Short term UV-B radiation-mediated transcriptional responses and altered secondary metabolism of in vitro propagated plantlets of Artemisia annua L.

In vitro propagated Artemisia annua plantlets were exposed to low dose (2.8 W m(-2)) of UV-B (280-315 nm) radiation for different short-term (1, 2, 3 and 4 h) durations that resulted in 103 and 100 % enhanced artemisinin and flavonoid yield respectively after 3 h of UV-B irradiation. UV-B irradiation resulted in reduced chlorophyll a, b and carotenoids while the concentrations of UV-B-absorbing anthocyanins and phenolics were induced. Under UV-B radiation, superoxide radical formation increased in a time dependent manner and was maximum at 4 h. This lead to differential responses of enzymatic antioxidants (superoxide dismutase, catalase, peroxidases and glutathione reductase) to withstand the UV-B radiation-induced reactive oxygen species burst. The transcript analysis through RT-PCR revealed the significant up-regulation of HMGR, DXR, IPPi, FPS, ADS, CYP71AV1 and RED1 gene transcripts leading to higher artemisinin accumulation. Differential expression of stress regulated genes (AOX1a, NDB2, UPOX, PAL, LOX) helped the plant towards better adaptability at least up to 3 h UV-B irradiation period. Simultaneous clustering of genes and UV-B treatments using complete linkage and a euclidean distance matrix revealed that 3 h irradiation period is optimum for high artemisinin yield with better tolerance. Histological studies revealed the larger trichome size in UV-B treated plants as compared to untreated plants. The present study conclude that short term UV-B treatment to in vitro raised A. annua may be a safe approach for continuous supply of high artemisinin producing plantlets with simultaneous action of stress- regulated genes to keep the plant healthy.