Ubiquitin, hormones and biotic stress in plants

Abstract

Background: The covalent attachment of ubiquitin to a substrate protein changes its fate. Notably, proteins typically tagged with a lysine48-linked polyubiquitin chain become substrates for degradation by the 26S proteasome. In recent years many experiments have been performed to characterize the proteins involved in the ubiquitylation process and to identify their substrates, in order to understand better the mechanisms that link specific protein degradation events to regulation of plant growth and development.

Scope: This review focuses on the role that ubiquitin plays in hormone synthesis, hormonal signalling cascades and plant defence mechanisms. Several examples are given of how targeted degradation of proteins affects downstream transcriptional regulation of hormone-responsive genes in the auxin, gibberellin, abscisic acid, ethylene and jasmonate signalling pathways. Additional experiments suggest that ubiquitin-mediated proteolysis may also act upstream of the hormonal signalling cascades by regulating hormone biosynthesis, transport and perception. Moreover, several experiments demonstrate that hormonal cross-talk can occur at the level of proteolysis. The more recently established role of the ubiquitin/proteasome system (UPS) in defence against biotic threats is also reviewed.

Conclusions: The UPS has been implicated in the regulation of almost every developmental process in plants, from embryogenesis to floral organ production probably through its central role in many hormone pathways. More recent evidence provides molecular mechanisms for hormonal cross-talk and links the UPS system to biotic defence responses.

Fig. 1.

An overview of the ubiquitylation process. RING- and U-box-domain-containing E3 ligases facilitate direct transfer of ubiquitin from the E2 to the substrate, as shown on the right branch of the pathway. In the case of CRLs, the E2 binds to the RBX RING domain-containing subunit that is linked to the substrate through a cullin and various adaptor proteins. HECT domain-containing E3 ligases form a thioester bond with the ubiquitin delivered by the E2 before passing the ubiquitin to the substrate, as shown on the left branch of the pathway.

Fig. 2.

Predicted phenotypic consequences of altering the levels of E3 ubiquitin ligases at the molecular level (A) and from the perspective of an overall pathway (B). The figure displays the predicted effects of altering the quantity of the E3 ligase components, but its predictions can be extended to mutations that affect the activity of the E3 ligase as well. The effects of mutations that decrease the activity of the E3 ligase, for example by impairing its ability to bind to the substrate or to the E2 enzyme, should resemble the effects of reducing the levels of the E3 protein. And similar outcomes might be expected when an E3 ligase is expressed at a higher level, or exhibits higher activity, for instance owing to an enhanced affinity for its substrate.