Plant protein inhibitors of invertases

In higher plants, cell wall (CWI) and vacuolar invertases (VI) are important metabolic enzymes, but are also key players during wound and pathogen defense reactions and in several developmental transitions. These multiple functions are implemented by small gene families. While induction of CWI and VI activities usually operates via increased transcription of the corresponding isoform gene, the equally important silencing of invertase activity depends on post-translational mechanisms, including inactivation by specific inhibitor proteins. Recently, the first cDNAs for plant invertase inhibitors were cloned, NtCIF and NtVIF (cell wall/vacuolar inhibitor of beta-fructosidase). The encoded proteins have been expressed in E. coli for functional studies and transgenic tobacco and potato plants were generated to explore the inhibitor function(s) in vivo. Mining the Arabidopsis thaliana genome revealed an inhibitor protein family of limited sequence conservation, some members grouping with tobacco CIF and VIF, while others showing a closer similarity with a recently identified inhibitor of pectin methylesterase. In vitro studies have confirmed target enzyme specificity for invertase and pectin methylesterase inhibitors (PMEI), respectively. The current status of research on invertase inhibitors and the perspectives for their use in plant biotechnology will be discussed.