriched by laser microdissection revealed three genes involved in auxin signal transduction, photosynthesis, and sucrose metabolism, namely ARF , POR , and CBH. These genes were co-expressed at different growth stages of twisty types, and among them, ARF is crucial regulating trunk twisting formation. The enzyme activities involved in sucrose metabolism, carbon fixation, and glycolysis were significantly increased after exogenous auxin was added to twisty types. When auxin signal transduction inhibitor (auxinole) and transport inhibitor (TIBA) were added, the plant height and related pathways were more obviously reduced in straight types. ARF can not only downregulate POR to block chlorophyll synthesis but also allows abundant sucrose to synthesize cellulose. Nevertheless, due to downregulated CBH expression and abnormal cellulolysis, cellulose accumulates and the lignin content decreases, eventually making the trunk highly prone to twisted growth. This study suggests that ARF can be vital in trunk shape screening during the early growth of Yunnan pines.