Small peptides can act as signaling molecules that coordinate development, growth and differentiation. In interaction with a receptor the ligand can trigger downstream pathways which induce cellular responses or regulation of gene expression. Recently, a novel group of putative ligands in plants, the IDA-LIKE (IDL) proteins, were identified based on their similarities to IDA, a putative ligand involved in floral organ abscission (Butenko et al., 2003). In this thesis three of the five AtIDL genes, AtIDL1, AtIDL2 and AtIDL3, have been studied.
Histochemical GUS-staining analyses of promoter-GUS constructs for IDL1, IDL2 and IDL3 in transgenic Arabidopsis thaliana have been performed. In the IDL1::GUS transgenic lines, GUS activity was detected in the two outermost cell layers of the columella root cap and in the epidermal tissues of the root differentiation zone. Based on the root expression pattern of IDL1, roots of transgenic Arabidopsis both downregulating and overexpressing IDL1 were investigated. While downregulation of IDL1 resulted in longer roots, our preliminary results showed that overexpression of IDL1 resulted in shorter roots. The IDL2 promoter directed GUS expression in lateral root caps, shoot meristems, floral organ abscission zones (AZs), and bases of the pedicels. IDL3::GUS activity was observed in roots, buds, floral AZs, and at the bases of the pedicels. Since IDA is involved in the cell separation process that induces the floral organs to be shed, it was interesting to find that the GUS expression of the three IDL genes in many cases was associated with AZs and other zones of cell separation. Overexpression of IDL1, IDL2 and IDL3 resulted in similar phenotypes featured by early senescence of rosette and cauline leaves, premature floral organ abscission, and shedding of organs that are normally not shed in Arabidopsis; pedicels and cauline leaves. SALK lines containing T-DNA insertions upstream of IDL2 and IDL3 have been investigated, and RNAi lines were generated for IDL1 and IDL3. Additional analyses will be needed to further understand the biological functions of the IDL genes.