Quantitative Proteomic and Phosphoproteomic Approaches for Deciphering the Signaling Pathway for Tension Wood Formation in Poplar

Trees
adjust their growth following forced changes in orientation
to re-establish a vertical position. In angiosperms, this adjustment
involves the differential regulation of vascular cambial activity
between the lower (opposite wood) and upper (tension wood) sides of
the leaning stem. We investigated the molecular mechanisms leading
to the formation of differential wood types through a quantitative
proteomic and phosphoproteomic analysis on poplar subjected to a gravitropic
stimulus. We identified and quantified 675 phosphopeptides, corresponding
to 468 phosphoproteins, and 3 763 nonphosphorylated peptides,
corresponding to 1 155 proteins, in the differentiating xylem
of straight-growing trees (control) and trees subjected to a gravitational
stimulus during 8 weeks. About 1% of the peptides were specific to
a wood type (straight, opposite, or tension wood). Proteins quantified
in more than one type of wood were more numerous: a mixed linear model
showed 389 phosphopeptides and 556 proteins to differ in abundance
between tension wood and opposite wood. Twenty-one percent of the
phosphoproteins identified here were described in their phosphorylated
form for the first time. Our analyses revealed remarkable developmental
molecular plasticity, with wood type-specific phosphorylation events,
and highlighted the involvement of different proteins in the biosynthesis
of cell wall components during the formation of the three types of
wood.