Hormone interplay in the regulation far-red-responsive stem elongation in tomato 4 109 4.1 INTRODUCTION Plants exhibit remarkable plasticity in response to their environment, and light perception is a critical factor influencing growth, development, and reproductive success, ultimately affecting plant fitness. The availability and quality of light are influenced by neighboring plants, leading to changes in the light spectrum, particularly the red to far-red (R:FR) light ratio. This phenomenon, known as shade avoidance syndrome (SAS), triggers a series of morphological changes that enable plants to optimize light capture and photosynthesis (Casal, 2012). Photoreceptors, such as cryptochromes and phytochromes, detect these light signals and initiate complex hormone pathways, contributing to the SAS. In our investigation, we focus on the SAS triggered by low R:FR conditions. 4.1.1 Role of hormones in shade avoidance Plants have developed intricate physiological reactions to shade, with various phytohormones playing a crucial role in the shade avoidance response. Among these, auxin, gibberellins (GAs), and brassinosteroids (BRs) have been identified as vital for controlling the elongation of hypocotyls induced by shading (Yang and Li, 2017). Auxin plays a pivotal role. Auxin is well known as a very important growth hormone in plant growth. The Dutch biologist Frits Went started the study of the secrets behind plant growth regulation by hormones in the 1920s. One of his early research topics was the influence of the plant tissues surrounding embryonic leaves on plant growth and development. Upon removal of these, plants were much shorter. When looking into the hormones involved, he confirmed the existence of auxin, a growth-promoting hormone (Went, 1928). Auxin is involved in a myriad of plant processes, including organ patterning, tropic responses, vascular development, and growth regulation (Davies, 2010). In Arabidopsis thaliana, a defective auxin biosynthesis taa1 gene was shown to impair the SAS, highlighting the significance of auxin in this process (Tao et al., 2008). In Arabidopsis, an increased concentration of auxin was found to hypocotyl and petiole elongation (Kohnen et al., 2016a; van Gelderen et al., 2018; Küpers et al., 2018), as well as hyponasty (Pantazopoulou et al., 2017, PNAS). Similarly, gibberellin (GA), initially discovered in rice plants with elongated seedlings due to fungal infection in an 1898 paper published by Shotaro Hori, also contributes to the shade avoidance response by promoting stem elongation through the degradation of DELLA proteins, which are growth repressors by inhibiting a set of growth-promoting transcription factors (Hedden and Sponsel, 2015). In earlier research, the treatment with IAA and GAs could stimulate stem elongation in pea (Pisum sativum) (Yang et al., 1996).
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