The amount of growth and the vaccine productivity of vaccine-producing lettuce with different cultivation periods were examined in order to determine the optimal harvesting time of transgenic lettuce cultivated in a closed plant factory. Lettuce was planted in a hydroponic system and harvested at 20, 30, 40, and 50 d, and the concentrations of the total soluble protein (TSP) and the double repeated B subunit of Shiga toxin 2e (2 Stx2eB) were measured. The dry-matter weight of leaves per plant

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... creased in a linear fashion until 50 d. Although the TSP concentration decreased continually from day 20 to 50 and 2 Stx2eB concentration decreased from day 40 to 50, the yield per plant of both TSP and 2 Stx2eB increased exponentially until day 50. According to the calculation based on these results, the optimal harvesting time to maximize the annual production of 2 Stx2eB was revealed to be 30 d. Since the optimal 30-d harvesting time is the same to the general harvesting time for commercial lettuce production in a closed plant factory, the capability to utilize the existing closed plant factory lettuce production system could be a big advantage for vaccine-producing lettuce in terms of the cost performance.