Radish sprouts as an efficient and rapidly available host for an agroinfiltration-based transient gene expression system.

Agroinfiltration, the infiltration of plants with Agrobacterium harboring a plasmid that contains a specific gene, is used to transiently express a gene in a heterologous organism. Using the "Tsukuba system", greater amounts of target protein accumulate compared with usual expression plasmids. Reported host plants, including Nicotiana benthamiana, a common plant for agroinfiltration, need several weeks after sowing to grow enough for infection. To shorten the culture period and, thereby, improve target protein production, we tested sprouts as host plants. Sprouts were grown in the dark to encourage elongation so that vacuum infiltration becomes easier, and this was followed by a few days of exposure to illumination before infection with pBYR2HS-EGFP, the EGFP expression plasmid of the Tsukuba system. Among six tested species of Fabaceae and Brassicaceae, radish showed the highest transient expression. Among six tested radish cultivars, Kaiware, Hakata, and Banryoku provided the best results. Culturing for 5 day, including 1 day of imbibition and 1 to 2 day of exposure to illumination resulted in EGFP fluorescence in 80% of the cotyledon area. Thus, a remarkable amount of EGFP was obtained only 8 day after seed imbibition. The EGFP amount in Kaiware cotyledons was comparable with Rubisco at ∼0.7 mg/g fresh weight. Kaiware sold in supermarkets could also be used, but resulted in lower expression levels.