THE OCCURRENCE AND SIGNIFICANCE OF DISPENSABLE PROTEINS IN PLANTS

S ummary Plants produce many different kinds of proteins. Enzymes, ribosomal proteins and storage proteins have clearly defined, ubiquitous, physiological roles and are indispensable for normal plant growth. Other proteins, such as urease, β‐amylase, lectins, proteinase inhibitors and some proteinases, have a varied distribution and no clear physiological role. These proteins are dispensable in that some plant species manage without them, or with only extremely low levels, whereas other species accumulate significant quantities of the proteins. Dispensable proteins all have defined biological activities, but in most cases these activities do not explain their physiological role. Urease is able to degrade urea, but many seeds contain little or no urea. Beta‐amaylase only attacks gelatinized starch, not intact starch granules. Lectins agglutinate red blood cells. Trypsin inhibitors specifically inhibit a proteolytic enzyme from animals. These proteins may fulfill a non‐specific role as part of the cellular protein complement and therefore could presumably be replaced by other proteins in other species. The production of dispensable proteins may also be another aspect of general plant secondary metabolism which already encompasses the biosynthesis of amino acids, amines, lipids and carbohydrates. Because they are dispensable, these proteins possibly could be manipulated through plant breeding or epigenetic procedures, without accompanying deleterious side effects. The presence of large quantities of these proteins or their absence appears to have little phenotypic effect on a particular species.