Chapter 6 208 This thesis explores the interplay between plants and their environment, with a specific focus on shade avoidance syndrome (SAS). This behavior in plants is triggered by the detection of a shift in the red (R) to far-red (FR) light ratio, typically indicating competition and impending shading by neighboring plants. Plants are sessile organisms; they are unable to walk away from an unfavorable situation. Because of their inability to move, they have evolved to acclimate to changes in their environment. Plants convert sunlight into energy that can be used by cellular processes, and it is the main source of energy for the plant. So, light is an environmental factor that influences a plant’s function to a great extent. Light availability and composition can alter a plant’s phenotype and photosynthetic capabilities (Kami et al., 2010). A changing light environment can be detected directly via light intensity, or by far-red or blue light enrichment depending on environmental circumstances and the plant species (Navrátil et al., 2007), and a widely researched of these is the supplemental far-red (FR) response. The FR response is initiated when plants grow with an increased abundance of FR light about red light, which happens when a plant grows near or underneath a canopy of leaves, creating a change in light composition (Navrátil et al., 2007). Overviews of this response are widely available as it has been researched extensively (Bongers et al., 2014; Courbier and Pierik, 2019; Wang et al., 2020) and will therefore only be touched upon briefly in this thesis. This thesis expands the knowledge of SAS into the crop species tomato, as understanding shade avoidance mechanisms outside of model species is important to see if findings are translatable to support global food production, in this case, tomato cultivation. In Chapter 2 we characterized the phenotypic changes in juvenile tomatoes in response to FR supplementation. We identified the 1st internode as a region that responded strongly by both elongation and radial growth. We then zoomed into its cellular morphology to discover strong changes in the pith parameters. We wanted to identify molecular processes underlying this phenotypic plasticity, so in Chapter 3 we carried out RNAseq profiling of internode 1 and its pith in response to FR treatment. We discovered gene expression changes that could be auxin-induced, so in Chapter 4, we delved into the hormone dynamics governing neighbor detection responses. While we started with a focus on auxin (IAA), we also explored the roles of gibberellins (GA), and brassinosteroids (BR). While auxin exhibited a role in stem elongation, its full impact required the involvement of other actors, potentially GA or BR, to achieve far-red (FR)- like elongation. GA and BR treatments in tomato FR-response induced comparable stem elongation, emphasizing their potency, and both also turned out to be necessary for FR-
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