Cloning and molecular characterisation of the maize retinoblastoma gene (ZmRBR2)

Retinoblastoma-related (RBR) proteins have been described in plant cells, and have been shown to be important in controlling plant growth, gametophyte and leaf development, and maintenance of stem cell identity. Whereas all dicotyledonous plants studied so far contain a single RBR gene, monocotyledonous species contain at least two genes and maize is the only known plant species with three RBR genes in its genome suggesting a recent gene duplication event. We report the genomic structure of the ZmRBR2 gene and its expression pattern. The ZmRBR2 transcript contained a 283-bp 5′-UTR and a putative canonical TATA box. The gene had a low GC content and showed conserved and unique features with other plant RBR genes. ZmRBR2 gene promoter-GUS fusions, introduced in Arabidopsis thaliana plants, drove high GUS activity in cotyledons, the shoot apical meristem and in flowering tissue, especially in anthers and pollen. ZmRBR2 transcript levels were consistently 50% lower in several tissues than those of ZmRBR1, suggesting a differential regulation of these two genes. Our results are consistent with the ubiquitous tissue distribution and function of plant RBR proteins and emphasize differences between the monocot and dicot RBR pathways.