Apical structure of actively growing fern rhizoids examined by DIC and confocal microscopy

The development and cytology of gametophyte primary rhizoids of the fern Dr yopteris affinis was examined using actively growing material. During devel opment an apical cytoplasmic 'accumulation' forms and is associated with ac tive tip growth. This accumulation deteriorates as terminal differentiation and cessation of growth approaches. During early development the nucleus m oves from the rhizoid cell base into the newly extending rhizoid. Later, du ring the active elongation phase, the nucleus takes up a relatively stable location approx. 100 mu m behind the extending apex. Towards terminal diffe rentiation the nucleus lags further behind the tip. In actively growing rhi zoids four distinct zones were distinguished: a richly cytoplasmic 'cap'; a n apical region with tubular vacuolar intrusions; a region distinguished by a peripheral sheath of cytoplasm and fine irregular cytoplasmic strands co nnecting to the nucleus; and the main subapical vacuole. Confocal microscop y of gametophytes stained with fluorescent vital dyes, not previously used to examine fern rhizoid structure, confirmed that the tubular vacuolar syst em extends well into the apical cytoplasm, and that the network of fine cyt oplasmic strands leads back from the apical cytoplasm to the nucleus. It al so revealed that mitochondria are distributed throughout the rhizoid and ar e not excluded from the extreme apex. Membrane staining by FM 4-64 suggeste d a high density of membrane vesicles within the cytoplasm of the extreme a pex. Uptake of this endocytosis marker into endomembranes also suggested ra pid plasma membrane turnover in the rhizoid. This study highlights the simi larity in the developmental stages and appearance of D. affinis rhizoids to angiosperm root hairs and their much less distinct apical zonation compare d to pollen tubes. (C) 2000 Annals of Botany Company.