Abstract
Plant fibres-cells with important mechanical functions and a widely used raw material-are usually identified in microscopic sections only after reaching a significant length or after developing a thickened cell wall. We characterized the early developmental stages of hemp (Cannabis sativa) stem phloem fibres, both primary (originating from the procambium) and secondary (originating in the cambium), when they still had only a primary cell wall. We gave a major emphasis to the role of intrusive elongation, the specific type of plant cell growth by which fibres commonly attain large cell length. We could identify primary phloem fibres at a distance of only 1.2-1.5 mm from the shoot apical meristem when they grew symplastically with the surrounding tissues. Half a millimeter further downwards along the stem, fibres began their intrusive elongation, which led to a sharp increase in fibre numbers visible within the stem cross-sections. The intrusive elongation of primary phloem fibres was completed within the several distal centimetres of the growing stem, before the onset of their secondary cell wall formation. The formation of secondary phloem fibres started long after the beginning of secondary xylem formation. Our data indicate that only a small portion of the fusiform cambial initials (<10 %) give rise directly or via their derivatives to secondary phloem fibres. The key determinant of final bundle structure, both for primary and secondary phloem fibres, is intrusive growth. Through bi-directional elongation, fibres join other fibres initiated individually in other stem levels, thus forming the bundles. Our results provide the specific developmental basis for further biochemical and molecular-genetic studies of phloem fibre development in hemp, but may be applied to many other species.
Highlights
A fibre is an individual plant cell belonging to the sclerenchyma
On longitudinal sections of the apical zone of developing hemp stems (Fig. 2A), procambial strands were identified at a distance of only 0.4 mm from the shoot apical meristem (SAM) (Fig. 2B)
For primary phloem hemp fibres such structures were identified at a distance of 1.8 – 2.0 mm from the SAM
Summary
A fibre is an individual plant cell belonging to the sclerenchyma. Its major characteristics are: (i) a very significant length (from several hundreds of micrometres up to many centimetres) with the ratio between cell lengthPublished by Oxford University Press on behalf of the Annals of Botany Company. A fibre is an individual plant cell belonging to the sclerenchyma. Snegireva et al — Intrusive growth of primary and secondary phloem fibres in hemp stems those of hemp (Cannabis sativa), have been used by humans since ancient times and currently have numerous applications, both traditional and modern (Reichert 1994; Ebskamp 2002; Thygesen et al 2006; John and Thomas 2008). Hemp fibres are successfully used in innovative technologies, e.g. as reinforcement in composite materials (Thygesen et al 2006; Abot et al 2013). Plant phloem fibres are interesting for fundamental studies of plant cell growth and cell wall formation, exhibiting a special (intrusive) type of elongation (Esau 1977; Lev-Yadun 2001; Snegireva et al 2010; Gorshkova et al 2012), and in various fibre types a special (gelatinous) tertiary cell wall that can serve as ‘plant muscles’ and pull upward trunks and branches by fibre-cell shortening (Mellerowicz et al 2008; Gorshkova et al 2010, 2012; Mikshina et al 2013)
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