, Volume 68, Issue 1-2, pp 107-133

“Transfer cells” Plant cells with wall ingrowths, specialized in relation to short distance transport of solutes—Their occurrence, structure, and development

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Summary

Plant cells possessing ingrowths of wall material, and hence having protoplasts with unusually high surface-to-volume ratios, may be found in a wide variety of anatomical situations and in most of the major taxa of multicellular plants. They are termed here “transfer cells”. Their function relates to any of four categories of trans membrane flux: 1. Absorption of solutes from the external environment (e.g. epidermis of submerged leaves), 2. Secretion of solutes to the external environment (e.g. nectaries and other glands), 3. Absorption of solutes from an internal, extracytoplasmic compartment (e.g. vascular parenchyma, haustorial-type connections, embryo sacs and embryos), 4. Secretion of solutes into an internal extracytoplasmic compartment (e.g. tapetum of anther, pericycle of root nodule). An overall assessment of their occurrence, structure, development and role in the plant is presented taking account of published information and new observations.

The wall ingrowths form just as intensive transport starts; they become best developed on those faces of the cell presumed to be most active in solute transport and their form, frequency and final degree of development are within limits characteristic of the plant species and of the location of its transfer cells. Numerous mitochondria and a conspicuous endoplasmic reticulum usually accompany this wall specialization, and the relevance of these features to the exchange of solutes across the plasma membrane is discussed.

Transfer cells are apparently restricted to situations where adverse surface area—volume relationships exist between donor and receptor compartments of the transport pathway and/or where the transported solutes are accompanied by a minimal flow of solvent. This suggests that selection pressures of a physiological nature may have shaped evolution of the transfer cell, its wall-membrane apparatus emerging as a module which may serve in any of several forms of intensive, short distance transport.