Enhancement of gene expression related to phytochemical accumulation in plant leaves via exposure to environmental stresses in a plant factory

Phytochemicals derived from secondary metabolites that are accumulated in plant leaves are used as functional foods, pharmaceutical materials, and raw materials for industry. Secondary metabolite biosynthesis pathways are influenced by environmental stresses including light, temperature, atmospheric gases, and nutrient solution. In this study, we determined the environment conditions that would increase or decrease the content of phytochemical compounds under totally controlled environments such as plant factories. We used vegetative-stage Brassica napus L. and Nicotiana benthamiana as model plants for Brassica leaf vegetables and for the production of pharmaceutical and industrial raw materials, respectively. At 18-25 days after sowing, plants were exposed to four environmental stress factors, alone or in combination for 3-6 days: UV-B irradiation (0-1.0 W m^-2), low nutrient solution temperature (5-15°C), and ozone gas exposure (100-400 ppb). For plants either exposed and not exposed (control) to stress, the third and fourth leaves were sampled for transcriptome analysis of secondary metabolite pathways using DNA microarrays, quantitative gene expression analysis using real-time RT-PCR, and measurement of the concentration of key phytochemicals using HPLC. Ascorbic acid, isoprenoid, and flavonoid pathways were activated in N. benthamiana upon exposure to UV irradiation or a combination of UV and low solution temperature. Low solution temperature increased expression of some alkaloid pathway genes. In B. napus, the phenylpropanoid and flavonoid pathways were activated following exposure to UV irradiation or ozone gas. Consequently, antioxidant capacity and total phenolic content increased under these conditions. However, the carotenoid pathway was not activated in either of the plant species following UV or ozone exposure. Thus, exposure to these environment stresses can be used to manipulate phytochemical compound accumulation targeted to the intended use.