Controlled Expression of Recombinant Proteins in <i>Physcomitrella patens</i> by a Conditional Heat-shock Promoter: a Tool for Plant Research and Biotechnology

The ability to express tightly controlled amounts of endogenous and recombinant proteins in plant cells is an essential tool for research and biotechnology. Here, the inducibility of the soybean heat-shock <i>Gmhsp17.3B</i> promoter was addressed in the moss <i>Physcomitrella patens</i>, using β-glucuronidase (GUS) and an F-actin marker (GFP-talin) as reporter proteins. In stably transformed moss lines, <i>Gmhsp17.3B</i>-driven GUS expression was extremely low at 25 °C. In contrast, a short non-damaging heat-treatment at 38 °C rapidly induced reporter expression over three orders of magnitude, enabling GUS accumulation and the labelling of F-actin cytoskeleton in all cell types and tissues. Induction levels were tightly proportional to the temperature and duration of the heat treatment, allowing fine-tuning of protein expression. Repeated heating/cooling cycles led to the massive GUS accumulation, up to 2.3% of the total soluble proteins. The anti-inflammatory drug acetyl salicylic acid (ASA) and the membrane-fluidiser benzyl alcohol (BA) also induced GUS expression at 25 °C, allowing the production of recombinant proteins without heat-treatment. The <i>Gmhsp17.3B</i> promoter thus provides a reliable versatile conditional promoter for the controlled expression of recombinant proteins in the moss <i>P. patens</i>.