| Previous work has identified many genes involved in the transcriptional regulation of floral organ identity specification in Arabidopsis. Among them, AGAMOUS (AG) encodes a key transcription factor acting in the center of flowers to specify the identities of the two types of reproductive organs, stamen and carpel, and to maintain the determinant state of the floral meristem. Mutations in AG result in severe consequences in plant reproductive development: stamen-to-petal transformation in the third whorl, carpel-to-sepal transformation in the fourth whorl, and loss of floral determinacy such that additional flowers appear in the center of flowers.; To better understand the underlying molecular mechanism governing the AG pathway, two enhancer mutagenesis screens have been carried out to look for additional components in this pathway. Several genes have been identified and appear to act redundantly in the AG pathway because only triply combined mutations in any of these genes result in strong ag-like floral homeotic and floral determinacy phenotypes.; HUA1 encodes a protein with a nuclear localization sequence (NLS) and six CCCH-type zinc fingers, and was demonstrated to be a nuclear RNA-binding protein, which for the first time suggested the importance of posttranscriptional regulation in floral organ identity specification. Indeed, more genes identified from the enhancer screens are involved in RNA metabolic events in flower development. HUA ENHANCERS (HEN5)/PAUSED (PSD) codes for the Arabidopsis orthologue of tRNA exportin in humans, which is a receptor for the nuclear export of tRNAs. Mutations in PSD results in reduced AG protein abundance presumably due to inefficient protein synthesis caused by limited tRNA abundance in the cytoplasm. HUA ENHANCER1 (HEN1) was demonstrated to be an essential component in the plant microRNA (miRNA) pathway and acts in the AG pathway by controlling the expression of a miRNA to regulate a floral homeotic gene, APETALA2 (AP2). HEN1 acts in the small RNA pathway by protecting the 3 ends of miRNAs and siRNAs. These studies revealed the wide utilization of posttransciptional regulation in plant development. |
