Abstract
Flax phloem fibers achieve their length by intrusive-diffusive growth, which requires them to penetrate the extracellular matrix of adjacent cells. Fiber elongation therefore involves extensive remodelling of cell walls and middle lamellae, including modifying the degree and pattern of methylesterification of galacturonic acid (GalA) residues of pectin. Pectin methylesterases (PME) are important enzymes for fiber elongation as they mediate the demethylesterification of GalA in muro, in either a block-wise fashion or in a random fashion. Our objective was to identify PMEs and PMEIs that mediate phloem fiber elongation in flax. For this purpose, we measured transcript abundance of candidate genes at nine different stages of stem and fiber development and found sets of genes enriched during fiber elongation and maturation as well as during xylem development. We expressed one of the flax PMEIs in E. coli and demonstrated that it was able to inhibit most of the native PME activity in the upper portion of the flax stem. These results identify key genetic components of the intrusive growth process and define targets for fiber engineering and crop improvement.
Highlights
Flax phloem fibers achieve their remarkable length through an extended period of intrusive growth
With reference to the precedent established by Gorshkova, we identified nine positions along the stem that represented progressive stages of fiber development (Figure 1)
In the vicinity of the shoot apex, very little vascular tissue maturation is expected to occur and it is only at node 3–5 that thickening starts [26], so if LuPME3 is involved in the cell wall stiffening of the xylem, it is expected that its expression is lower in point SA, which we found, and as more xylem is produced along the stem the maturation of the xylem is a constant process which is observed in the expression of this gene
Summary
Flax phloem fibers achieve their remarkable length through an extended period of intrusive growth. The demethylesterification of the cell wall plays a major role in the elongation and development of the phloem fibers of flax. 105 putative flax pectin methylesterases (LuPMEs) and 95 putative pectin methylesterase inhibitors (LuPMEIs) have been identified. The majority of these genes (77 LuPMEs and 83 LuPMEIs) have been demonstrated to be transcribed in at least one of the following tissues and developmental stages: floral buds, flowers, green capsules, early cortical peels, early fibers, late fibers, shoot apices, xylem, roots, leaf, senescent leaves [3]. Having defined the LuPME and LuPMEI families, we have the opportunity to more precisely characterize these genes in the context of flax bast fiber development
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