Inducible protein knockdowns are excellent tools to test the function of essential proteins in short time scales and to capture the role of proteins in dynamic events. Current approaches destroy or sequester proteins by exploiting plant biological mechanisms such as the activity of photoreceptors for optogenetics or auxin-mediated ubiquitination in auxin degrons. It follows that these are not applicable for plants as light and auxin are strong signals for plant cells. We describe here an inducible protein degradation system in plants named E3-DART for E3-targeted Degradation of Plant Proteins. The E3-DART system is based on the specific and well-characterized interaction between the Salmonella secreted protein H1 (SspH1) and its human target protein kinase N1 (PKN1). This system harnesses the E3 catalytic activity of SspH1 and the SspH1-binding activity of the Homology Region 1b (HR1b) domain from PKN1. Using Nicotiana benthamiana and Arabidopsis (Arabidopsis thaliana), we show that a chimeric protein containing the Leucine-Rich Repeat (LRR) and novel E3 ligase (NEL) domains of SspH1 efficiently targets protein fusions of varying sizes containing HR1b for degradation. Target protein degradation was induced by transcriptional control of the chimeric E3 ligase using a glucocorticoid transactivation system and target protein depletion was detected as early as 3 h after induction. This system could be used to study the loss of any plant protein with high temporal resolution and may become an important tool in plant cell biology.
