ENOD40 affects phytohormone cross-talk

Only a small number ofphytohormonesare sufficient to regulate virtually every aspect of plant growth and development. Hence, adequate communication betweenphytohormonesis important to coordinate cellular processes during development. In this thesis we show that the plant gene ENOD40 plays a role in the communication betweenphytohormonesignaling pathways.Mainly on the basis of its complex expression patterns, it has been suggested that ENOD40 could affect thephytohormonestatus of cells, thus participating in regulation of plant growth and development but direct evidence was lacking. We have developed bioassays to study the function of ENOD40 in relation tophytohormonesignaling, using tobacco BY-2 cell-cultures. In these filamentous cell suspensions, quantification of cell-growth and cell division is straightforward and these morphological growth parameters can be used to monitor responses tophytohormones. In wild-type cells that have no or very low levels of ENOD40 expression, both processes are by-en-large independent of each other. By quantifying the parameters cell-elongation growth and cell division frequency under variousphytohormoneregimes in transgenic, 35S: NtENOD40 containing, tobacco BY-2 cell suspensions, we showed that the relative rates of both cellular processes become coupled byoverexpressionof ENOD40 . Since elongation growth is regulated by ethylene and cell division by thecytokinin/auxinratio, our data indicate that ENOD40 is involved in cross-talk between ethylene,auxinandcytokinin.Furthermore, we present an overview of ENOD40 expression patterns and analyze the structure of ENOD40 transcripts. The transcripts are characterized by two short conserved nucleotide sequence regions, which both contain biological activity in the bio-assay, and one of the two regions encodes a remarkably shortoligopeptide.In conclusion, the observation that ENOD40 affectsphytohormoneresponses in BY-2 cells is consistent withspatio-temporal ENOD40 expression patterns, which correlates with developmental stages involving the activity ofphytohormones. We propose that coupling between cell division, cell growth and cell differentiation may be achieved by transient and local expression of plant factors like ENOD40 that are involved inphytohormonecross-talk pathways. The coupling of hormone responses may be crucial to many developmental programs, and this may explain the complexity of ENOD40 expression patterns and the variety of tissues with which ENOD40 expression is associated in different plant species.