Physiochemical ultrastructural and transcriptomic insights into adaptive responses of Bletilla striata seedlings to drought stress

Drought is a major abiotic stress that restricts plant development, especially at seedling stage, which stimulates reduction of shoot, root absorb ability, and biomass accumulation of plant. Drought effects have been investigated extensively in food crop; however, drought stress on Bletilla striata ( Thunb. ) Reichb.F. is still unclear.As a more cultivated medicinal plant in Yunnan, Bletilla striata will still cause plant dwarfing and root atrophy under drought conditions, resulting in yield loss. In this study, we analyzed physiological responses and transcriptional changes of Bletilla striata, exposed to different levels of drought stress. The morphology of the aboveground and underground portions of Bletilla striata changed under drought stress. Additionally, the antioxidants activity and content in leaves and roots changed dramatically, and The contents of chlorophyll a and chlorophyll b also decreased significantly. We performed analysis of expressed genes (DEGs) and regulatory pathways involved in drought resistance in Bletilla striata. A total of 52 associated with drought resistance were identified in leaves. These genes included 8 responsive transcription factors (TFs) belonging to WRKY, MYB, TGA, bZIP, IIA, TFIID, and TRAB1 families that play critical roles in plant responses to abiotic stresses.Furthermore, many receptor protein kinase encoding genes including IPMK, BSK and PP2C involved in the stress signaling responses such as BR signaling pathway, MAPK signaling pathway, and Ca2+ signal transduction, were significantly upregulated under drought stress.Moreover, more responsive genes participated in multiple metabolic pathways in leaves. Our study provided new insights into the regulatory mechanism of the acclimation responses to drought stress in Bletilla striata and suggested that these genes may be used in molecular breeding to develop new varieties tolerant to drought stress.