Chapter 4 112 directional auxin flow from cell to cell. These in turn prevent auxin-induced functions in plant tissues (Křeček et al., 2009; Abas et al., 2020). BBo is a chemical which inhibits auxin biosynthesis by targeting YUCCA (YUC). YUC-type flavin-containing monooxygenases (FMO) facilitate the oxidative decarboxylation of IPyA to generate IAA (Zhao et al., 2001; Zheng et al., 2013). BBo suppresses recombinant YUC activity in vitro and thus likely decreases endogenous IAA content (Kakei et al., 2015). PEO-IAA is an auxin antagonist that competitively binds the auxin receptor, TIR1/AFB proteins (TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALING F-BOX PROTEINS) (Šenkyřík et al., 2023). To study gibberellins (GAs), we use gibberellic acid 3 (GA3) as the exogeneous application, and we use paclobutrazol (PBZ) as an inhibitor of gibberellin signaling. PBZ hinders plant growth by inhibiting GA synthesis, as it blocks the oxidation of ent-kaurene to ent-kauronoic acid. This interference occurs through the inactivation of cytochrome P450-dependent oxygenase (Gallardo et al., 2002; Desta and Amare, 2021). And finally, we studied brassinosteroids (BRs) which play a crucial role in normal plant growth and are essential for shade avoidance responses in Arabidopsis seedlings (Asami et al., 2000). To test the effects of BRs, we treated plants with brassinolide- the active BR form. To inhibit the brassinosteroid signalling, we investigated the effects of brassinazole (BZ), an inhibitor of brassinosteroids biosynthesis. By exploring the interactions and effects of these exogenous hormone treatments and inhibitors, we aim to deepen our understanding of the hormonal regulation underlying the shade avoidance response in tomato stems and shed light on the molecular mechanisms governing their growth and development in varying light conditions. In this chapter, our primary focus was to study the roles of auxin, brassinosteroids and gibberellins in tomato stem elongation responses to light. We used a pharmacological approach and applied the chemicals described above. As these treatments were not previously published on tomato, we first optimized the chemical application methods, precise timings and frequencies of treatment, and chemical concentrations. Then, we sought to recapitulate the intricate FR response by applying one or multiple hormones or hormone inhibitors. Our aim was to construct a hormone network of SAS in tomato, and indeed, we found evidence for all three hormones in FR-responsive tomato stem elongation.
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