Mechanisms of intrusive growth of secondary xylem fibers in flax (Linum usitatissimum L.).

This study explores the mechanisms of intrusive growth in secondary xylem fibers, a process determining their length and future mechanical performance. The flax variety CDC Bethune and the derived from it rdf mutant with reduced fiber length were analyzed to identify factors influencing intrusive growth. The structure and development of secondary xylem, as well as the composition, architecture, and mechanical properties of cell walls in xylem fibers, were studied using microscopy. Hypotheses of intrusive growth initiation were tested using computer modeling. Biomechanical differences between cell walls of tips, extensions and bodies of the fibers were identified. The extensions were found to be the softest, while the tips were the stiffest. The cell walls of the extensions were found to be thinner than those in the bodies. These findings suggest extensions as the most actively growing parts of fibers. Fiber cell walls in rdf were stiffer than those in the control, consistent with a more chaotic orientation of cell wall macrofibrils. The increased cell wall stiffness may have contributed to the reduction in fiber length. Computational modeling shows that intrusive growth initiation depends on cell wall properties and breakability of intercellular contacts, rather than increased turgor.