Summary
The survival of seed plants in natural environments requires the successful emergence from the soil. In this process, the ethylene signaling pathway is utilized by plants to sense and respond to the mechanical resistance of the soil. Here, we report that CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1), a central repressor of light signaling, is a key component required for seedlings to sense the depth of soil overlay. Mutation in COP1 causes severe defects in penetrating soil, due to decreased level of EIN3, a master transcription factor in ethylene pathway that mediates seedling emergence. We show that COP1 directly targets the F-box proteins EBF1 and EBF2 for ubiquitination and degradation, thus stabilizing EIN3. As seedlings grow towards the surface, the depth of soil overlay decreases, resulting in a gradual increase of light fluences. COP1 channels the light signals while ethylene transduces the information on soil mechanical conditions, which cooperatively control EIN3 protein levels to promote seedling emergence from the soil. The COP1-EBF1/2-EIN3 module reveals a mechanism by which plants sense the depth to surface and uncovers a novel regulatory paradigm of an ubiquitin E3 ligase cascade.
Summary
Plants germinating under subterranean darkness assume skotomorphogenesis, a developmental program strengthened by ethylene in response to mechanical pressure of soil. Upon reaching the surface, light triggers a dramatic developmental transition termed de-etiolation that requires immediate termination of ethylene responses. Here, we report that light-activation of photoreceptor phyB results in rapid degradation of EIN3, the master transcription factor in ethylene signaling pathway. As a result, light rapidly and efficiently represses ethylene actions. Specifically, phyB directly interacts with EIN3 in a light-dependent manner and also physically associates with F-box protein EBFs. The light-activated association of phyB, EIN3, and EBF1/EBF2 proteins stimulates robust EIN3 degradation by SCFEBF1/EBF2 E3 ligases. We reveal that phyB manipulates substrate-E3 ligase interactions in a light-dependent manner, thus directly controlling the stability of EIN3. Our findings illustrate a mechanistic model of how plants transduce the light information to immediately turn off ethylene signaling for de-etiolation initiation.
In buried seedlings, chloroplasts are arrested at the etioplast stage, but they rapidly mature upon emergence of the seedling. Etioplast-chloroplast differentiation is halted through the integration of soil-induced signals, including pressure and the absence of light, although the details on how this information converges to regulate cellular decisions remain unclear. Here, we identify an interdependent transcription module that integrates the mechanical pressure and darkness signals to control chloroplast development in Mutations of ETHYLENE-INSENSITIVE3 (EIN3), the primary transcription factor in the ethylene signaling pathway that is activated in response to mechanical pressure, cause early development of etioplasts in the dark and severe photobleaching upon light exposure. Genetic studies demonstrate that repression of etioplast differentiation by EIN3 requires PHYTOCHROME INTERACTING FACTOR3 (PIF3), a darkness-stabilized bHLH transcription factor. EIN3 and PIF3 directly interact and form an interdependent module to repress the expression of most () genes; overexpressing even one could cause premature development of etioplasts. The EIN3-PIF3 transcription module synergistically halts chloroplast development by interdependently co-occupying the promoters of genes. Thus, our results define a transcriptional regulatory module and provide mechanistic insight on the concerted regulation of chloroplast development by multiple soil-induced signals.
The amorphous calcium carbonate (ACC) or polycrystalline vaterite, which has long-term water stability and thermal stability, can be induced by bacteria. These biogenic CaCO3 are organo-mineral complexes.
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