Transcriptome changes of tomato internode elongation induced by far-red light 3 57 3.1 INTRODUCTION Shade is one of the most common environmental challenges plants come across throughout their development. Plants’ morphology changes in response to neighbor cues, such as enrichment of far-red (FR) light in order to escape impending shade. This phenomenon, which is called shade avoidance syndrome (SAS), is observed in many plants and includes examples such as elongation of the petiole and the internodes in mustard (Sinapis alba) in response to local FR exposure (Child and Smith, 1987; Casal and Smith, 1988), and elongation of the hypocotyl in oil seed rape (Brassica rapa) in response to FR (Procko et al., 2014). In tomato (Solanum lycopersicum cultivars Moneymaker and M82), whole-plant FR treatment promotes shoot growth as described in Chapter 2 of this thesis. Stem, the plant’s support organ connecting leaves and flowers, functions in transportation of water, minerals, and other nutrients from roots to leaves, where they can be used for photosynthesis and plant growth (Esau, 1953). In dicotyledonous plants, the stem is the major vertical shoot which has maintenance, support, and nutrient transport functions. Generally, dicot stems support their longitudinal (primary) growth through radial (secondary) growth. Most dicots have two major transitions in stem growth. The first transition is from primary growth that mainly happens in shoot and root apical meristems to secondary growth which is driven by vascular (pro)cambium, a lateral meristem (Tonn and Greb, 2017) The second transition is from radial growth to extensive wood formation driven by another lateral meristem, cork cambium (Sanchez et al., 2012). During secondary growth, plants remodel the structures of their vascular tissues via vascular cambium (Savidge, 1983; Friml et al., 2002; Randall et al., 2015; Ragni and Greb, 2018) Cell division, expansion, and differentiation in secondary growth are coordinated by a complex network of hormones and other signals. Woody plants usually have a hard stem that continues secondary growth every year, such as evergreen species and deciduous trees, leading to very thick stems. Some dicots undergo secondary growth but have a limited scope, for example, tomato (Solanum lycopersicum) (Figure 3.1), potato (S. tuberosum), and Brassicaceae such as Arabidopsis (Moazzeni et al., 2014; Hardwick and Elliott, 2016)
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