Early transcriptional response to gravistimulation in poplar without phototropic confounding factors.

Jérôme Franchel,Patricia Roeckel-Drevet,Joël R Drevet,Philippe Label,Jean-Stéphane Venisse,David Lopez,Catherine Coutand

doi:10.1093/aobpla/plaa071

Abstract

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 signalling 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. After 30 min 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 min after tilting at 35°, with candidate genes. 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 min 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. Our experimental set-up allowed the identification of genes regulated in early gravitropic response without the bias introduced by phototropic and stem bending responses.

Highlights

The perception of Earth’s gravity leads to a gravity-driven growth process called gravitropism
This study aimed to better understand the molecular processes that occur in the secondary xylem of the woody stem after short-term tilting while separating the phototropic effect from gravitrophic stimulation
We used an innovative experimental design (Coutand et al 2019) to identify a list of differentially expressed genes responding to gravitropism without interference with the phototropic response

Summary

Introduction

The perception of Earth’s gravity leads to a gravity-driven growth process called gravitropism. Several plant species were used to study the response to gravistimulation of aerial parts including maize coleoptiles, pulvini barley, Pisum hypocotyls, Arabidopsis inflorescence stems and hypocotyls (for a review, see Hashiguchi et al 2013). In these herbaceous species, vertical root or shoot movements are caused by differential elongation between the upper and lower parts of the gravistimulated organs. The ‘reaction wood’ is called ‘tension wood’ (TW) and forms on the upper face of the inclined stem when the tree begins the reorientation process (for a review, see Du and Yamamamoto 2007). TW is characterized by the presence of gelatinous fibres (G-layer) of almost pure cellulose

Objectives

Methods

Results

Conclusion