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

The development and cytology of gametophyte primary rhizoids of the fern Dryopteris affinis was examined using actively growing material. During development an apical cytoplasmic 'accumulation' forms and is associated with active tip growth. This accumulation deteriorates as terminal differentiation and cessation of growth approaches. During early development the nucleus moves from the rhizoid cell base into the newly extending rhizoid. Later, during the active elongation phase, the nucleus takes up a relatively stable location approx. 100 mu m behind the extending apex. Towards terminal differentiation the nucleus lags further behind the tip. In actively growing rhizoids four distinct zones were distinguished: a richly cytoplasmic 'cap'; an apical region with tubular vacuolar intrusions; a region distinguished by a peripheral sheath of cytoplasm and fine irregular cytoplasmic strands connecting to the nucleus; and the main subapical vacuole. Confocal microscopy of gametophytes stained with fluorescent vital dyes, not previously used to examine fern rhizoid structure, confirmed that the tubular vacuolar system extends well into the apical cytoplasm, and that the network of fine cytoplasmic strands leads back from the apical cytoplasm to the nucleus. It also revealed that mitochondria are distributed throughout the rhizoid and are not excluded from the extreme apex. Membrane staining by FM 4-64 suggested a high density of membrane vesicles within the cytoplasm of the extreme apex. Uptake of this endocytosis marker into endomembranes also suggested rapid plasma membrane turnover in the rhizoid. This study highlights the similarity in the developmental stages and appearance of D. affinis rhizoids to angiosperm root hairs and their much less distinct apical zonation compared to pollen tubes. (C) 2000 Annals of Botany Company.