Until recently, most intercellular communication in plants has been explained by the signaling and perception of non-peptide plant hormones. It is, however, becoming increasingly clear that plant cell communication also makes use of small peptide signals. One example is the IDA (INFLORESCENCE DEFICIENT IN ABSCISSION) peptide that is believed to signal through the receptor-like kinases HAESA (HAE) and HAESA-LIKE2 (HSL2) to induce the cell separation process necessary for floral organ abscission. Several genes closely related to IDA and HAE have been identified, and it is conceivable that the IDL (IDA-LIKE) proteins interact with HSL proteins to control different cell-separation processes. A main aim of this thesis has been to match the expression of IDL and HSL genes as a first approach to identifying putative ligand-receptor pairs. Furthermore, the roles of IDA, HAE and HSL2 in root development have been examined by mutant studies, and finally, the interaction between IDA and HAE, HSL1 and HSL2 has been investigated using yeast direct mating and a genetic approach.
Based on overlapping promoter:reporter gene expression patterns, several novel IDL-HSL ligand-receptor pairs have been suggested in this thesis. HSL2 may for instance influence the growth of the main root together with IDL1. Furthermore, IDA has been shown to facilitate the emergence of lateral roots, which also involves a cell-separation process. No direct, biochemical interaction between IDA and HAE, HSL2 or the closely related HSL1 was detected in a yeast direct mating experiment. However, by performing a genetic study it was revealed that IDA has a higher in vivo affinity for HAE than for HSL2. Finally, a genotype lethal during seed development in the presence of ectopic IDA expression was identified that may explain the strongly reduced fertility seen in plants overexpressing IDA.