Sucrose Synthase Activity as a Potential Indicator of High Rice Grain Yield

TheChinese rice cultivarGuichao2 is a potentially valuable resource for breeding rice (Oryza sativa L.) with many valuable characteristics including upright leaves, prolific tillering ability, and high yields. The objectives of this study were to compare grain yields and yield components between Guichao2 and U.S. rice lines and to determine if the enzyme sucrose synthase was associated with the observed differences in grain yield and yield components. Grain yield, grain dry matter accumulation, yield components, and endosperm sucrose synthase activity were measured in field and greenhouse experiments with Guichao2 and ‘Lemont’ rice, as a U.S. standard, and various other lines. In three of four field tests, Guichao2 had higher grain yields than Lemont and other U.S. rice cultivars. Guichao2 also usually had higher sucrose synthase activity per grain and per unit of protein, in three of four field tests. The cultivar Qiguizao, derived from a cross of Guichao2 and Qiyouzhan, also had higher sucrose synthase activity on a protein basis. Since the grains of Qiguizao weigh less by approximately one half of the other cultivars, the sucrose synthase activity per grain was somewhat lower. Greenhouse tests supported these findings. In one test, the F1 hybrids of Guichao and Lemont had higher sucrose synthase activity than either Guichao2 or IR36. From this research, we conclude that sucrose synthase activity, and perhaps the genes coding for sucrose synthase, will be a valuable resource in transferring the yield characteristics of Guichao2 and Qiguizao into U.S. rice cultivars. SUCROSE is the primary transport carbohydrate in rice and most other higher plants (Avigad and Dey, 1996; Taiz and Zeiger, 1998). Sucrose is actively loaded into the rice grain and subsequently converted into starch by a series of enzymatic steps (Avigad and Dey, 1996). In most actively filling sinks, sucrose is broken down by sucrose synthase to form uridine diphosphoglucose (UDPG) and fructose, the first step in starch synthesis. Greater than 80% of the rice grain consists of starch. The conversion of sucrose into starch molecules can potentially limit grain filling. There are three sucrolytic enzymes in higher plants: sucrose synthase, acid invertase, and neutral invertase. In most rapidly filling sinks such as seeds, sucrose synthase has the highest activity (Sung et al., 1989). Sucrose synthase is first in the line of conversion of sucrose to starch and thus, the activity of sucrose synthase is an excellent predictor of sink strength (Sung et al., 1989). Sink strength is the ability of a sink to attract or import carbohydrate and, in the case of rice, fill grain. Accordingly, sucrose synthase may be valuable to assess relative sink strength among rice breeding lines. Guichao2 and Other Chinese Rice Cultivars Guichao2 is a potentially valuable resource for U.S. rice breeding programs.HuangYueXiang inGuangdong Province, People’s Republic of China (PRC), developed Guichao2. The grain yields of Guichao2 are high throughout the world including in Arkansas (Table 1). Qiguizao is a cultivar also developed in the Guangdong Province, PRC from the cross Guichao2 3 Qiyouzhan (/ 3 ?). The valuable attributes of Guichao2 and Qiguizao include high grain yield and some resistance to stress (other than photooxidative stress). Another positive attribute of Guichao2 is its greater competitiveness with weeds compared with U.S. rice cultivars (Gealy et al., 2003), possibly a result of allelopathy in Guichao2 (Mattice et al., 2001). In a number of situations, the photosynthetically active radiation that powers photosynthesis also generates active oxygen species such as superoxide and oxygen singlets. Guichao2 and Qiguizao are prone to photooxidative damage (Tu et al., 1988). Part of the photooxidative damage sustained by Guichao and Qiguizao appears to be a smaller pool of reduced xanthophyll cycle carotenoids (Black et al., 1995b). Consequently, Guichao2 and Qiguizao are less resistant than U.S. rice cultivars to high light and low temperature stresses. Other negative characteristics of Guichao2 and Qiguizao, for U.S. rice cultivar improvement, include pubescence, relatively long growing season, proclivity for lodging, and poor milling yields, which results from low percentages of whole kernels after milling removes the hull and bran layers. Counce et al. (1998) found Qiguizao, Guichao2, and Teqing all had increased photooxidative damage relative to American rice lines Lemont, Bengal, and Adair and relative to the F1 hybrid of Lemont and Qiguizao. The objectives of this study were to compare grain yields and yield components betweenGuichao2 and U.S. rice lines and to determine if the enzyme sucrose synthase was associated with the observed differences in grain yield and yield components. MATERIALS AND METHODS Greenhouse Experiments The experiments were conducted in a greenhouse in Keiser, Arkansas (358409 N, 90859 W) from January to June of 1992, 1993, 1995, and 1996. Plants were grown in pots filled with Sharkey silty clay loam soil (very-fine, smectitic, thermic P.A. Counce, Univ. of Arkansas, Rice Res. and Ext. Ctr., 2900 Highway 130 East, Stuttgart, AR 72160; K.A. Gravois, Louisiana State Univ. Agric. Ctr., St. Gabriel Res. Stn., 5755 LSUAg. Road, St. Gabriel, LA 70776. Paper published with the approval of the Director, Arkansas Agric. Exp. Station, Univ. of Arkansas, Fayetteville, AR 72701. This research was supported by a grant from the Arkansas Rice Res. and Promotion Board. Received 18 Mar. 2005. *Corresponding author