Cloning and characterization of the gene for a phloem-specific glutathione S-transferase from rice leaves

The N-terminal amino-acid sequence was determined for a major rice phloem protein with a molecular mass of 31 kDa, named RPP31. The corresponding full-length rice EST-clone was cloned based on the amino acid sequence. The predicted total amino-acid sequence of RPP31 shared high similarity with plant glutathione S-transferases (GSTs). Recombinant RPP31 produced in Escherichia coli and rice phloem sap showed GST activity. Immunocytological analysis indicated that RPP31 is localized in the phloem region of leaves. In mature leaves, the signal was restricted to sieve element-companion cell complexes, and was stronger in sieve elements than in companion cells. Although some plant GSTs are known to be induced by xenobiotics, the amount of RPP31 was not affected by treatments with an herbicide, pretilachlor, and/or its safener, fenclorim. These results suggest that RPP31 is an active GST restricted to the phloem region of normal rice leaves.