Abstract
Plants both generate and shed organs throughout their lifetime. Cell separation is in function during opening of anthers to release pollen; floral organs are detached after pollination when they have served their purpose; unfertilized flowers are shed; fruits and seeds are abscised from the mother plant to secure the propagation of new generations. Organ abscission takes place in specialized abscission zone (AZ) cells where the middle lamella between adjacent cell files is broken down. The plant hormone ethylene has a well-documented promoting effect on abscission, but mutation in ethylene receptor genes in Arabidopsis thaliana only delays the abscission process. Microarray and RNA sequencing have identified a large number of genes differentially expressed in the AZs, especially genes encoding enzymes involved in cell wall remodelling and disassembly. Mutations in such genes rarely give a phenotype, most likely due to functional redundancy. In contrast, mutation in the INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) blocks floral organ abscission in Arabidopsis. IDA encodes a small peptide that signals through the leucine-rich repeat receptor-like kinases HAESA (HAE) and HAE-LIKE2 (HSL2) to control floral organ abscission and facilitate lateral root emergence. Untimely abscission is a severe problem in many crops, and in a more applied perspective, it is of interest to investigate whether IDA-HAE/HSL2 is involved in other cell separation processes and other species. Genes encoding IDA and HSL2 orthologues have been identified in all orders of flowering plants. Angiosperms have had enormous success, with species adapted to all kinds of environments, adaptations which include variation with respect to which organs they shed. Here we review, from an evolutionary perspective, the properties of the IDA-HAE/HSL2 signaling module and the evidence for its hypothesized involvement in various cell separation processes in angiosperms.
Highlights
Cell separation processes, especially abscission, are in general of great importance in plants, in relation to reproduction, development and adaptation to the environment, but the sites of cell separation differ between species
Abscission is a severe problem in many crops, and in a more applied perspective, it is of interest to investigate whether IDA-HAE/HSL2 is involved in other cell separation processes and other species
The abscission process can be divided in four stages, namely formation of the abscission zone (AZ), conditioning of the AZ so it can respond to signaling molecules, cell wall loosening followed by the actual cell separation and generation of a protective layer [2]
Summary
Especially abscission, are in general of great importance in plants, in relation to reproduction, development and adaptation to the environment, but the sites of cell separation differ between species. During the last couple of decades the invention of technologies for genome-wide determination of gene expression levels, such as microarrays and, later, RNA sequencing, have facilitated a transcriptomic approach to understand these processes This approach gives unbiased access to the gene expression pattern in a given organ at a given time. The genetic approach, that is, the identification of mutants impaired in the relevant process, can pick up important genes with low activity From this perspective we review the evidence for the importance of the signaling peptide IDA and its receptors in different cell separation processes both above and below ground in Arabidopsis, and in abscission of diverse organs in a variety flowering plants. Control of Cell Separation Processes by IDA and IDA-LIKE Peptides in Arabidospsis
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