Annals of Botany

Evergreen exception proved in protein storage story

Mention of plant storage proteins generally leads us to think about the more obvious examples, namely those in seeds. However, storage proteins are also laid down in vegetative organs in many plants. In addition to their storage function, many of these vegetative storage proteins (VSPs) have other roles; these include enzyme activities that may facilitate breakdown of other storage materials and defence, often mediated by protease inhibition. In some species, VSPs are deposited in preparation for a metabolically costly event such as flowering or seed set. In many deciduous trees, proteins are deposited as a seasonal nitrogen store; they are much rarer in evergreen trees, in many of which VSPs have never been detected. However, the work of Tian et al. (Hainan, Haikou and Xi’an, China, pp. 1199-1208) shows that amongst evergreen trees, lychee (Litchi chinensis) is an exception. Classic protein storage cells with protein-packed central vacuoles are present in roots, and in the trunk, branches and twigs. The protein is mobilized during the development of young shoots and during fruiting, pointing clearly to its storage function. Extraction and initial characterization of the protein showed that it is a single 22-kDa polypeptide. More detailed analysis revealed several very similar iso-forms encoded by a small gene family; homologous storage proteins occurring in fruits and seeds. Messenger RNA encoding one of the VSP isoforms was used for synthesis of cDNA that was then expressed to produce the ‘recombinant ‘ protein in vitro. Comparison of the protein sequence with others in the databases revealed a strong homology to other plant proteins that inhibit the proteases trypsin and chymotrypsin. This inhibitory activity was tested in vitro with the recombinant L. chinensis VSP. The VSP does indeed inhibit these enzymes and, like VSPs from herbaceous plants, but not like the majority of those from deciduous trees, may thus also have a role in defence.