Transport processes in pea seed coats

Abstract

The research described in this thesis concerns transport processes in coats of developing pea seeds. The scope of the investigation ranges from seed coat anatomy, via transport studies to the cloning of cDNA encoding proteinaceous membrane pores, and the heterologous expression of these proteins to analyse their function. Pea seed coats do not have an extensive vascular web. Nevertheless, assimilates imported from the mother plant are distributed throughout the entire seed coat. The transport pathway of assimilates through the seed coat towards the embryo was envisaged by using the fluorescent symplast tracer pyranine. Besides, cryo scanning electron microscopy revealed an extensive web of liquid filled intercellular spaces, which facilitate transport from the site of unloading to the cotyledons. It has been postulated that the unloading of sugars and amino acids from pea seed coat parenchyma cells is mediated by poorly selective, proteinaceous pores. To investigate whether electrically charged molecules can be transported via the same pathway, the uptake of the organic cation choline, and the amino acid histidine were characterised. The results indicate that cations may indeed be transported by the same pore as neutral solutes. The idea of poorly selective pores lead to the formulation of the Supply follows Demand model, which offers a simple concept for the regulation of assimilate unloading in the seed coat. Solutes that are unloaded into the apoplast are transported from the mother plant to the seed coat by the phloem. Together with these solutes, water is imported into the seed coat. This has to be released from the seed coat parenchyma cells again. Therefore, membrane transport of solutes and water across the seed coat parenchyma plasma membrane strongly influence each other. Major Intrinsic Proteins (MIPs) may be involved in the entwined processes of water and solute transport. To examine the presence of MIPs in pea seed coats, a seed coat cDNA library was screened for MIP clones. Four clones were isolated, and their expression patterns in the plant were investigated. To determine the permeability for water and solutes, the four pea MIPs were functionally assayed after heterologous expression in Xenopus oocytes.