Ultrastructural analysis of cell component distribution in the apical cell ofCeratodon protonemata

Summary A distinctive feature of tip-growing plant cells is that cell components are distributed differentially along the length of the cell, although most ultrastructural analyses have been qualitative. The longitudinal distribution of cell components was studied both qualitatively and quantitatively in the apical cell of dark-grown protonemata of the mossCeratodon. The first 35 m of the apical cell was analyzed stereologically using transmission electron microscopy. There were four types of distributions along the cell's axis, three of them differential: (1) tubular endoplasmic reticulum was evenly distributed, (2) cisternal endoplasmic reticulum and Golgi vesicles were distributed in a tip-to-base gradient, (3) plastids, vacuoles, and Golgi stacks were enriched in specific areas, although the locations of the enrichments varied, and (4) mitochondria were excluded in the tipmost 5 m and evenly distributed throughout the remaining 30 m. This study provides one of the most comprehensive quantitative, ultrastructural analyses of the distribution of cell components in the apex of any tip-growing plant cell. The finding that almost every component had its own spatial arrangement demonstrates the complexity of the organization and regulation of the distribution of components in tip-growing cells.Abbreviations CER cisternal endoplasmic reticulum - ER endoplasmic reticulum - N_d numerical density - SE standard error - S_v surface density - TEM transmission electron microscopy - TER tubular endoplasmic reticulum - V_v volume fraction