Development and functions of seed transfer cells.

In secretion or absorption processes, solutes are transported across the plasmalemma between the symplastic and apoplastic compartments. For this purpose, certain plant cells have developed a specialised transfer cell morphology characterised by wall ingrowths, which amplify the associated plasmalemma surface area up to 20-fold. Detailed studies on the function and development 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 legumes. The major solutes transferred are amino acids, sucrose and monosaccharides. The contributions of recently identified symporter proteins to solute transfer are reviewed here, as is the role of apoplastic invertases in promoting solute assimilation. Expression of invertase and monosaccharide transporters early in both cereal and legume seed development orchestrates the distribution of free sugars which play an important role in regulating transfer cell function and determining final endosperm or embryo cell number. Transfer cell differentiation is subject to developmental control, and may also be modulated by sugar levels. The most abundant genes specifically expressed in the transfer layer of maize endosperm encode small antipathogenic proteins, pointing to a role for these cells in protecting the developing endosperm against pathogen ingress. The functional characterisation of the corresponding transfer layer-specific promoters has provided a tool for dissecting transfer cell functions. Transfer cells are highly polar in their organisation, the characteristic cell wall ingrowths developing on one face only. The presence of cytoskeletal components bordering wall ingrowths is documented, but their role in establishing transfer cell morphology remains to be established.