A flax mutant with impaired intrusive growth of phloem and xylem fibres demonstrates constitutive gravitropic response.

Intrusive growth is a type of growth in which a cell exceeds the growth rate of its neighbours and intrudes between them, reaching a much greater length. This process provides plant fibres with their exceptional length. Fibres are the most abundant cell type in the mechanical tissues of plants. At the same time, the plant fibres are of fundamental importance for the production of textiles, paper, biocomposites, etc. Here we describe a mutant of flax (reduced fibre 1, rdf) in which intrusive growth of fibres is impaired in both phloem and xylem. In addition to the intrinsic differences in fibre length, the mutant is characterized by a constitutive gravitropic response, mechanical aberrations at the macro- and nanolevels, disruption of the cambium and uneven transition of xylem cells to secondary cell wall formation. Gelatinous cell walls in both phloem and xylem of mutant plants have disturbed structure and reduced elasticity. The existence of this mutant-control pair offers both prospects for finding the molecular players involved in triggering intrusive growth, cell wall thickening and for understanding the principles of plant mechanical tissue functioning.