General introduction 23 1 Figure 1.7. The cell types of tomato stem. The diagram shows combined cross section and longitudinal section, both taken from the middle of the first internode. In SAS research, some attention has been on how cellular morphology changes in response to shade or low R:FR. The most well-known hypothesis is epidermal growth-control: the epidermal layer is controlled by the tension and stress coming from the expansion of the inner layers of the hypocotyl in the stage of unidirectional growth (Kutschera and Niklas, 2007; Robinson and Kuhlemeier, 2018). Epidermis cell numbers and cell length increase in the petiole in response to shade, as found in Trifolium repens (Huber et al., 2008). In a soybean study, pith cell elongation in particular was related to internode elongation, as well as cell division of all cell types was enhanced to result in internode elongation (Beall et al., 1996). However, there is only limited results from tomato: the cellular morphology changes of stem in SAS have not been characterized previously. 1.4 GENERAL SUMMARY AND OUTLOOKS In this general introduction, it was discussed what are the intricate regulation networks that contribute to the regulation of plant architecture as part of SAS, and the coordination of hormone interplay in shade avoidance in multiple crop species and Arabidopsis was explored. In this thesis, we explore the shade avoidance responses of tomato and other dicots, focusing on stems and the hormonal regulation of internode elongation, involving different cell layers.
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