Screenplay of flax phloem fiber behavior during gravitropic reaction.

Flax phloem fibers act as constitutively formed "muscles" that support the vertical position of the high but narrow stem. The specific mechanical properties of flax fibers and of similar fibers in other plant species are provided by the development of tertiary cell wall with tensed cellulose microfibrils. The work of phloem fibers becomes especially pronounced during the restoration of stem vertical position if it was disturbed. Gravistimulation of flax plants induces considerable modification of phloem fibers at the pulling stem side - the lumen diameter increases, while the cell wall thickness goes down. Here we show that the action of phloem fibers as motors of stem vertical position restoration is coupled to the cell wall remodelling as well as the increase of osmolytes (mainly potassium and malate) content, and accumulation of the γ-amino-butyric acid that may be involved in signalling events. The molecular players that take part in these processes are suggested.