Chapter 4 108 ABSTRACT Understanding shade avoidance mechanisms is crucial for global tomato cultivation. This study explored hormone dynamics underlying neighbor detection responses, focusing on auxin (Indole-3-Acetic Acid (IAA)), gibberellins (GA), and brassinosteroids (BR). While the origin of auxin during far-red (FR)-response remained elusive, auxin treatment on the internode and the first true leaf induced some elongation. IAA increased cell elongation and division but failed to recapitulate the far-red-induced internode elongation fully. Auxin inhibitors Peo-IAA and BBo had weak effects. Auxin combined with its polar transport inhibitor NPA had an unexpected effect. These results indicate auxin has a role in regulating stem elongation, but there is a need for other actors, potentially GA or BR, to achieve FR-like elongation. Examining also GA and BR in tomato FR-response showed stem elongation in response to of GA and BR treatment comparable with FR treatment. While IAA-induced transcriptional changes resembled FR-responsive gene expression, GA and BR induced greater elongation, emphasizing their potency. GA or BR inhibition during FR treatment halted elongation, indicating their essential roles. Additionally, inhibition of GA or BR during FR treatment were rescued by BR+IAA or GA+IAA, respectively. In conclusion, this study uncovers some of the hormonal interactions in regulating FRinduced stem elongation in tomato. IAA, GA, and BR play distinct roles. The findings shed light on dynamic hormone networks governing plant growth responses to light signals informing the plant of its neighbors. Further research into molecular mechanisms promises deeper insights into optimizing plant growth across environments.
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