Viral-based expression cassettes ensure high level production of recombinant green fluorescent protein (GFP) in sweet basil ( Ocimum basilicum ) plants

Sweet basil ( Ocimum basilicum ) is a widespread edible aromatic herb used in many countries for medicinal and culinary purposes. Here, we disclose that basil plants accumulate high levels of recombinant protein GFP through Agrobacterium -mediated transient gene expression. Three different expression systems carrying the gfp gene coding the reporter GFP were investigated. When the gfp gene was inserted in a simple expression cassette with 35S CaMV promoter, GFP production was low, and fluorescence was poorly detectable. Improved expression systems based on potato virus X and tobamoviruses (turnip vein clearing virus and crucifer-infecting tobacco mosaic virus) genome elements increased notably the GFP yield. The highest GFP levels were observed 2–3 weeks after agroinfiltration, and the GFP content reached up to 20–30% of the total soluble proteins (1–3 mg GFP/g of fresh leaf weight). We revealed that unidentified basil cell soluble compounds quench notably GFP fluorescence in protein extracts but not in undamaged leaves. Typical viral symptoms (ringspots or systemic infection) were observed under UV light due to GFP fluorescence when viral-based expression vectors were used assuming that these viruses can be potential pathogens for basil plants. This study promotes further investigation of basil plants as a promising host for recombinant protein production through transient gene expression.