Strategies to improve low copy transgenic events in Agrobacterium-mediated transformation of maize
Studies on distantly related dicot plant species have identified homeotic genes that specify floral meristem identity and determine the fate of floral organ primordia. Most of these genes belong to a family characterized by the presence of a structural motif, the MADS-box, which encodes a protein domain with DNA-binding properties. As part of an effort to understand how such genes may have been recruited during the evolution of flowers with different organ types such as those found in maize, two members of this gene family in maize, ZAG1 and ZAG2, have been characterized previously. Here, the isolation and characterization of four new members of this gene family, designated ZAP1, ZAG3, ZAG4 and ZAG5, are described and the genetic map position of these and 28 additional maize MADS-box genes is determined. The first new member of this family appears to be the Zea mays ortholog of the floral homeotic gene APETALA1 (AP1) and has been designated ZAP1. One of these genes, ZAG4, is unusual in that its deduced protein sequence includes the MADS domain but lacks the K-domain characteristically present in this family of genes. In addition, its copy number and expression varies among different inbreds. A large number of maize MADS-box genes map to duplicated regions of the genome, including one pair characterized here, ZAG3 and ZAG5. These data underscore the complexity of this gene family in maize, and provide the basis for further studies into the regulation of floral organ morphogenesis among the grasses.