The
plant hormone ethylene is believed to be responsible for the ability of rice to grow in the deepwater regions of Southeast Asia.
Ethylene production is induced by
hypoxia, which is caused by flooding, because of enhanced activity of
1-aminocyclopropane-1-carboxylic acid (
ACC) synthase, the key
enzyme in the
ethylene biosynthetic pathway. We have cloned three divergent members, (OS-ACS1, OS-ACS2, and OS-ACS3), of a multigene family encoding
ACC synthase in rice. OS-ACS1 resides on chromosome 3 and OS-ACS3 on chromosome 5 in the rice genome. The OS-ACS1 and OS-ACS3 genes are induced by anaerobiosis and
indoleacetic acid (IAA) + benzyladenine (BA) + LiCl treatment. The anaerobic induction is differential and tissue specific; OS-ACS1 is induced in the shoots, whereas OS-ACS3 is induced in the roots. These inductions are insensitive to
protein synthesis inhibitors, suggesting that they are primary responses to the inducers. All three genes are actually induced when
protein synthesis is inhibited, indicating that they may be under negative control or that their mRNAs are unstable. The OS-ACS1 gene was structurally characterized, and the function of its encoded
protein (M(r) = 53 112 Da, pI 8.2) was confirmed by expression experiments in Escherichia coli. The
protein contains all eleven invariant
amino acid residues that are conserved between
aminotransferases and ACC synthases cloned from various dicotyledonous plants. The amino acid sequence shares significant identity to other ACC synthases (69-34%) and is more similar to sequences in other plant species (69% with the tomato LE-ACS3) than to other rice ACC synthases (50-44%). The data suggest that the extraordinary degree of divergence among
ACC synthase isoenzymes within each species arose early in plant evolution and before the divergence of monocotyledonous and dicotyledonous plants.