Euphytica 121: 137–143, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 137 Rapid induction of a novel ACC synthase gene in deepwater rice seedlings upon complete submergence Zhongyi Zhou 1,2 , Wim Vriezen 1 , Wim Van Caeneghem 1,2 , Marc Van Montagu 1 & Dominique Van Der Straeten 1 1 Vakgroep Moleculaire Genetica & Departement Plantengenetica, Vlaams Interuniversitair Instituut voor Biotech- nologie (VIB), Universiteit Gent, K.L. Ledeganckstraat 35, B-9000 Gent, Belgium; 2 Present address: CropDesign N.V., B-9052 Zwijnaarde, Belgium; ( ∗ author for correspondence; e-mail: dostr@gengenp.rug.ac.be) Received 8 February 2000; accepted 15 December 2000 Key words: ACC synthase, ethylene, rice, submergence Summary Elongation growth is a typical feature of deepwater rice plants in response to submergence. The growth phe- nomenon is known to be induced by hypoxia, which results in the expression of genes implicated in ethylene biosynthesis. Ethylene is considered to trigger the growth response as it accumulates in the submerged tissues and submergence enhances the expression of 1-aminocyclopropane-1-carboxylate (ACC) synthase. However, ACC concentration in rice plants increases much faster after submergence than the activity of the ACC synthase genes studied previously. Here, we studied the expression characteristics of the fifth member of this gene family, OS- ACS5, and show that submergence induces the messenger concentration of this gene before the accumulation of ACC could be observed. OS-ACS5 may play a fundamental role in the growth-promoting increase in ethylene biosynthesis during the first hours of submergence in deepwater rice. Introduction Plants respond to submergence with a series of physiological and biochemical changes that alter growth and development. Terrestrial species show a reduced elongation upon submergence. In contrast, aquatic and semi-aquatic plants, such as deepwater rice and some Rumex species, respond with a growth promotion (Jackson, 1985; Voesenek et al., 1990). Rapid growth of stems or petioles helps these plants to keep part of their foliage above the rising water level and to survive long-term flooding. In rice, the growth response upon submergence has been investig- ated at various developmental stages (Jackson, 1985; Kende et al., 1998). Rice is grown in five differ- ent ecosystems. According to its water requirement and tolerance, rice can be classified in three different ecotypes: highland, lowland and deepwater rice (Taka- hashi, 1984). Deepwater rice has when cultivated in areas with water depths of more than 50 cm, the most pronounced elongation capacity, in terms of growth rate and of sustainability of the response (Vergara et al., 1976; Keith et al., 1986). Submergence leads to a decrease of gas diffusion between the tissue and the surrounding atmosphere. This results in low endogenous oxygen levels and an increase of the ethylene concentration (Jackson, 1985). The low partial pressure of oxygen in sub- merged organs is the key factor initiating the signalling cascade that triggers the subsequent growth response (Kende et al., 1998; Voesenek & Blom, 1999). Sim- ulation of submergence by exposing plants to an at- mosphere that consists of 3% (v/v) oxygen, 6% (v/v) carbon dioxide and 1 ppm of ethylene in nitrogen gas induces comparable effects on plant growth and on the expression of certain marker genes (Raskin & Kende, 1984a; Vriezen et al., 1997, 1999). This gas composition is thought to come close to the en- dogenous atmosphere in plants during submergence (Setter et al., 1987). In rice, low oxygen concentration causes an increase in ethylene biosynthesis through the