Blue LED light promotes indican accumulation and flowering in indigo plant, Polygonum tinctorium

Abstract The photoperiod is a day-length-dependent seasonal change of physiological or developmental activities, such as flowering, in many plant species. Polygonum tinctorium (P. tinctorium) is an important industrial crop producing indigo blue dyes, and indican is an important substance as a precursor of indigo. Here, we report the day-length-dependent responses of growth, flowering, and indican synthesis in P. tinctorium. Indigo plants were grown in a hydroponic system under artificial light conditions in a completely-controlled plant factory. The growth parameters and indican content of leaves were measured and compared. Certain growth parameters (such as fresh weight and number of leaves) under 24-h continuous irradiation were significantly higher than those under other day-length conditions. Under 12-h photoperiod conditions, the flowering rate of plants with blue LED treatment increased six-fold compared with fluorescent white light treatment, while none of the plants flowered with red LED treatment. In the leaves, the relative expression levels of Pt’IGS and Pt’BGL were significantly higher in newer leaves compared to older ones. Indican content was greatly enhanced by blue light under 24-h continuous irradiation, which was reflected by increased expression levels of Pt’IGS. These findings demonstrate that there is a new regulatory mechanism for the indican synthesis pathway through blue light signalling. Blue light provides feasible strategy for artificially regulating indican synthesis and flowering in P. tinctorium.