Exploring conservation of cellular-level traits in shade avoidance syndrome among species 5 163 5.1 INTRODUCTION Shade avoidance syndrome is a prevalent response observed in various species such as tomato, as described at length in Chapters 2 to 4. Shade tolerance levels differ among species, but the full extent of diversification remains unclear. In this Chapter. we expanded our work to include a broader spectrum of dicotyledonous species and investigate the conservation of transcription factor responsiveness to far-red (FR), unveiling some underlying conserved mechanisms of the shade avoidance syndrome response. Plants display shade avoidance responses that help them reach the light at high planting density, upon light-mediated detection of neighbors. Plants have evolved mechanisms to sense changes in light quality and adjust their growth strategy accordingly. Phytochromes serve as key indicators by measuring the ratio of red to far-red light, allowing plants to detect whether they are, or soon will be, in the shade of neighboring plants (GreenTracewicz et al., 2011; Pierik and De Wit, 2014; Ranade et al., 2019). In response to FR enrichment, plants adopt various common and well-studied growth strategies, termed shade avoidance syndrome (SAS). This response involves a suite of morphological changes, including the elongation of petioles and stems, alterations in flowering time, and a reduction in chlorophyll content (Green-Tracewicz et al., 2011; Pierik and De Wit, 2014; Ranade et al., 2019). The SAS represents a strategy of considerable adaptive significance in plants, particularly in dense natural communities, where increased shoot elongation can enhance relative fitness (Schmitt, 1997). However, it can also result in reduced crop productivity as plants invest their resources to growth in non-harvestable parts of the plant at the cost of the harvestable parts. In the preceding chapters, we delved into the cellular (Chapter 2) and transcriptomic changes (Chapter 3) occurring in the stem, with a focus on the transcription factors (TFs) associated with these changes. In this chapter, we will compare the FR-responses of different dicotyledonous species and look into the conservation of the FR-responsiveness of the transcription factors that we identified from tomato RNAseq data in Chapter 3. To provide a better framework to consider shade avoidance in an evolutionary context, we will first provide an overview of relevant background, starting with the ecological aspects of playing into shade avoidance and its adaptive value in different environments. 5.1.1 Shade avoidance, shade tolerance and their evolution Exploring the dynamic strategies of plant species in different ecosystems reveals contrasting approaches to competing for light. Pioneer species and shade-tolerant species
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