ANAEROBIOSIS AND PLANT-GROWTH HORMONES INDUCE 2 GENES ENCODING 1-AMINOCYCLOPROPANE-1-CARBOXYLATE SYNTHASE IN RICE (ORYZA-SATIVA L)

The plant hormone ethylene is believed to be responsible for the abili ty of rice to grow in the deepwater regions of Southeast Asia. Ethylen e production is induced by hypoxia, which is caused by flooding, becau se of enhanced activity of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, the key enzyme in the ethylene biosynthetic pathway. We hav e cloned three divergent members, (OS-ACS1, OS-ACS2, and OS-ACS3), of a multigene family encoding ACC synthase in rice. OS-ACS1 resides on c hromosome 3 and OS-ACS3 on chromosome 5 in the rice genome. The OS-ACS 1 and OS-ACS3 genes are induced by anaerobiosis and indoleacetic and ( IAA) + benzyladenine (BA) + LiCl treatment. The anaerobic induction is differential and tissue specific; OS-ACSI is induced in the shoots, w hereas OS-ACS3 is induced in the roots. These inductions are insensiti ve to protein synthesis inhibitors, suggesting that they are primary r esponses to the inducers. All three genes are actually induced when pr otein synthesis is inhibited, indicating that they may be under negati ve control or that their mRNAs are unstable. The OS-ACSI gene was stru cturally characterized, and the function of its encoded protein (M(r) = 53 112 Da, pI 8.2) was confirmed by expression experiments in Escher ichia coli. The protein contains all eleven invariant amino acid resid ues that are conserved between aminotransferases and ACC synthases clo ned from various dicotyledonous plants. The amino acid sequence shares significant identity to other ACC synthases (69-34%) and is more simi lar 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 e xtraordinary degree of divergence among ACC synthase isoenzymes within each species arose early in plant evolution and before the divergence of monocotyledonous and dicotyledonous plants.