Participation of Cob Tissue in the Uptake of Medium Components by Maize Kernels Cultured In Vitro

Abstract

Summary The role of the cob tissue in the absorption of sugars from the culture medium by maize ( Zea mays L.) kernels cultured in vitro was examined. Growth of kernels was proportional to the size of cob piece attached. Slicing the cob piece to disrupt symplastic but not apoplastic continuity greatly reduced uptake of 14 C-sugars into the kernels. Uptake of 14 C from L-glucose into cob and kernel tissues was much lower than that from D-glucose, sucrose, or fructose. Similar amounts of 14 C from sucrose and 1'-fluorosucrose (an invertase-resistant sucrose analog) entered cob and endosperm tissues, but more 14 C entered pedicel, pericarp, and embryo tissues from sucrose than from 1'-fluorosucrose. The non-penetrating sugar transport inhibitors p-chloromercuribenzene sulfonate and phlorizin reduced sugar uptake by cob and kernel tissues. 3-Hydroxy-5,8,10-pyrenetrisulfonate (PTS), a fluorescent apoplastic tracer, did not permeate the cob tissue or enter the kernels in 24 h, while fluorescein, a symplastic tracer, permeated all tissues. These results indicate that phloem loading or uptake by cob parenchyma cells is a prerequisite to transfer of compounds from the incubation medium into the endosperm, and that invertase hydrolysis in the cob tissue does not contribute to that process. The active role of the cob tissue in absorption of compounds into the endosperm, as well as the observed barrier to apoplastic diffusion, needs to be considered in any experiments in which this culture technique is used.