Cell-to-cell communication is a vital characteristic of multicellular organisms, making it possible for cells to coordinate their physiological behaviour. Small peptides may act as signalling molecules that control processes such as growth, differentiation and response to the environment. When such a ligand interacts with a receptor, it can trigger downstream effects like intracellular responses or change of gene expression. Recently, a novel group of putative peptide ligands, the IDA-LIKE (IDL) proteins was discovered, based on their similarity to IDA. IDA is a putative ligand involved in floral abscission (Butenko et al. 2003). For this thesis, the IDA-LIKE genes AtIDL4 and AtIDL5 in the model plant Arabidopsis thaliana have been studied.
Histochemical analysis of promoter-GUS constructs for IDL4 and IDL5 have been performed in transgenic Arabidopsis plants. Promoter activity showed gene function in a wide range of tissues to be probable. Noticeably the IDL4 promoter was activated in tissues throughout the life span of the plant, whereas IDL5 activity was shown exclusively in young parts.
An insertional mutant line with an immobilised transposable element situated in the IDL5 coding region was analysed. No striking phenotype was observed, but microarray expression profiling was undertaken in search for molecular phenotypes.
Over expression of the two genes led to similar aberrant phenotypes. Plants were generally smaller, and had curled leaves that appeared water-soaked. They also showed increased guttation and white, crystalline patches often appeared on leaf margins. Some transformants developed abnormal siliques and displayed reduced fertility. A minority of the individual plants over expressing IDL5 developed enlarged floral abscission zones and cells there were covered in a white substance.
One could tentatively suggest that the IDL genes show activity at sites where cell separation processes occur, however expression was not restricted to such regions and this hypothesis would require further assessment. IDL4 and IDL5 may also be involved in the formation and/ or function of hydathodes. Both genes’ promoter showed activity in these pores, and the gain-of-function mutants displayed increased guttation. Additional analyses will be required to further understand the functions of the IDL genes.