Ethylene plays essential roles in adaptive growth of rice plants in water-saturating environment; however,
ethylene signaling pathway in rice is largely unclear. In this study, we report identification and characterization
of ethylene-response mutants based on the specific ethylene-response phenotypes of etiolated rice seedlings, including
ethylene-inhibited root growth and ethylene-promoted coleoptile elongation, which is different from the ethylene
triple-response phenotype in Arabidopsis. We establish an efficient system for screening and a set of rice mutants have
been identified. Genetic analysis reveals that these mutants form eight complementation groups. All the mutants show
insensitivity or reduced sensitivity to ethylene in root growth but exhibit differential responses in coleoptile growth. One
mutant group mhz7 has insensitivity to ethylene in both root and coleoptile growth. We identified the corresponding
gene by a map-based cloning method. MHZ7 encodes a membrane protein homologous to EIN2, a central component of
ethylene signaling in Arabidopsis. Upon ethylene treatment, etiolated MHZ7-overexpressing seedlings exhibit enhanced
coleoptile elongation, increased mesocotyl growth and extremely twisted short roots, featuring enhanced ethyleneresponse
phenotypes in rice. Grain length was promoted in MHZ7-transgenic plants and 1000-grain weight was reduced
in mhz7 mutants. Leaf senescent process was also affected by MHZ7 expression. Manipulation of ethylene signaling may
improve adaptive growth and yield-related traits in rice.