Abstract:To confirm the role of miR1917 in plant response to microgravity, the ethylene level in the culture environment, the phenotypes responded to ethylene of tomatoes, as well as the expression quantity of miRNA1917 and its target gene LeCTR4 was detected . In the experiment, ethylene related phenotypes including hypocotyl bending geotropically and developing spirals were observed. The quantity of malonaldehyde (MDA) and superoxide anion in the environment increases along the time. These reactions are clearly affected by the microgravity treatment. The difference of ethylene level in the culture environments is significant only in the initial period (day 10 to day 20). After 20 days, there is almost no difference. The relative expression quantity of miRNA1917 increases along the time, while the relative expression quantity of its target gene keeps decreasing. Therefore, the phenotypes of hypocotyl bending geotropically and developing spirals are related to the ethylene level in the initial microgravity treatment period. In the later period (after 20 days), the accumulation of MDA and superoxide anion are related to the expression quantity of miRNA1917. This is because the increase of the expression quantity of miRNA1917 leads to the decrease of its target gene, which negatively controls the phenotype response to ethylene. Hence, the plant is more sensitive to the ethylene level in the environment. miRNA1917 play a role in plant microgravity response through reducing the amount of its target gene to increasing the sensitivity to ethylene,. The study laid a foundation for the establishment of plant microgravity response process mediated by miRNA.