Ethylene Regulates The Stability Of HY5and Hypocotyl Growth In Arabidopsis Thaliana

Plant can not grow without light, besides as energy for photosynthesis, light can also act as asignaling that regulates plant growth and development. In the light conditions, the Arabidopsis seedlingshows photomorphogenesis with shorter hypocotyl, opened and green cotyledon, while in the darkconditions, Arabidopsis seedling displays skotomorphogenesis with elongated hypocotyl, bended top,small and yellow cotyledon. Plant can detect the changes of the external environment, subsequentlytriggering the regulation coordinating with endogenous plant hormones. For example, ethylene, as oneof the plant hormones, can enable the plant to produce the typical "triple response" in Arabidopsisetiolated seedlings, showing the shorter and thicker hypocotyl, the shorter root and the intensified hooktop. In contrast with the dark conditions, ethylene, in the light conditions, promotes the elongation ofArabidopsis hypocotyl. Current research shows that ethylene activates two opposite pathways throughETHYLENE INSENSITIVE3(EIN3) at transcriptional level. One pathway is promoted byPHYTOCHROME INTERACTING FACTOR3(PIF3) to mediate hypocotyl elongation in the light,and the other one is controlled by ETHYLENE RESPONSE FACTOR1(ERF1) to inhibit hypocotylelongation in the dark. In the present investigation, we further explored the process ofethylene-regulated hypocotyl growth in the light at posttranscriptional level, revealing a novel pathwaythat the interaction of light and ethylene coordinately modulates hypocotyl growth.Through phenotypic observations, combining with biochemical and genetic experiments, we foundthat a novel photomorphogenic factor LONG HYPOCOTYL (HY5) mediates the ethylene-regulatedArabidopsis hypocotyl growth in the light. Firstly, we observed with the phenotype of ethylenesignaling and hy5mutants that the negative factor of photomorphogenesis. The result displayed that theeffect of ethylene on HY5protein degradation was suppressed in hy5mutant. it was proved that HY5isessential for ethylene-enhanced hypocotyl growth in the light. The conclusion was confirmed by thephenotype that hypocotyl of hybrid of eto2and35S::HY5showed shorter than eto2. Further analyseswith Western blot of HY5protein revealed that ethylene precursor ACC improves the degradation ofHY5protein. And the regulation of ethylene in HY5stability is does and cumulative-dependent oftreatment with different concentrations and time course of ethylene precursor ACC. The effect of ACCon HY5degradation could be inhibited by the proteasome inhibitor MG132, indicating that thedegradation was mediated by26S proteasome pathway. Biochemical results also revealed that the HY5protein in ethylene overproduction or signal activation mutants was decreased, while there was littlechange but the degradation of HY5protein was suppressed in ethylene signal blocking mutants. Theseresults indicated that ethylene regulated HY5in protein level through its signal pathway. Analysis ofbiochemical and genetic experiments also found that HY5operates downstream of EIN3inethylene-regulate hypocotyl growth. In summary, ethylene regulates the degradation of HY5through the26S proteasome pathway,consequently affecting the growth of Arabidopsis hypocotyl, revealing a novel regulatory pathway thatArabidopsis HY5integrates the exogenous light signal and endogenous ethylene signal.