Deepwater rice: A model plant to study stem elongation

Semiaquatic plants grow mostly in flood plains and along river beds and are adapted to survive partial submergence during periods of flooding (Blom and Voesenek, 1996). Among their adaptive features are the development of internal air channels (aerenchyma) that facilitate aeration of submerged organs and the capacity for rapid elongation when the plants become partially covered by floodwaters. Submergence-induced growth enables semiaquatic plants to keep part of their foliage above the rising waters and to avoid drowning. Rice (Oryza sativa L.) is a semiaquatic plant whose growth, at both the seedling and adult stages, is well investigated. It is cultivated in five ecosystems where the source of water supply and the degree of flooding are the major environmental determinants. The rice types corresponding to these ecosystems are rain-fed lowand upland rice, rice grown under controlled irrigated conditions, deepwater rice, and rice in tidal wetlands. Rice grown in the deepwater ecosystem distinguishes itself from most modern rice varieties by its ability to survive in water depths of more than 50 cm for at least 1 month (Catling, 1992). Among the deepwater rice types, the so-called floating rices exhibit extreme elongation capacity. They can grow at rates of 20 to 25 cm/d when partially submerged and can reach a length of up to 7 m in water depths of up to 4 m (for a detailed description of the deepwater rice ecosystem, see Vergara et al. [1976]; Catling [1992]). Figure 1A illustrates the growth habit of deepwater rice. Seedlings are allowed to establish themselves before the onset of flooding. The potential for submergence-induced rapid internodal elongation develops with the differentiation of internodes (Métraux and Kende, 1983). As the floodwaters rise, the internodes elongate and adventitious roots are formed at the nodes. When the waters recede, the rice plant sinks to the ground and gravitropic stimulation causes the top of the stem and the panicle to grow upward. Figure 1B illustrates the growth response of one internode during 2 d of submergence. Deepwater rice is a subsistence crop for about 100 million people in areas of Southeast Asia, where severe flooding occurs during the monsoon season. Whereas yields of modern rice cultivars average 6 tons/ha, the average yield of deepwater rice is only 2 tons/ha (Vergara et al., 1976; Catling, 1992). Efforts to improve yield and grain quality of deepwater rice have been met with limited success. Increases in yield and retention of floating ability have not yet been combined in one single cultivar. Since deepwater rice is the only crop that can be grown in many flood-prone areas of Southeast Asia, developing cultivars with increased yield and growth potential is of major agronomic importance. The genetic basis for submergence-promoted internodal elongation of deepwater rice has received relatively little attention. It appears that this trait is controlled by a number of minor and perhaps as few as two major genes (Catling, 1992). Suge (1987) proposed that elongation during submergence is based on the capacity of an internode to elongate, as well as the degree of elongation, and identified one gene with incomplete dominance that determined elongation ability. Deepwater rice is of the Indica type and can be crossed with Japonica rice. It can be transformed (Alam et al., 1998), and investigating its unique biological properties such as the signal transduction pathways leading to accelerated internodal growth is greatly aided by the rapidly expanding genetic database of rice. In addition to its importance as a crop plant, deepwater rice is also excellent for studying basic aspects of plant growth. The growth response is induced by an environmental signal and is mediated by at least three interacting hormones, namely ethylene, ABA, and GA. Internodal elongation is based on increased cell-division activity and enhanced cell elongation in well-delineated zones of the internode. This allows one to study both processes of growth in an integrated manner. Also, the unusually high growth rates magnify growth-related cellular, physiological, biochemical, and molecular processes, thereby facilitating their analysis. In addition to yielding fundamental insights into the growth process, studies of internodal elongation in deepwater rice may ultimately help to identify genes that could confer at least limited elongation capacity onto modern, high-yielding cultivars. 1 This work was supported by the National Science Foundation and the U.S. Department of Energy. This paper is dedicated to the memory of Paul B. Green (1931–1998). 2 Present address: Department of Plant Breeding and Biometry, Cornell University, Ithaca, NY 14853–1673. * Corresponding author; e-mail [email protected]; fax 1–517–353–9168. Abbreviations: CMF, cellulose microfibril; DZ, differentiation zone; EZ, elongation zone; IM, intercalary meristem. Plant Physiol. (1998) 118: 1105–1110