Identification of putative GS BD PROs. Embryos were isolated from 20 DAP immature maize kernels. Each construct was introduced twice by particle gun bombardment with six embryos per bombardment. GS-n represent the promoters cloned randomly from 1696 putative BD PROs; CK: Embryos bombarded with empty vector plasmid. Putative BD PROs that showed promoter activity in both directions are indicated by a red box.