Degradation of storage proteins in germinating seeds

This review deals with the main proteins of white lupin seed (Lupinus albus, L.) and reports on the current knowledge of the structural and functional properties of these proteins with the aim of providing the first comprehensive, accurate and up-to-date survey on this topic. Lupin seed's four main protein families of globulins, termed α-, β-, γ- and δ-conglutins, are reviewed with specific regard to their molecular and biological features. Their nutritional, technological, nutraceutical and allergenic potentials are also considered. The review is intended to provide nutritionists, food technologists and various stakeholders with the molecular background for a better exploitation of this valuable and accessible protein-rich source.

The major proteins of lupin seed : Characterisation and molecular properties for use as functional and nutraceutical ingredients

Seeds are very diverse in their shape and size. In the mature state they contain an embryo, the next generation of plant, surrounded by a protective structure (the seed and/or fruit coat) and, in species in which the nutritive reserves are not stored within the cotyledons, by an alternative storage tissue (endosperm, perisperm, or megagametophyte). Most seeds contain large and characteristic quantities of polymeric reserves. The major ones are carbohydrates, oils, and proteins, with minor amounts of phosphate-rich phytin. Starch, a polymer of glucose, is the most common form of stored carbohydrate, contained within cytoplasmic starch granules; less common are the hemicelluloses, stored in secondary cell walls, usually as mannan polymers. Oils are triacylglycerols, each composed of glycerol and three fatty acids that are specific to the oil; these are present within oil bodies. Storage proteins, of which there are three types, albumins, globulins, and prolamins, are sequestered in protein storage vacuoles. These reserves are vital components of human and animal diets, and their production in crops is a basis of agriculture.

Structure and Composition

We identify new organelles associated with the vacuolar system in plant cells. These organelles are defined biochemically by their internal content of three integral membrane proteins: a chimeric reporter protein that moves there directly from the ER; a specific tonoplast intrinsic protein; and a novel receptor-like RING-H2 protein that traffics through the Golgi apparatus. Highly conserved homologues of the latter are expressed in animal cells. In a developmentally regulated manner, the organelles are taken up into vacuoles where, in seed protein storage vacuoles, they form a membrane-containing crystalloid. The uptake and preservation of the contents of these organelles in vacuoles represents a unique mechanism for compartmentalization of protein and lipid for storage.

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Biogenesis of the Protein Storage Vacuole Crystalloid

Among the commercially cultivated Brassicaceae (Cruciferae) plants, Brassica juncea, Brassica napus, Brassica rapa, and Sinapis alba store significant amounts of oil and protein in the seed. At present, Brassica seed proteins are primarily used for livestock feeding based on the nutritional value. The point of curiosity is whether the present knowledge on the protein structure, biochemical characteristics, nutritive value, and the recovery processes are inadequate to develop Brassica proteins into a usable plant protein source or these proteins are of substandard for uses beyond animal nutrition applications. Cruciferin (11S) and napin (2S) are the predominant storage proteins of Brassicaceae seeds that contribute to different properties and functions. A gamut of information is available on the chemistry, nutritional value, as well as the functionality in foods, and associated non-protein components of canola/rapeseed storage proteins. The intention of this article is to critically review what is known about the predominant storage proteins of commercially produced Brassicaceae seeds relative to the above aspects and identify the knowledge gaps.

D. Neumann

Papers

Protein bodies, storage organelles in plant seeds