Figure 5.

The Seedling Emergence Signaling Networks Regulated by Light and Mechanical Pressure.

(A) Upon germination deep in the soil, the seedlings covered by soil face two stress stimuli: darkness and mechanical pressure. The active COP1/SPA complexes promote skotomorphogenesis by regulating three transcription factors (HY5, PIFs, and EIN3). In addition, mechanical impedance boosts endogenous ethylene accumulation, which induces crosstalk between ethylene and light signals.

(B) During the course of hypocotyl elongation under soil, dim light penetrates into soil and reduces the activity of COP1/SPA complexes. However, ethylene responses remain highly variable depending on soil conditions.

(C) After breaking out of the soil, seedlings free from mechanical pressure and under sunlight switch to photomorphogenic development. The activity of COP1/SPA complexes is completely repressed.