Plant cell biology: Shedding light on plant growth

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Phytochrome interacting factors (PIFs) coordinate plant development by integrating the light and gibberellin hormone signaling pathways.

Researchers have uncovered how plants can adapt their growth under varying environmental conditions by showing that phytochrome interacting factors (PIFs) are key components of a signalling network that coordinates plant development in response to light and gibberellin (GA) hormones.

Seedlings undergo alternative developmental programmes depending on whether they germinate in the light (known as photomorphogenesis) or dark (known as etiolated growth). During etiolated growth, the primary organ of extension of the young plant (the hypocotyl) elongates, the first leaves of the embryo (the cotyledons) are small and closed with undifferentiated chloroplasts, and light-regulated genes are repressed. By contrast, during photomorphogenesis, hypocotyl growth is inhibited, whereas cotyledon opening and expansion, chloroplast differentiation and the activation of light-regulated genes are promoted. These processes are also regulated by GAs through a crosstalk mechanism that has long been elusive.

To reveal the molecular mechanism underlying the coordinated regulation of plant development by light and GAs, two research teams analysed the in vivo function of DELLA proteins, which are known GA-signalling repressors. Feng et al. treated transgenic plants that expressed DELLA proteins with an active form of GA and confirmed previous observations that GA promotes the degradation of DELLA proteins through the ubiquitin–proteasome pathway. Using chromatin immunoprecipitation (ChIP) assays, the authors also confirmed that DELLA proteins do not function by directly binding to GA-responsive genes, thereby suggesting that these factors control gene expression indirectly by regulating the function of other transcription factors.

Both groups independently identified PIF3 and PIF4 as DELLA-interacting factors. Using a transient expression system, de Lucas et al. showed that DELLA proteins repress PIF4-mediated gene expression and that treatment with GA ameliorates the DELLA-mediated inhibitory effect.

Next, the two teams carried out ChIP analysis on seedlings that had been treated with an inhibitor of GA biosynthesis to induce DELLA protein accumulation. The binding of PIF3 and PIF4 to their target genes was inhibited, and overexpression of PIF4 rescued the hypocotyl growth restraint induced by DELLA accumulation.

So, in the absence of GA, DELLA proteins interact with PIF proteins and prevent them from binding to their target genes and regulating their expression. As a result, PIF-mediated control of hypocotyl elongation is inhibited. By contrast, in the presence of GA, DELLA proteins are degraded and cell elongation can occur. The authors propose that PIFs function as an integration node for light and GA signalling pathways, thereby providing a regulatory mechanism by which plants adapt their growth to changing environmental conditions.

Francesca Cesari

References

de Lucas, M. et al. A molecular framework for light and gibberellin control of cell elongation. Nature 451, 480–484 (2008) | Article | PubMed |
Feng, S. et al. Coordinated regulation of Arabidopsis thaliana development by light and gibberellins. Nature 451, 475–479 (2008) | Article | PubMed |
Reddy, G. V., Gordon, S. P. & Meyerowitz, E. M. Unravelling developmental dynamics: transient intervention and live imaging in plants. Nature Rev. Mol. Cell Biol. 8, 491–501 (2007) | Article | PubMed |