Red light promotes cotton embryogenic callus formation by influencing endogenous hormones, polyamines and antioxidative enzyme activities

Genetic transformation through somatic embryogenesis is a major tool for improving cotton's (Gossypium hirsutum L.) development and defenses. However, the low efficiency of somatic embryogenesis seriously limits cotton's biotechnology-based breeding and gene functional analyses. In this study, the effects of different light qualities on callus morphology and embryogenic callus induction in cotton were evaluated. Embryogenic callus formation was affected by different light regimes during the induction and expression phases. Red light treatments during the induction phase significantly promoted the formation of embryogenic calli, reaching 94.0%, and the differentiation period was reduced to 30days compared with more than 60days under white light conditions. Calli were of a moderate size and had moderate peroxidase, superoxidase dismutase, and catalase activity levels under red light conditions, which may maintain the balance between callus growth and differentiation. The highest total polyamine, spermidine, and spermine contents, and the lowest putrescine content, were detected during red light treatments. Additionally, endogenous auxin levels were significantly greater in red light-treated calli, and the expression levels of the somatic embryogenesis marker genes AGAMOUS-like 15, LEAFY COTYLEDON1, LEAFY COTYLEDON2, BABY BOOM, and WUSCHEL were up-regulated under red light compared with white light conditions. Thus, red light applied in the induction phase favored embryogenic callus differentiation, possibly through physiological responses to light quality. A high-efficiency cotton somatic embryogenesis protocol using red light applications will help clarify the mechanism of somatic embryogenesis.