Development and functions of seed transfer cells

In secretion or absorption processes, solutes are transported across the pl asmalemma between the symplastic and apoplastic compartments. For this purp ose, certain plant cells have developed a specialised transfer cell morphol ogy characterised by wall ingrowths, which amplify the associated plasmalem ma surface area up to 20-fold. Detailed studies on the function and develop ment of transfer cells in the context of seed filling have been carried out mainly in cereal endosperm, and for the cotyledon and seed coat cells of l egumes. The major solutes transferred are amino acids, sucrose and monosacc harides. The contributions of recently identified symporter proteins to sol ute transfer are reviewed here, as is the role of apoplastic invertases in promoting solute assimilation. Expression of invertase and monosaccharide t ransporters early in both cereal and legume seed development orchestrates t he distribution of free sugars which play an important role in regulating t ransfer cell function and determining final endosperm or embryo cell number . Transfer cell differentiation is subject to developmental control, and ma y also be modulated by sugar levels. The most abundant genes specifically e xpressed in the transfer layer of maize endosperm encode small antipathogen ic proteins, pointing to a role for these cells in protecting the developin g endosperm against pathogen ingress. The functional characterisation of th e corresponding transfer layer-specific promoters has provided a tool for d issecting transfer cell functions. Transfer cells are highly polar in their organisation, the characteristic cell wall ingrowths developing on one fac e only. The presence of cytoskeletal components bordering wall ingrowths is documented, but their role in establishing transfer cell morphology remain s to be established. (C) 2001 Elsevier Science Inland Ltd. All rights reser ved.