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The plant hormone ethylene exerts dual effects on various plants and separate organs. The dual-and-opposing-effect (DOE or "Yin and Yang Effect") is one of the five different types of dual-effects reported for ethylene in regulation of the plant growth and development as well as tropic response. DOE is defined as such that a plant hormone or a growth regulator exerts both stimulatory and inhibitory effects on a growth response (or a tropic response) simultaneously at any given concentrations found naturally from plant tissues, and the strengths of the two opposing effects are changing with respect to each other. The ultimate phenotypic expression of a response depends on the dynamic interplays and balancing of the two effects in a plant organ studied. Ethylene has dual-and-opposing-effect on shoot negative gravitropism. The ethylene-dependent gravicurvature of inflorescence stem and light-grown hypocotyls, either faster or slower, depends on the length of ethylene exposure and organs studied. A longer term ethylene exposure (>=12?h) enhances both inflorescence stem and light-grown hypocotyl gravicurvatures of Arabidopsis, whereas a short-term ethylene exposure (<=0.5?h) has an inhibitory effect on the gravibending rate of inflorescence and hypocotyls. A basal level of ethylene biosynthesis occurs during the entire life cycle of Arabidopsis and its production rate increases significantly in response to developmental cues and environmental inducers (such as gravistimulation). The co-existence of both the stimulatory and inhibitory effect in the same organ enables both effects to oppose each other, resulting in retarded gravicurvature. Because the strength of each effect changes with respect to the opposing one during ethylene exposure, the eventual gravicurvature phenotype of an organ i.e. inflorescence stem and light-grown hypocotyls results from the dynamic balancing of the two effects of ethylene. In light-grown hypocotyls of wild-type Arabidopsis, exogenous ethylene has to be removed to observe a faster hypocotyl gravicurvature because the inhibitory effect of ethylene is stronger in this organ, whereas in the flowering Arabidopsis plant, ethylene does not need to be removed to observe a faster inflorescence stem gravicurvature, suggesting that ethylene plays a stronger stimulatory role in this organ. Study on ethylene biosynthesis and perception mutants as well as auxin response mutant confirms the important role of ethylene in regulation of light-grown hypocotyl gravitropism. The two opposing effects of ethylene are time-dependent and the strength of theirs is positively correlated to ethylene concentrations. The stimulatory effect (Yang effect) of ethylene takes place slowly and persists even after ethylene has been removed from the surroundings, whereas the inhibitory effect (Yin effect) commences abruptly and ceases to function once ethylene is removed. Forward genetic screen designed according to the DOE of ethylene in light-grown hypocotyls has led to isolation of an ethylene-dependent gravitropism-deficient yellow-green 1 (egy1) Arabidopsis mutant. EGY1 gene encodes a 59?kDa metalloprotease and regulates biogenesis and replication of endodermal amyloplast, the gravity sensor. EGY1 gene expression is enhanced by both ethylene and light. Further studies on the DOE of ethylene with the cutting-edge phosphoproteomics in combination with reverse genetics and genetic approaches should be able to reveal more valuable information about molecular mechanisms underlying ethylene and gravity signaling as well as cross-talks among hormone, light and gravity and bring it to a level of system biology.
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