Cellular pathway of photosynthate transport in coats of developing seed ofVicia fabaL. andPhaseolus vulgarisL. II. Principal cellular site(s) of efflux

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Abstract
The cells responsible for the photosynthate efflux from coats of developing seed of Vicia faba L. and Phaseolus vulgaris L. were elucidated using known properties of the efflux mechanism. Sensitivity of sucrose efflux to NEM and high potassium concentrations was retained by seed-coat halves of Phaseolus following pectinase removal of the branch parenchyma cell layer. In contrast, removal of the thin-walled parenchyma transfer cell layer from Vicia seed-coat halves abolished this sensitivity. The membrane-impermeant thiol-binding fluorochrome, qBBr, selectively stained the surface of the thin-walled parenchyma transfer cells. This phenomenon was inhibited by the slowly permeable sul-phydryl agent, PCMBS, indicating that the plasma membranes of these cells are enriched in sulphydryl groups characteristic of membrance porter proteins. On the basis that carrier-mediated sucrose efflux from seed coats appears to be proton coupled, the putative plasma membrane H+-ATPase was used as a marker for the cells responsible for carrier-mediated photosynthate efflux. When seed-coat halves were exposed briefly at pH 8.5 to the weak acid fluorochrome, SRG, the ground parenchyma and thin-walled parenchyma transfer cell layers selectively accumulated the dye. The apparent low pH environment in the walls of these cells that renders SRG membrane permeant appeared to be maintained by a VAN-sensitive proton pump. The findings with SRG were corroborated by the cyto-chemical localization of plasma membrane ATPase activity to the ground parenchyma and thin-walled parenchyma transfer cells using precipitation of cerium phosphate. Together, our observations provide qualified support for the conclusion that carrier-mediated photosynthate efflux from coats of Phaseolus and Vicia seed is primarily restricted to the ground parenchyma and thin-walled parenchyma transfer cell layers, respectively.

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