Abstract
The morphology of characean algae could be mistaken for a higher plant: stem-like axes with leaf-like branchlets anchored in the soil by root-like rhizoids. However, all of these structures are made up of giant multinucleate cells separated by multicellular nodal complexes. The excised internodal cells survive long enough for the nodes to give rise to new thallus. The size of the internodes and their thick cytoplasmic layer minimize impalement injury and allow specific micro-electrode placement. The cell structure can be manipulated by centrifugation, perfusion of cell contents or creation of cytoplasmic droplets, allowing access to both vacuolar and cytoplasmic compartments and both sides of the cell membranes. Thousands of electrical measurements on intact or altered cells and cytoplasmic droplets laid down basis to modern plant electrophysiology. Furthermore, the giant internodal cells and whole thalli facilitate research into many other plant properties. As nutrients have to be transported from rhizoids to growing parts of the thallus and hormonal signals need to pass from cell to cell, Characeae possess very fast cytoplasmic streaming. The mechanism was resolved in the characean model. Plasmodesmata between the internodal cells and nodal complexes facilitate transport of ions, nutrients and photosynthates across the nodes. The internal structure was found to be similar to those of higher plants. Recent experiments suggest a strong circadian influence on metabolic pathways producing indole-3-acetic acid (IAA) and serotonin/melatonin. The review will discuss the impact of the characean models arising from fragments of cells, single cells, cell-to-cell transport or whole thalli on understanding of plant evolution and physiology.
Highlights
From all the charophytes, Characeae morphology appears most similar to embryophytes
The early measurements of intercellular transport and electrical conductance provided basis to what is a large field mainly centered on structure and evolution of plasmodesmata (Burch-Smith and Zambryski, 2012; Evkaikina et al, 2014)
In gymnosperms and angiosperms primary plasmodesmata develop at the time of cell division, while secondary plasmodesmata can form between any adjacent cells after cell division
Summary
Characeae morphology appears most similar to embryophytes (land plants). Perfusion, Permeabilized Plasma Membrane and Cytoplasmic Droplets: Tonoplast Transporters. To resolve tonoplast electrical characteristics Moriyasu et al (1984b) increased the conductance of the plasma membrane by including 110 mM KCl in the external medium, opening large conductance K+ channels. Cl−/H+ Symporter With the negative membrane PD across the plasma membrane, Cl− needs active transport into the cell even with the low concentration in the cytoplasm. Using perfused tonoplast-less cells he established that this flux stimulation resulted from drop of cytoplasmic Cl− concentration. He observed a strong dependence of Cl− influx on cytoplasmic and external pH (Figure 4B; Sanders, 1980b). The model addresses the main features of the symporter: (i) Michaelis– Menten kinetics, (ii) cytoplasmic Cl− concentration and pH effects on Vmax but not Km, and (iii) Cl− concentration and pH interaction
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