Intercellular Transport of Ions and Photoassimilates

Chara provides a simple cell system for studying the intercellular transport of ions and photoassimilates. Moreover, in characean cells, the distribution of ions and photoassimilates between the vacuole, the streaming endoplasm, and the chloro-plast-rich cortical gel layer can be determined by using the vacuolar perfusion. The cell sap pushed out by a Ca2+ -containing perfusion solution can be used to obtain the vacuolar sample and the remaining cell can be used as the cytoplasmic sample. The cytoplasm can be further separated into the sol endoplasm and the cortical gel. After the first perfusion, the vacuole is perfused with a solution containing EGTA, which disrupts the tonoplast. The extract pushed out by the second perfusion contains the sol endoplasm. The remaining cell without the vacuolar sap and the sol endoplasm can be used as the chloroplast-rich gel layer sample.

Using the one step perfusion, we analyzed the Rb+ transport between two internodes that were connected by the node (Ding and Tazawa 1989). The rate of transnodal symplastic transport of Rb+ was strictly dependent on the temperature. The rate of the transport was correlated with the rate of cytoplasmic streaming, which was also temperature-dependent. The Rb+ transport was impeded by imposing a gradient of turgor pressure imposed between the two internodes. Thus, the plasmodesmata may be equipped with a valve mechanism that is sensitive to the pressure gradient (Ding and Tazawa 1989).

Using the two-step perfusion, we succeeded for the first time in the simultaneous measurement of the amino acids distribution in subcellular compartments under light and dark conditions (Mimura et al. 1990). Both the vacuole and the endoplasm showed the same distribution pattern and this pattern differs from that of the cortical gel. Upon illumination, the concentrations of the amino acids decreased in the vacuole and the cortical gel but remained nearly constant in the endoplasm.

In the isolated upper shoot composed of apex-internode-branchlet complex, we found that the transport of photoassimilates consisting of mainly sucrose and amino acids was polar, i.e., the rate of transport from the branchlet to the internode was five times greater than the rate of transport from the internode to the brachlet. The polar transport is supported by the gradient of cytoplasmic concentrations of photoassi-milates between the branchlet and the internode (Ding et al. 1991a). The polarity of the transport vanished when the apex was removed (Ding et al. 1991b).

While closing my article, I would like to cite a sentence from a paper of my mentor, Noburo Kamiya, saying that "Characean cells give us an opportunity to perform various kinds of unusual cell manipulations and surgeries which would be impracticable with other material. As a matter of fact, the progress of research in this field since the middle of this century (twentieth century) owes much to the development of these novel methods which are not usual in plant physiology and plant cell biology" (Kamiya 1986).

Acknowledgments I would like to express my cordial thanks to my coworkers cited above. A wide variety of productive researches mentioned above could surely not have been achieved without their cooperation.

My thanks are also due to Professor Ulrich Liittge, the Editor of "Progress in Botany," for his invitation to this review article, and Professor Randy Wayne, one of my former coworkers and now in the Department of Plant Physiology, Cornell University, for his kindness to read my manuscript and to give valuable suggestions in polishing the style of my English.

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