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Analysis of evolutionary conserved signalling pathways mediating polar leaf development in dicotyledonous plants

Final Report Summary - EVOSIGNAL (Analysis of evolutionary conserved signalling pathways mediating polar leaf development in dicotyledonous plants)

The body plan of seed plants consists of two distinct systems, the shoot system and the root system. Both the root and the shoot system harbor populations of stem cells that divide asymmetrically and thus produce the cells required for the formation of new organs. Leaves derive from the shoot system. During early development a pre-pattern is established dividing the young leaf (which is termed primordium) into an adaxial (future upper leaf side) and an abaxial (future lower leaf side) domain. Outgrowth, and correct tissue organization along the adaxial/abaxial axis depends on the juxtaposition of adaxial and abaxial tissue. Several genes have been isolated that are involved in specifying either adaxial or abaxial cell fate. Using the model species Arabidopsis thaliana, we have started to unravel the downstream target genes of the adaxial specifyer REVOLUTA (REV) and the abaxial specifyer KANADI1 (KAN1). In order to identify evolutionary conserved target genes, inducible versions of REV and KAN1 were also introduced into Cardamine hirsuta, another Brassicaceae species and are currently being transformed into Brachypodium distanchion, a model monocotyledonous plant species. Using a combination of mRNA-Seq and ChIP-Seq we aim to identify the set of evolutionary conserved leaf regulators in the near future.
In Arabidopsis thaliana, we were the first to describe the sets of genes regulated by the REVOLUTA/KANADI1 module on a genome-wide basis. Our analysis revealed targets underlying control by either KANADI1 or REVOLUTA and a set of genes underlying regulation by both REV and KANADI1. REV is a transcriptional activator and these targets are thus active in adaxial tissue. Since KAN1 acts as a transcriptional repressor, KAN1 targets are actively repressed in abaxial tissue. Thus REV and KAN1 act to establish gradients of gene expression along which tissue differentiation occurs. The analysis of REV targets revealed several target genes that are involved in the shade avoidance response pathway. We could show that REV promotes growth in response to shade while KAN1 counteracts this growth promotion and strongly suppresses shade growth. Using RNA-Seq, we could show that a plethora of growth promoting factors is actively repressed by KAN1. Current work is focusing on the downstream pathways regulated by REV and KAN1. Once Brachypodium transgenic plants are available, we will investigate which genes the REV/KAN1 module controls in the monocot system. Furthermore, we have laid the foundation to study physiological processes controlled by REV/KAN1 in Arabidopsis. This enables us to functionally assess whether the regulation of evolutionary conserved target genes likely affects the same physiological responses in the monocot system.