FIGURE 1.

A diagram of ethylene signal transduction pathway in rice. In the dark, rice etiolated seedlings have “double response” with ethylene treatment. Ethylene promotes coleoptile elongation but inhibits root growth in rice. A linear ethylene signaling pathway has been found in rice etiolated seedlings which is similar to that in Arabidopsis. Rice has homologs of Arabidopsis ethylene signaling pathway, such as ethylene receptors, OsCTRs, MHZ7/OsEIN2, and MHZ6/OsEIL1 and OsEIL2. In contrast to that in Arabidopsis, the AtEIN3/EIL1 homologs OsEIL1 and OsEIL2 have divergent functions in rice coleoptile and root growth. (A) Ethylene promotes coleoptile/mesocotyl elongation through OsEIL2 which inhibits GY1/EG1-mediated jasmonate (JA) biosynthesis. JA pathway acts downstream of ethylene signaling pathway to inhibit cell elongation. On the other hand, MHZ5/CRTISO and MHZ4/ABA4-mediated ABA pathway acts upstream of ethylene signaling pathway to inhibit transcription of MHZ7/OsEIN2 and inhibit coleoptile elongation. (B) Ethylene regulates root growth through MHZ6/OsEIL1 function. In Arabidopsis, AtEIN3 and AtEIL1 have functional redundancy in root inhibition. Moreover, ethylene inhibits root growth partially through MHZ5/CRTISO and MHZ4/ABA4-mediated ABA pathway. Arrows and T-bars indicate direct or indirect activation and inhibition, respectively. Dotted lines indicate several steps involved that are not shown in the diagram.