Chapter 4 130 4.2.10 Inhibiting GA signalling abolished FR-responsive stem elongation After showing that GA application can mimic supplemental FR-induced elongation, we wanted to see if GA was necessary for the FR-responsive stem elongation. In this experiment, we used paclobutrazol (PBZ), a known antagonist of gibberellin, acting by inhibiting gibberellin biosynthesis (Kamoutsis et al., 1999). After trial of different application methods, namely brushing (Figure S4.9) and soil penetration (Figure 4.11), we established a working protocol. With the soil penetration treatment, we observed that blocking GA synthesis in supplemental FR led to WL-like internodes, thus indicating that GA is required for the FR-responsive stem elongation (Figure 4.14). 4.2.11 Exogenous brassinosteroid treatment induced elongation comparable to FR Although the RNAseq data showed only mild effects on the expression of brassinosteroidrelated genes, Auxin and brassinosteroids (BR) are interconnected in numerous growth processes, with cell elongation serving as the driving force behind hypocotyl elongation for both (Keuskamp et al., 2010b; Keuskamp et al., 2012). BR are known to play a crucial role in normal plant growth and are essential for shade avoidance responses in Arabidopsis seedlings (Asami et al., 2000). First, we tested effects of different concentrations of BR in WL (Figure 4.16). In this experiment, we utilized brassinolide as a representative of BR. The treatment commenced with brushing the internode of 14-day-old seedlings. With a high concentration of BR, the elongation observed in the BR-treated internode reached a level comparable to that induced by supplemental FR (Figure 4.16). BR also triggered cell elongation in correspondence with the changes in concentration (Figure 4.17).
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