Identification of three shikimate kinase genes in rice: characterization of their differential expression during panicle development and of the enzymatic activities of the encoded proteins

The shikimate pathway is common to the biosynthesis of the three aromatic amino acids and that of various secondary metabolites in land plants. Shikimate kinase (SK; EC 2.7.1.71) catalyzes the phosphorylation of shikimate to yield shikimate 3-phosphate. In an attempt to elucidate the functional roles of enzymes that participate in the shikimate pathway in rice (Oryza sativa), we have now identified and characterized cDNAs corresponding to three SK genes—OsSK1, OsSK2, and OsSK3—in this monocotyledenous plant. These SK cDNAs encode proteins with different NH2-terminal regions and with putative mature regions that share sequence similarity with other plant and microbial SK proteins. An in vitro assay of protein import into intact chloroplasts isolated from pea (Pisum sativum) seedlings revealed that the full-length forms of the three rice SK proteins are translocated into chloroplasts and processed, consistent with the assumption that the different NH2-terminal sequences function as chloroplast transit peptides. The processed forms of all three rice proteins synthesized in vitro manifested SK catalytic activity. Northern blot analysis revealed that the expression of OsSK1 and OsSK2 was induced in rice calli by treatment with the elicitor N-acetylchitoheptaose, and that expression of OsSK1 and OsSK3 was up-regulated specifically during the heading stage of panicle development. These results suggest that differential expression of the three rice SK genes and the accompanying changes in the production of shikimate 3-phosphate may contribute to the defense response and to panicle development in rice.