Terrestrial rhizophytes and H⁺ currents circulating over at least a millimetre: an obligate relationship?

Abstract

SUMMARYCirculating ionic currents are apparently ubiquitous in growing and diflferentiating eukaryotes. In many the current is carried by H+, in which case the medium around the apex of growing organs is alkalinized while the medium adjacent to subapical portions is acidified. These two zones may help in, respectively, molybdenum acquisition and reduction of aluminium toxicity, and phosphorus and iron acquisition, by rhizophytic plants. An analysis of the taxonomic distribution of H*-borne circulating current shows that all rhizophytes (terrestrial, freshwater and, probably, marine) except those in the green algal class Ulvophyceae have such currents; the Ulvophyceae have circulating currents carried by Cl". The Ulvophyceae do not seem to have any alternative means of causing pH zonation in the rhizosphere. Their marine habitat, with the possibility of molybdenum acquisition from the alkaline bulk water phase, may mean that the Ulvophyceae are not disadvantaged by the absence of rhizosphere acid-alkaline zonation as would be a land plant; similarly, they are less likely to suffer aluminium toxicity to the growing zone of rhizoids. Ulvophyceae do not occur as terrestrial plants; this may relate not only to the absence of rhizosphere pH zonation in these organisms but also to mechanical problems related to their coenocytic habit, which in turn results from their inability to produce plasmodesmata.

Electrophysiology of the Detached Cell Under Stress

“…The source of the current is located in the acid root subapical zone with the sink at the alkaline tip (Raven 1991). Protoplasts from wheat roots were found to change from pump-dominated to H + /OH À channel-dominated state (Tyerman et al 2001).…”

“…As it is the nodes of the Chara cells that sense difference in turgor (Shimmen 2008;Staves and Wayne 1993), experiments should be done with nodeless constructs (Beilby and Shepherd 1989;Beilby and Shepherd 1991 (Raven 1991;Tyerman et al 2001). The source of the current is located in the acid root subapical zone with the sink at the alkaline tip (Raven 1991).…”

Salt tolerance at single cell level in giant-celled Characeae

“…This complex electrophysiological motif of pH banding can be observed in higher plants: aquatic angiosperms (Prins et al, 1980), pollen tubes (Feijo et al, 1999) and, most importantly, roots of land plants (Raven, 1991). In roots the source of the current is located in the acid root subapical zone with the sink at the alkaline tip (Raven, 1991). The author suggests that the acid and alkaline zones facilitate acquisition of molybdenum, phosphorus and iron as well as reduction of aluminum toxicity.…”

Understanding Membrane Function

“…This Cl − ATPase in Acetabularia , like the H + ATPase in the plasmalemma of Charales (and embryophytes), is the energizer of electrical currents circulating over millimeters or centimeters in the cytosol and the external medium, with active Cl − influx (H + efflux in Charales) and passive Cl − efflux (H + influx in Charales) (see Bowles and Allen 1986, Raven 1991. Despite the lack of obvious involvement of H + transport in these circulating currents, there are pronounced pH differences, associated with different metabolic rates in different parts of the acellular organism for both photosynthesis and respiration (Serikawa et al 2001).…”