Floral organ abscission is regulated by a positive feedback loop

Abscission is the process by which plants shed unwanted organs, either as part of a natural developmental program or in response to environmental stimuli. Studies in Arabidopsis thaliana have elucidated a number of the genetic components that regulate abscission of floral organs, including a pair of related receptor-like protein kinases, HAESA and HAESA-like 2 (HAE/HSL2) that regulate a MAP kinase cascade that is required for abscission. HAE is transcriptionally up-regulated in the floral abscission zone just before cell separation. Here, we identify AGAMOUS-like 15 (AGL15; a MADS-domain transcription factor) as a putative regulator of HAE expression. Overexpression of AGL15 results in decreased expression of HAE as well as a delayed abscission phenotype. Chromatin immunoprecipitation experiments indicate that AGL15 binds the HAE promoter in floral receptacles. AGL15 is then differentially phosphorylated through development in floral receptacles in a MITOGEN-ACTIVATED PROTEIN KINASE KINASE 4/5-dependent manner. MAP kinase phosphorylation of AGL15 is necessary for full HAE expression, thus completing a positive feedback loop controlling HAE expression. Together, the network components in this positive feedback loop constitute an emergent property that regulates the large dynamic range of gene expression (27-fold increase in HAE) observed in flowers when the abscission program is initiated. This study helps define the mechanisms and regulatory networks involved in a receptor-mediated signaling pathway that controls floral organ abscission.