Early transcriptional response to gravistimulation in poplar without phototropic confounding factors

Background and Aims In response to gravistimulation under anisotropic light, tree stems showing an active cambium produce reaction wood that redirects the axis of the trees. Several studies have described transcriptomic or proteomic models of reaction wood relative to the opposite wood. However, the mechanisms leading to the formation of reaction wood are difficult to decipher because so many environmental factors can induce various signaling pathways leading to this developmental reprogramming. Using an innovative isotropic device where the phototropic response does not interfere with gravistimulation we characterized the early molecular responses occurring in the stem of poplar after gravistimulation in an isotropic environment, and without deformation of the stem. Methods After 30 minutes tilting at 35° under anisotropic light, we collected the upper and lower xylems from the inclined stems. Controls were collected from vertical stems. We used a microarray approach to identify differentially expressed transcripts. High throughput real-time PCR allowed a kinetic experiment at 0, 30, 120 and 180 minutes after tilting at 35°, with candidate genes. Key Results We identified 668 differentially expressed transcripts, from which we selected 153 candidates for additional fluidigm qPCR assessment. Five candidate co-expression gene clusters have been identified after the kinetic monitoring of the expression of candidate genes. Gene-ontology analyses indicate that molecular reprogramming of processes such as “wood cell expansion”, “cell wall reorganization” and “programmed cell death” occur as early as 30 minutes after gravistimulation. Of note is that the change in the expression of different genes involves a fine regulation of gibberellin and brassinosteroid pathways as well as flavonoid and phosphoinositide pathways. Conclusions Our experimental setup allowed the identification of genes regulated in early gravitropic response without the bias introduced by phototropic and stem bending responses.