RT-qPCR analysis of HDR1 and HDR2 expression in different organs of Populus × canescens and Picea abies. PcHDR2 expression in Populus × canescens is many-fold greater than that of PcHDR1 in all organs tested, making it virtually the only active isoform in the plant A). In Picea abies, PaHDR2 Is mainly expressed in spruce stems and roots, while PaHDR1 shows highest expression in needles B). These differences in HDR isoform expression point towards species-specific regulation of the production of DMADP and IDP. Values are given as means ± standard deviation of three biological replicates, measured as technical triplicates. Statistical analysis was performed by using Student's t-test, letters represent statistical differences between the tissues; P ≤ 0.05.