Intrusive growth of flax phloem fibers.Dr. André van Lammeren (associate professor)
Collaboration with Dr. Gorshkova
Flax (Linum usitatissimum L.) phloem fibers elongate considerably during their development and intrude between existing cells. We questioned whether fiber elongation is caused by cell tip growth or intercalary growth. Cells with tip growth are characterized by having two specific zones of cytoplasm in the cell tip, one with vesicles and no large organelles at the very tip and one with various organelles amongst others longitudinally arranged cortical microtubules in the sub-apex. Such zones were not observed in elongating flax fibers. Instead, organelles moved into the very tip region, and cortical microtubules showed transversal and helical configurations known from cells growing in intercalary way. In addition, pulse–chase experiments with calcofluor white resulted in a spotted fluorescence in the cell wall all over the length of the fiber. Therefore, we concluded that fiber elongation is not achieved by tip growth but by intercalary growth. The intrusively growing fiber is a coenocytic cell that has no plasmodesmata, making the fibers a symplastically isolated domain within the stem.
Figure 5: Immunocytochemical localization of microtubules in whole mount preparations of flax fibers, depicted as Z-projections. a: Flax fiber in coordinated growth stage with cortical microtubules in an orientation transversal to the long cell axis. b: Flax fiber in coordinated growth stage with cortical microtubules with an orientation markedly deviating from transversal. c: Flax fiber in intrusive growth stage with tapered ends, one of which is shown in the micrograph. Cortical microtubules have transverse orientation in the tapered end region (arrow) and helical orientation in the central zone (arrow-head). d: Two flax fibers in the intrusive growth stage showing helical cortical microtubules with an orientation greatly deviating from transversal. e: Flax fiber in the region below snap point with microtubules parallel to the long axis of the cell. f-g: Two images of the same part of a fiber below snap point showing the cortical microtubules parallel to the long axis of the fiber (f) and the thickened cell wall at dim bright field (g). SCW secondary cell wall, C cytoplasm. Bar a-d, f-g 10 µm; e 25 µm. Taken from Ageeva? et al., Planta 2005 (in press).