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.

Tissue-specific GAL4 expression patterns as a resource enabling targeted gene expression, cell type-specific transcript profiling and gene function characterization in the Arabidopsis vascular system.

Cell type-specific fluorescent gene expression markers are a prerequisite for various strategies in functional genomics and developmental biology. To increase the resolution of vascular tissue analysis and to identify more genes expressed in the vasculature, we searched for expression in vascular tissues within a new collection of transactivated enhancer trap lines. Among 33 lines with vascular expression, we identified five lines with expression profiles marking procambial or procambium-associated cell states. In stem cross-sections we identified one line with phloem- and four with xylem-specific expression, as well as nine lines with expression in both phloem and xylem and two with cambial expression. For all lines, we also report the expression patterns in different organs and developmental stages. Special features of expression patterns include a line with auxin-dependent expression domains. We determined the flanking sequences of 21 enhancer trap insertions, 16 of which are found in, or in close proximity to, annotated genes and thus may reflect the expression patterns of natural promoters. Finally, we analyzed the loss-of-function phenotypes of 14 putatively affected genes. Remarkably, mutations in a gene encoding a putative F-box protein were associated with an auxin-hypersensitive hypocotyl elongation response. Our compendium provides a diverse selection of markers for different vascular cell states, which can be used for targeted gene expression, cell type-specific transcript profiling and gene function assignment in the plant vascular system.