Sugar Efflux from Maize (Zea mays L.) Pedicel Tissue

Abstract

Sugar release from the pedicel tissue of maize (Zea mays L.) kernels was studied by removing the distal portion of the kernel and the lower endosperm, followed by replacement of the endosperm with an agar solute trap. Sugars were unloaded into the apoplast of the pedicel and accumulated in the agar trap while the ear remained attached to the maize plant. The kinetics of "C-assimilate movement into treated versus intact kernels were comparable. The rate of unloading declined with time, but sugar efflux from the pedicel continued for at least 6 hours and in most experiments the unloading rates approximated those necessary to support normal kernel growth rates. The unloading process was challenged with a variety of buffers, inhibitors, and solutes in order to characterize sugar unloading from this tissue.Unloading was not affected by apoplastic pH or a variety of metabolic inhibitors. Although p-chloromercuribenzene sulfonic acid (PCMBS), a nonpenetrating sulfhydryl group reagent, did not affect sugar unloading, it effectively inhibited extracellular acid invertase. When the pedicel cups were pretreated with PCMBS, at least 60% of sugars unloaded from the pedicel could be identified as sucrose. Unloading was inhibited up to 70% by 10 millimolar CaCl2. Unloading was stimulated by 15 millimolar ethyleneglycol-bis(6-aminoethyl ether)-N,N,N',N'-tetraacetic acid which partially reversed the inhibitory effects of Ca21. Based on these results, we suggest that passive efflux of sucrose occurs from the maize pedicel symplast followed by extracellular hydrolysis to hexoses.

Monitoring Phloem Unloading and Post-Phloem Transport by Microperfusion of Attached Wheat Grains

“…Com, however, contrasts with other species in both respects. Solute release from the com pedicel is not affected by metabolic inhibitors or PCMBS (Porter et al, 1985) and is decreased by osmotic treatments (Porter et al, 1987).…”

“…These goals have been partially achieved by the surgical remova1 of embryonic tissues from the seed coat (Patrick, 1983;Thorne and Rainbird, 1983;Wolswinkel and Ammerlaan, 1983) or from maize pedicels (Porter et al, 1985) to intercept solutes being released from matemal tissues into the apoplast. Results from this approach have implicated transport, within matemal tissues as a controlling factor for the rate of seed growth (Wolswinkel, 1992).…”

Transport and Metabolism of a Sucrose Analog (1′-Fluorosucrose) into Zea mays L. Endosperm without Invertase Hydrolysis

“…It has been suggested that in maize and sorghum the apoplast ofthe placenta and chalazal regions, which contain many dead cells, functions as a temporary storage site for sugars before absorption into the endosperm (2, 6). Evidence for this is as follows: (a) high hexose concentrations are found in the placental sac, for sorghum 300 to 400 mm (6), and for maize 470 to 800 mm of glucose equivalents (12); (b) there is a buildup of radioactivity in the pedicel of maize after a pulse of "4CO2 to the leaf ear (2).In maize (7,10) Here we have used an analog of sucrose, FS,3 which is a poor substrate for invertase (4,8), to elucidate the role of invertase hydrolysis in uptake of phloem-supplied sucrose into maize endosperm. FS behaves similarly to sucrose in phloem-loading in leaf tissue (4) and in translocation in soybean and sugar beet plants (8).…”

“…In maize (7,10) and sorghum (6), incoming sucrose is inverted to hexoses in the placental-chalazal, pedicel, or basal endosperm regions during movement into endosperm tissues. In maize, the highest kernel activities of acid invertase are extracted from the placental-chalazal and pedicel tissue (11).…”

“…In maize (7,10) Here we have used an analog of sucrose, FS,3 which is a poor substrate for invertase (4,8), to elucidate the role of invertase hydrolysis in uptake of phloem-supplied sucrose into maize endosperm. FS behaves similarly to sucrose in phloem-loading in leaf tissue (4) and in translocation in soybean and sugar beet plants (8).…”