Comparison of the relative levels of GUS under the control of different promoters in switchgrass (A, C) and rice (B, D) callus cultures. Each promoter construct contained the same pHLucGWgus DNA backbone and the relative expression levels of GUS under the control of CaMV 35S were set to 1 for both switchgrass and rice. All other promoter values are shown relative to this CaMV 35S control. Bars represent the mean value of six independent replicates. Treatments that share the same letter are not significantly different as calculated by LSD (P ≤ 0.05). Error bars represent standard error.