Ethylene plays important roles in plant growth, development and stress responses. Its receptor genes have been studied in dicots such as Arabidopsis, tobacco and tomato. However, no research has been reported for the ethylene receptors from monocots currently. In the present study, we cloned an ethylene receptor gene OSPK2 from rice and found that its encoded protein was divergent from the ethylene receptors from dicots. OSPK2 had a long extension in its N- terminal, followed by three transmembrane segments, a GAF domain, a putative kinase domain and a putative receiver domain. Although most of the domains were conserved, the expected phosphorylation site His and the phosphate receiver Asp have been replaced by Gln and Asn, respectively. This fact indicates that OSPK2 may not function as a histidine kinase in a phosphorelay manner, but rather play roles by other mechanism, probably through Ser/Thr kinase activity. The expression of the OSPK2 gene was investigated by RT-PCR method under different conditions. We found that this gene was apparently induced by wounding and PEG treatment, but not significantly affected by salt and ABA treatments. The differential expression of the OSPK2 gene may reflect its roles in mediating different abiotic stress responses, consistent with our previous studies on tobacco ethylene receptors.
Previously the partial sequence of an ethylene receptor gene NTHK2 was isolated from tobacco (Nicotiana tabacum L. var. Xanthi) plants and it was wound and drought inducible. In the present study full-length cDNA of NTHK2 was cloned by 5'-RACE method. NTHK2 gene has 3 216 bp, with 509 bp of 5'-non-coding region and 427 lip of 3'-non-coding region, and encodes an ethylene-receptor homolog of 760, amino acids. NTHK2 protein has a putative signal peptide, three transmembrane domains, a histidine kinase domain and a receiver domain. In the putative histidine kinase domain, the histidine at the phosphorylation site was replaced by an asparagine. To study the biochemical property of NTHK2, its kinase domain was expressed as a fusion protein with glutathione S-transferase (GST) using yeast Schizzosaccharomyces pombe as an expression system. In vitro kinase assay showed that NTHK2 kinase domain can autophosphorylate in the presence of Mg2+, indicating that NTHK2 may function as a kinase. Further studies will elucidate the function of NTHK2 in plant.
