Before they reach the soil surface, young seedlings shunt resources to hypocotyl (embryonic stem) elongation at the expense of cotyledon (seed leaf) growth, a phenomenon known as skotomorphogenesis (see Stepanova and Alonso). After seedlings break the surface, hypocotyl elongation slows while cotyledon growth accelerates, a mode of development called photomorphogenesis. The hormone ethylene promotes skotomorphogenesis, and light inhibits ethylene responses to promote photomorphogenesis. In the dark, the E3 ubiquitin ligase constitutive photomorphogenic 1 (COP1) promotes the activity of the ethylene-responsive transcription factor ethylene insensitive 3 (EIN3) by stimulating degradation of two ubiquitin ligase complex components that target EIN3 for degradation, EIN3-binding F-box 1 and 2 (EBF1 and EBF2). As seedlings are exposed to dim light near the soil surface, a gradual decrease in COP1 abundance reduces ethylene signaling. Shi et al. used a combination of genetic experiments, in planta colocalization assays, coimmunoprecipitation, and in vitro biochemical and binding assays to delineate a role for the red light photoreceptor phytochrome b (phyB) in the rapid inhibition of ethylene signaling in Arabidopsis thaliana. Activation of phyB by red light caused phyB to bind to both EIN3 and EBF1 or EBF2 in the nucleus, triggering the degradation of EIN3. Red light and the binding of phyB to EIN3 and EBF1 or EBF2 were required for the transition from skotomorphogenesis to photomorphogenesis. Furthermore, phyB-induced, EBF-dependent degradation of EIN3 was rapid, occurring within 30 minutes of exposing dark-grown seedlings to light. Phytochrome-interacting factors (PIFs), the canonical targets of phyB, are also involved in regulating skotomorphogenesis, implying that phyB may regulate the switch to photomorphogenesis through multiple targets. Thus, in addition to the gradual inhibition of ethylene signaling mediated by a decrease in COP1 abundance during hypocotyl growth, the rapid inhibition of ethylene signaling mediated by phyB enables seedlings to adopt a photomorphogenetic mode of growth as soon as they break the soil.
H. Shi, X. Shen, R. Liu, C. Xue, N. Wei, X. W. Deng, S. Zhong, The red light receptor phytochrome B directly enhances substrate-E3 ligase interactions to attenuate ethylene responses. Dev. Cell 39, 597–610 (2016). [PubMed]
A. N. Stepanova, J. M. Alonso, Cutting out the middle man in light-hormone interactions. Dev. Cell 39, 524–526 (2016). [PubMed]