Multi-Scale Characean Experimental System: From Electrophysiology of Membrane Transporters to Cell-to-Cell Connectivity, Cytoplasmic Streaming and Auxin Metabolism

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.

Dissecting the subcellular membrane proteome reveals enrichment of H+ (co-)transporters and vesicle trafficking proteins in acidic zones of Chara internodal cells

“…The participation of PM H + ATPases in the pH banding of characean internodal cells is undisputable, but the question of how the cells maintain cytoplasmic pH homoeostasis is still under debate. Electrophysiological measurements suggest that H + export at the acidic region is balanced by an efflux of OH - at the alkaline bands but the necessary OH - /H + channels have not been identified so far [ 2 , 35 ]. A possible alternative is a large net H + influx via symporter or antiporter preferentially localised in alkaline regions.…”

“…Since more than 6 decades, characean internodal cells have been used to study transport processes and to characterize transporter proteins by means of (electro-) physiological techniques [ 1 , 2 ], microscopy [ 3 ] and biochemistry including proteomics [ 4 ] and metabolomics [ 5 ]. Characean internodal cells have a cylindrical shape and are up to several centimetres long.…”

“…The plasma membrane is thus equipped with numerous H + ATPases, which acidify the external environment and convert HCO 3 - into easily permeable CO 2 , either directly or indirectly via carbonic anhydrase [ 8 – 12 ]. Characean internodal cells produce extended acidic regions (H + efflux) along their surface with alternating smaller alkaline areas with probably local OH - efflux [ 2 ], required to achieve pH homoeostasis of the cytoplasm. This pH banding pattern can be visualized by phenol red [ 13 ] and acidic bands correlate with an enhanced photosynthesis compared to alkaline regions [ 14 , 15 ] as well as high abundance and activity of cortical mitochondria [ 3 ] ( S1 Fig ).…”

Dynamical Patterning Modules, Biogeneric Materials, and the Evolution of Multicellular Plants

“…The case of auxin was briefly mentioned above. Indeed, currently available evidence shows that auxin biosynthesis was already present in the unicellular ancestors of multicellular eukaryotes ( Beilby, 2016 ; Khasin et al, 2017 ; Ishizaki, 2017 ; Kato et al, 2017 ; among many other lines of evidence). However, this is not the case for the auxin transporters that have been identified and thoroughly studied in angiosperm model systems.…”

Charophytes: Evolutionary Giants and Emerging Model Organisms

“…Rather, the mineral-rich statoliths must settle onto specific regions of the plasma membrane for gravitropic morphological effects to proceed ( Braun, 2002 ). Chara has also been used to study other phenomena including electrophysiology and the role and movement of hormones ( Belby, 2016 ).…”