Abstract
Vegetative propagation relies on the capacity of plants to regenerate de novo adventitious roots (ARs), a quantitative trait controlled by the interaction of endogenous factors, such as hormones and environmental cues among which light plays a central role. However, the physiological and molecular components mediating light cues during AR initiation (ARI) remain largely elusive. Here, we explored the role of red light (RL) on ARI in de-rooted Norway spruce seedlings. We combined investigation of hormone metabolism and gene expression analysis to identify potential signaling pathways. We also performed extensive anatomical characterization to investigate ARI at the cellular level. We showed that in contrast to white light, red light promoted ARI likely by reducing jasmonate (JA) and JA-isoleucine biosynthesis and repressing the accumulation of isopentyl-adenine-type cytokinins. We demonstrated that exogenously applied JA and/or CK inhibit ARI in a dose-dependent manner and found that they possibly act in the same pathway. The negative effect of JA on ARI was confirmed at the histological level. We showed that JA represses the early events of ARI. In conclusion, RL promotes ARI by repressing the accumulation of the wound-induced phytohormones JA and CK.
Highlights
Vegetative propagation through stem cuttings is widely used in breeding programs for propagation of elite genotypes
Three-week-old de-rooted seedlings were treated with 1 or 5 μM of Indole 3-Acetic Acid (IAA), 1, 5 or 10 μM 1-Naphthalene 3-Acetic Acid (1-NAA) or Indole 3Butyric Acid (IBA), but none of the exogenously applied auxins could induce AR initiation (ARI) under constant White Light (cWL) conditions (Figures 1A– C). These results indicate that auxin is not sufficient to induce ARI in de-rooted Norway spruce seedlings kept under cWL
In the case of Norway spruce de-rooted hypocotyls, our data suggest that the positive effect of red light (RL) is unlikely due to an increase of auxin content since the de-rooted seedlings kept under constant Red Light (cRL) accumulated less indole acetic acid (IAA) compared to those kept under cWL during the early stages of ARI
Summary
Vegetative (or clonal) propagation through stem cuttings is widely used in breeding programs for propagation of elite genotypes. Successful clonal propagation requires de novo regeneration of adventitious roots (ARs), a process governed by a multitude of developmentally programmed and environmentally induced cues including wounding, light, and nutrient availability (Bellini et al, 2014; Steffens and Rasmussen, 2016). Phytohormones, mainly auxin, jasmonate (JA), cytokinins (CKs), ethylene, Light Control of Adventitious Rooting and abscisic acid (ABA), play major roles in AR formation. It has been shown that there is interplay among these phytohormones in complex networks and loops that ensure proper AR development (Lakehal and Bellini, 2019). It remains difficult to address the molecular mechanisms controlling AR formation in perennial species, the rapid development of sequencing technologies giving access to most genome and transcriptome sequences offers the possibility of further exploring the degree of evolutionary conservation or divergence of the known molecular networks
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