Abscission is a developmental program that leads to the separation of specific organs from the body of a plant. The objective of the our research is to elucidate the gene regulatory networks that control abscission of Arabidopsis floral organs. Currently, we have a general understanding of the genes and proteins involved in abscission but know little about how they integrate the physiological processes required for organ separation. Abscission includes the production of hydrolytic enzymes that are secreted in a specialized layer of cells, known as the abscission zone (AZ). The AZ consists of a few layers of small, cytoplasmically dense cells at the boundary between an organ and the plant body. Secretion of AZ hydrolytic enzymes results in hydrolysis of the pectin-rich middle lamella, weakening of the cell wall, and subsequent separation of the abscising organ.
Development of the AZ and activation of the abscission program proceeds throughout multiple stages of floral development. Establishment of the AZ cells at the boundary between a developing floral organ and the receptacle occurs early in Arabidopsis floral development. Expression of enzymes required for dissolution of the middle lamella is activated at later stages of development. HAESA (HAE) and HAESA-LIKE 2 (HSL2), which encode functionally redundant receptor-like protein kinases (RLK), are required for abscission. Double hae hsl2 mutant plants fail to abscise their floral organs (Figure). Expression of these hydrolytic genes requires, at least in part, establishment of the HAE HSL2 signaling pathway. Some components of this signaling pathway (e.g., HAE, HSL2, IDA) are transcriptionally activated in the AZ around the time of anthesis but prior to expression of the AZ hydrolytic enzyme genes. Meta-analysis of expression profiling studies also suggests both HAE HSL2-dependent and independent processes are involved in abscission. We hypothesize that multiple gene regulatory networks are involved in establishment of AZ competence and subsequent activation of the abscission program (e.g., AZ hydrolase gene expression). To begin to understand the emergent properties of the molecular networks that underlie physiological processes regulating plant development and environmental responses we are investigating the gene regulatory networks that establish the HAE HSL2 abscission program and developing computational models that predict the regulatory networks controlling abscission.