Chapter 4 114 We found strong foldchange responses to FR in our auxin and GA gene lists, namely in SAUR (small auxin up-regulated RNA) genes and GA oxidase genes, while BR-related genes exhibited a more subdued response (Figure 4.2). SAURs regulate growth by regulation of cell activity, and they are responsive to multiple hormones not limited to auxin (Stortenbeker and Bemer, 2019). GA oxidases catalyse the conversion of active GAs to inactive forms, thereby regulating the levels of bioactive GAs in plants. Gibberellin 2-oxidases (GA2oxs) regulate plant growth by inactivating endogenous bioactive GAs (Lo et al., 2018), while gibberellin 20-oxidase (GA20ox) is responsible for catalyzing consecutive oxidation steps in the late phase of the GA biosynthetic pathway. These observations highlight the sensitivity of these genes to FR conditions. The FR-induced gene expression changes suggest interplay between light signaling and hormone pathways, warranting further exploration into the specific mechanisms governing these responses. 4.2.2 Signal triggering internode 1 elongation might derive from leaf 1 or internode 1 Based on the findings from Chapter 2, we focused our investigation on internode 1, which corresponds to the internode between the cotyledons and the first true leaf. We wanted to dissect the hormone signalling pathway in more detail, but we wanted first to identify what was the location of FR light perception triggering the internode elongation. In previous research on Datura ferox and Sinapis alba, local FR applied only on a leaf can induce stem elongation response in the adjacent internode (Ballaré et al., 1990). Similarly, local FR treatment on tomato can induce a local response from the terminal leaflet (Courbier et al., 2020). First, we conducted a local FR experiment (Figure 4.3) to test which local FR treatments would induce internode 1 elongation. When we exposed internode 1, leaf 1, or the meristem to a localised R:FR=0.01 FR with a spot lamp, we observed elongation in the first internode, akin to the effect seen in the entire plant under FR light. This suggests the mobility of the signal from this localized FR treatment that was not always on the responding tissue.
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