Journal of Experimental Botany, Vol. 52, No. 364, pp. 2169-2179,
November 1, 2001
© 2001 Oxford University Press
Original Papers |
Activities of starch hydrolytic enzymes and sucrose-phosphate synthase in the stems of rice subjected to water stress during grain filling
1 College of Agriculture, Yangzhou University, Yangzhou, Jiangsu, China
2 Department of Biology, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China
Received 15 January 2001; Accepted 12 July 2001
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Abstract |
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To understand the effect of water stress on the remobilization of prestored carbon reserves, the changes in the activities of starch hydrolytic enzymes and sucrose-phosphate synthase (SPS) in the stems of rice (Oryza sativa L.) during grain filling were investigated. Two rice cultivars, showing high lodging-resistance and slow remobilization, were grown in the field and subjected to well-watered (WW,
soil=0) and water-stressed (WS, soil=-0.05 MPa) treatments 9 d after anthesis (DAA) till maturity. Leaf water potentials of both cultivars markedly decreased during the day as a result of WS treatment, but completely recovered by early morning. WS treatment accelerated the reduction of starch in the stems, promoted the reallocation of prefixed 14C from the stems to grains, shortened the grain filling period, and increased the grain filling rate. More soluble sugars including sucrose were accumulated in the stems under WS than under WW treatments. Both 
- and ß-amylase activities were enhanced by the WS, with the former enhanced more than the latter, and were significantly correlated with the concentrations of soluble sugars in the stems. The other two possible starch-breaking enzymes, -glucosidase and starch phosphorylase, showed no significant differences in the activities between the WW and WS treatments. Water stress also increased the SPS activity that is responsible for sucrose production. Both Vlimit and Vmax, the activities of the enzyme at limiting and saturating substrate concentrations, were enhanced and the activation state (Vlimit/Vmax) was also increased as a result of the more significant enhancement of Vlimit. The enhanced SPS activity was closely correlated with an increase of sucrose accumulation in the stems. The results suggest that the fast hydrolysis of starch and increased carbon remobilization were attributed to the enhanced -amylase activity and the high activation state of SPS when the rice was subjected to water stress. Key words: Rice, starch hydrolytic enzymes, sucrose-phosphate synthase, remobilization, water stress.
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Introduction |
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Monocarpic plants need the initiation of whole plant senescence to remobilize the prestored carbon from stems to grains (Gan and Amasino, 1997
; Noodén et al., 1997). In rice (Oryza sativa L.), senescence is often unfavourably delayed due to the high lodging-resistant feature of a particular cultivar (Yuan, 1997; Zhu et al., 1997) or the heavy use of nitrogen fertilizers (Yang et al., 1996). In both cases, unused carbon is left in the straws and results in low harvest index (Cao et al., 1992; Zhu et al., 1997). Grain filling in cereals depends on carbon from two sources: current assimilation and remobilization of either pre- or post-anthesis reserves stored in the culm and other parts (mainly the sheath) (Yoshida, 1972; Kobata et al., 1992). In rice, the contribution of reserves to grain filling is about one-third of the final grain weight (Cock and Yoshida, 1972; Kobata and Takami, 1981; Rahman and Yoshida, 1985) and has been reported to range from 0–40%, depending on the growth conditions and rate of nitrogen application (Yoshida, 1972).
The main storage form of non-structural carbohydrate (NSC) in the stem (culm+sheath) of rice is starch (Murayama et al., 1961; Murata and Matsushima, 1975; Cao et al., 1992). Starch is degraded to glucose first and then sucrose is resynthesized when the carbon is remobilized from the stem to the grains (Venkateswarlu and Visperas, 1987; Beck and Ziegler, 1989). Starch degradation can occur via hydrolytic and phosphorolytic reactions and, probably, the concerted action of several enzymes (Beck and Ziegler, 1989; Nielsen et al., 1997). These enzymes include: -amylase (1,4--D-glucan glucanohydrolase) [EC 3.2.1.1, ß-amylase (1,4--D-glucan maltohydrolase; EC 3.2.1.2, -glucosidase (-D-glucoside glucohydrolase; EC 3.2.1.20, and starch phosphorylase (1,4--D-glucan:orthophosphate -D-glucosyltransferase; EC 2.4.1.1) (Preiss, 1982; Gallagher et al., 1997). Based on its high affinity, sucrose-phosphate synthase (SPS, or the 6-phosphate-fructose-glycosyltransferase, EC 2.4.1.14) is expected to play a major role in the resynthesis of sucrose (Whittingham et al., 1979; Wardlaw and Willenbrink, 1994), and sustain the assimilatory carbon flux from source to sink (Isopp et al., 2000).
It has been reported that starch degradation in stolons of white clover was controlled by the -amylase activity (Gallagher et al., 1997). Water stress can induce -amylase in barley leaves (Jacobsen et al., 1986) and enhance ß-amylase activity in cucumber cotyledons (Todaka et al., 2000). Our early work (Yang et al., 2000, 2001) has shown that a controlled water deficit during grain filling can enhance whole plant senescence and improve grain filling in wheat plants where heavy nitrogen was applied. This research was to test the hypothesis that a controlled water deficit during grain filling may enhance the grain filling rate by accelerating the remobilization of prestored carbon in the stem of rice through regulating the key enzymes involved. The changes in activities of starch hydrolytic enzymes and SPS in rice stems and their relationships with the remobilization of prestored carbon were investigated.
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Materials and methods |
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Plant materials
The experiment was conducted at a farm of Yangzhou University, Jiangsu Province, China (32°30' N, 119°25' E) during the rice growing season (May to October) of 1999, and repeated in 2000. Two lodging-resistant cultivars currently used in local rice production, Wuyujing 3 (japonica) and Yangdao 4 (indica), were grown in the paddy field. Seedlings were raised in the field with the sowing date on 10–11 May and transplanted on 10–11 June at a hill spacing of 0.20x0.16 m with two seedlings per hill. The soil was a sandy loam [Typic fluvaquents, Etisols (US taxonomy)] with 24.5 g kg-1 organic matter and available N-P-K at 105, 33.5 and 66.0 mg kg-1, respectively. N (60 kg ha-1 as urea), P (30 kg ha-1 as single superphosphate) and K (40 kg ha-1 as KCl) were applied and incorporated before transplanting. N as urea was also applied at mid-tillering (40 kg ha-1) and at panicle initiation (25 kg ha-1). Both cultivars headed on 20–22 August (50% of plants), flowered on 23–24 August, and were harvested on 9–10 October. Except for drainage at the end of tillering (9–12 July), the field was kept at the 1–2 cm water level until 9 d after anthesis (DAA) when water stress treatments were initiated.
Water stress treatments
The experiment was a 2x2 (two cultivars and two levels of soil moisture) factorial design with four treatment combinations. Each treatment had three plots as replicates in a completely randomized block design. Plot dimension was 4.2x3.2 m and plots were separated by a ridge (40 cm in width) wrapped with plastic film. From 9 DAA to maturity, two levels of soil water potential (soil) were imposed by controlling water application. The well-watered (WW) treatment was kept at a 1–2 cm water depth (soil=0 MPa) in the field by manually applying tap water every day, and the water-stressed (WS) treatment was maintained at soil at -0.05 MPa, monitored with tension meters buried in the soil at a depth of 15–20 cm. Five tension meters were installed in each plot and readings were recorded twice a day at 10.00 h and 16.00 h. When the readings dropped to the designated value, 60 l of tap water per plot was added manually. A rain shelter made of a steel frame covered with plastic sheet was used to protect the plot during rain.
Radioactive labelling
At the boot stage (30 July for Wuyujing 3 and 31 July for Yangdao 4), 50 plants from each treatment were labelled with 14CO2. Flag leaves of the main stems were used for labelling between 09.00 and 11.00 h on a clear day with photosynthetically active radiation at the top of the canopy ranging between 1000–1100 µmol m-2 s-1. The whole flag leaf was placed into a polyethylene chamber (25 cm length and 4 cm diameter) and sealed with tape. Six ml of air in the chamber was drawn out and the same volume of mixed gas containing 14CO2 was injected into the chamber (0.01 mol CO2 concentration at specific radioactivity of 14C at 1.48 MBq l-1). The chamber was removed after 30 min.
Labelled plants were harvested at 0 (50% anthesis), 9 (the initiation of water withholding), and from 12 to 36 DAA at 4 d intervals, respectively. Harvested plants were divided into leaf blades, stem plus sheaths, and panicles. Carbon-14 in the plants was assayed by the method described previously (Ge et al., 1994). Briefly, samples were dried in an oven at 80 °C to constant weight, ground to a powder, and then extracted by shaking in 80% (v/v) boiling ethanol. The residue was extracted in 2:1 of 60% (v/v) HClO4 to 30% (v/v) H2O2 for 4 h at 60 °C. The radioactivity of 14C in both the extracted aliquots was counted using a liquid scintillation counter (Beckman Instruments Inc., Fullerton, California, USA). Radioactivity distribution to each part of the plant was expressed as a percentage of total radioactivity remaining in the above-ground portion of the plant.
Sampling and harvesting
Three hundred panicles that headed on the same day were chosen and tagged for each treatment. The flowering date and the position of each spikelet on the tagged panicles were recorded. Thirty tagged panicles from each treatment were sampled at 4 d intervals from anthesis to maturity. The sampled panicles were divided into three groups (10 panicles each) as subsamples. Grains that developed from the spikelets that flowered on the same day were removed, dried at 70 °C to constant weight for 72 h, dehulled and weighed. The grain-filling process was fitted by the growth equation (Richards, 1959) as described earlier (Zhu et al., 1988):
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Twenty plants from each plot were sampled at 0, 9 and every 4 d from 12 to 36 DAA. The stems (culm+sheath) of half the sampled plants were frozen in liquid nitrogen for 1 min and then stored at -80 °C for enzymatic measurement. The other half of the stems was used for the measurement of dry weight and non-structural carbohydrate (NSC). Each measurement had six replicates with two subsamples in each plot.
In each plot all plants from a 2 m2 site (except border ones) were harvested at maturity for the determination of grain yield. Yield components, i.e. the panicles per m2, spikelets per panicle, the percentage of ripened grains, and grain weight, were determined from 50 plants (excluding the border ones) sampled randomly from each plot. The percentage of ripened grains was defined as the ripened grains (specific gravity 
1.06) as a percentage of total spikelets.
Measurement of leaf water potential
The measurement of leaf water potential was made at 2 h intervals at 19 and 20 DAA for both cultivars. Well-illuminated flag leaves were chosen randomly for the measurement. A pressure chamber (Model 3000, Soil Moisture Equipment Corp., Santa Barbara, CA, USA) was used with six leaves for each treatment.
Enzyme extraction and assays
The frozen culms and sheaths were ground and extracted (in the ratio 4 ml buffer per 1 g of tissue) in cold 0.1 M phosphate buffer (pH 6.5) at 4 °C, and centrifuged at 15 000 g for 30 min. The supernatant was used for the assay of starch hydrolytic enzymes. The enzyme activities were determined as described previously: ß-amylase (exo-amylase) (Sirou et al., 1990); -amylase (endo-amylase) (McCleary and Sheehan, 1987); -glucosidase (Gallagher et al., 1997); and starch phosphorylase activities (Agarwala et al., 1965). The amylase activities were expressed as µmol maltase equivalent released mg-1 protein h-1, and the starch phosphorylase activity as µmol Pi liberated mg-1 protein h-1.
A modified method for SPS extraction was used (Hirano et al., 1997). The frozen stems (3–4 g FW) was ground with a mortar and pestle in 10 ml of ice-cold 40 mM Hepes-NaOH (pH 7.5) buffer containing 10 mM MgCl2, 1.5 mM EDTA, 3 mM DTT, 0.5 mg ml-1 BSA, and 0.05% (v/v) Triton X-100. The homogenate was centrifuged at 12 000 g for 10 min (<4 °C). The supernatant was immediately desalted by centrifugal filtration on a Sephadex G-25 (Pharmacia, Freiburg, FRG) column equilibrated with the grinding buffer minus EDTA and Triton X-100. SPS activity was assayed under both limiting (Vlimit) and saturated (Vmax) substrate conditions (according to the method of Huber and Huber, 1990). The enzyme activity was expressed as µmol sucrose synthesized mg-1 protein h-1. The activation state of SPS in percentage was calculated as Vlimit/Vmaxx100.
The method for sucrose synthase (SS, EC 2.4.1.13) extraction was modified from Ranwala and Miller (Ranwala and Miller, 1998). Stems were homogenized with a mortar and pestle (5 ml buffer g-1 FW) in 100 mM HEPES (pH 7.5) containing 10 mM isoascorbate, 3 mM MgCl2, 5 ml DTT, 2 ml EDTA, 5% (v/v) glycerol, 3% (w/v) polyvinylpyrrolidone, and 0.01% Triton X-100. After centrifugation at 12 000 g for 30 min, the supernatant was desalted on a Sephadex G-25 column (Pharmacia, Freiburg, FRG) and the proteins were eluted by the reaction buffer that contained 50 mM HEPES (pH 7.5), 10 mM MgCl2, 2 mM EDTA, and 3 mM DTT. SS was assayed in the synthesis direction as described previously (Wardlaw and Willenbrink, 1994). The enzyme activity was expressed as µmol sucrose synthesized mg-1 protein h-1. Protein content was determined using bovine serum albumin as a standard (Bradford, 1976).
NSC measurement
The method for extraction of NSC in stems was modified according to the method described earlier (Yoshida et al., 1976). The sample was dried in an oven and ground into fine powder. In a 15 ml centrifuge tube, 100 mg of ground sample was added with 10 ml of 80% (v/v) ethanol and kept in a water bath at 80 °C for 30 min. After cooling in cool water the tube was then centrifuged at 2000 rpm for 20 min. The supernatant was collected and the extraction was repeated three times. The alcohol in the supernatant was evaporated on a water bath at 80 °C until most of the alcohol was removed and the volume was reduced to about 3 ml. The sugar extract was diluted to 25 ml with distilled water. The concentration of sugars in the extract was analysed as described earlier (Somogyi, 1945). Sucrose content was determined according to Gong and Zhang (Gong and Zhang, 1995).
The residue left after extracting sugars in the centrifuge tube was dried at 80 °C for starch extraction. Two ml of distilled water was added to the tube containing the dried residue. The tube was then shaken in a boiling water bath for 25 min. Two ml of 9.36 M HClO4 was added to the tube after cooling in cool water. The solution was shaken further for 15 min. The extract was then made up to about 10 ml and centrifuged at 2000 rpm for 20 min. The supernatant was collected and a further 2 ml of 4.68 M HClO4 was added to the residue. The extraction was repeated as above. The supernatants were combined and made up to 50 ml with distilled water. The starch was analysed by the method of Pucher et al. (Pucher et al., 1948).
The effects of rice cultivars and level of soil moisture were tested by analysis of variance (ANOVA). Data from each sampling date were analysed separately. Means were tested by least significant difference at P0.05 level (LSD0.05). Linear correlation analysis was used to evaluate the relationships of starch hydrolytic enzyme and SPS activities with tissue sugar and starch concentrations.
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Results |
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Leaf water potential and grain filling
The leaf water potential ranged from -0.06 at pre-dawn (06.00 h) to -0.65 MPa at midday (12.00 h) for the plants under WW treatments. It was greatly reduced for the plants under WS treatments during the day, and reached -1.38 to -1.49 MPa at midday (Fig. 1
). However, the differences in leaf water potential in the early morning between WS and WW treatments were small, indicating that plants subjected to water deficit stress could rehydrate overnight.
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) treatments. Measurements were made on the flag leaves 19 DAA for Wuyujing 3 and 20 DAA for Yangdao 4. Vertical bars represent ±SE of the mean (n=6) where these exceed the size of the symbol.The WS treatment increased the grain-filling rate and shortened the grain-filling period (Fig. 2
; Table 1). The active grain filling period was shortened by 2.7 to 5.5 d and grain filling rate increased by 0.18 to 0.29 mg d-1 grain-1, respectively, compared to the WW treatment. Neither the panicles m-2, nor the spikelets per panicle, were influenced by the water stress treatment in this experiment, and the effects of water stress on the number of grains m-2 were too small to be statistically significant (Table 1). The percentage of ripened grains, grain weight and grain yield decreased by 0.6 to 1.6%, 1.1% and 0.3 to 3.0% respectively, under WS, compared to WW treatments, but such differences were not significant (Table 1).
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Changes in NSC in the stems
The WS treatments accelerated the reduction in dry weight per stem (Fig. 3). At 36 DAA, 0.15–0.18 g of the dry weight per stem was more reduced under the WS than under WW treatments.
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The total sugars (soluble sugars+starch) as NSC in stems were decreased during grain filling (Fig. 4a
, b). The disappearance was faster under WS than under WW treatments. Starch content sharply decreased from 9 to 28 DAA, and the WS treatment accelerated this reduction (Fig. 4c, d). In contrast to the decrease in total sugars and starch contents, both soluble sugars and sucrose in the stems were increased from 9 to 28 DAA, and the WS enhanced their accumulation (Fig. 4e–h).
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Water stress promoted the reallocation of pre-anthesis assimilates from the stems to grains. Figure 5
shows the disappearance of pre-anthesis assimilated 14C in the stems and its appearance in the grains during grain filling. Radioactive 14C was fed to the flag leaves at booting stage. At the start of water withholding (9 DAA), about 80% of the recovered 14C was located in the stems, and about 10% in the grains. After 31 d (40 DAA), 14C in the stems was reduced to 8–10% under WS and to 41–43% under WW treatments. At the same time, 14C in grains increased to 81–83% under WS, and only to 39–46% under WW treatments at 40 DAA (Fig. 5), opposite to that observed in the stems.
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Changes in starch hydrolytic enzyme activities
The activity of -amylase in the stems under WW treatments changed little during grain filling. However, it was remarkably enhanced by the water stress, reached its peak at 28 DAA, and decreased thereafter (Fig. 6a, b). The changing pattern of ß-amylase activity was somewhat similar to that of -amylase (Fig. 6c, d). However, ß-amylase activity was lower and less enhanced by the WS treatment, when compared to -amylase.
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The activity of
-glucosidase in the stems was very low, and ranged from 0.28 to 0.41 µmol mg-1 protein h-1. There were no significant differences in the enzyme activity between WW and WS treatments (Fig. 6e
, f). The starch phosphorylase activity changed little from 0 to 20 DAA, but sharply decreased thereafter, with no significant difference between the WW and WS treatments (Fig. 6g, h).
The changing patterns of - and ß-amylase activities coincided with that of soluble sugars in the stems (refer to Fig. 4e, f). The content of soluble sugars very significantly correlated with both -amylase (r=0.95** to 0.98**, P=0.01) and ß-amylase activities (r=0.87** to 0.89**, P=0.01), while neither -glucosidase nor starch phosphorylase activity correlated with the soluble sugars (r=-0.32 to 0.28, P>0.05).
Changes in SPS and SS activities
SPS activities in the stems, assayed under saturating (Vmax) and limiting (Vlimit) substrate conditions, showed a similar changing pattern under both WW and WS treatments. They increased first and then declined from 20 or 24 DAA (Fig. 7a–d). Both activities, however, were much higher under WS than under WW treatments from 16 to 28 DAA. The activity enhanced by the WS was more at Vlimit than at Vmax, when compared to their respective WW treatments. As a result, WS increased SPS activation state as expressed by Vlimit/Vmax (Fig. 7e, f).
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SPS activities were very significantly correlated with the sucrose contents in the stems (cf. Fig. 2g
, h). The correlation coefficients were 0.91** to 0.96** for the Vlimit and 0.83** to 0.86** (P=0.01) for the Vmax, suggesting that the increase of sucrose in the stems was associated to the enhanced SPS activities.
SS activity in the stems (assayed in synthesis direction) declined during grain filling (Fig. 8). The difference in the activity was not significant between WS and WW treatments.
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Relationships between enzymatic activities and remobilization
The disappearance of starch in the stems (starch at anthesis minus starch at 9, 12, 16, 20, 24, and 28 DAA, respectively) and 14C transfer to the grains (14C at 9, 12, 16, 20, and 28 DAA, respectively minus 14C at anthesis) were calculated as remobilization parameters. The correlations of starch hydrolytic enzyme and SPS activities during a rapid period of starch remobilization in the stems (9–28 DAA) with the parameters were determined (Table 2). SPS activities (at Vlimit, Vmax and activation state) and - and ß-amylase activities were all significantly and positively correlated with the remobilization of starch and 14C transfer to the grains (r=0.66* to 0.97**, P=0.05 and 0.01, respectively). No correlation of -glucosidase activity was observed with the remobilization parameters (r=-0.15 to 0.19, P>0.05). Starch phosphorylase activity in the stems negatively correlated with the remobilization of starch and the reallocation of 14C (r=-0.63* to -0.87*, P=0.05) (Table 2).
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Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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If a water deficit imposed during rice grain filling is properly controlled, i.e. the plants can rehydrate completely overnight, remobilization of prestored carbon in the stems can be substantially enhanced (Figs 3
, 4, 5) and grain filling rate greatly increased (Fig. 2; Table 1). Such an enhancement of remobilization may improve harvest index and shorten the grain filling period in areas where a cold front in later autumn often abruptly terminates the rice growing season and results in a low harvest index in senescence-delayed rice.
It was reported that air temperature affects the mobilization of photosynthate in wheat and rice, and a possible rise in temperature due to stomatal closure under water stress may account for the enhanced movement of carbon to the grains (Wardlaw, 1971; Sofield et al., 1977; Chowdhury and Wardlaw, 1978). In this experiment it was observed that the average day time air temperature in the canopy (the top 20 cm) during the grain filling period was 26.7 °C under the WW and 27.4 °C under the WS treatments (data not shown). The difference was not so dramatic, possibly because the soil drying was relatively mild. It is concluded that such a small increase in the day temperature under the WS treatment may not explain the observed enhancement of carbon remobilization and increase in the grain filling rate.
The results showed that enhanced remobilization of starch in the stems under water stress was closely associated with an increase of the -amylase activity (Fig. 6a, b). Though ß-amylase activity was also associated with both soluble sugars and starch remobilization, it was less enhanced by the WS treatment (Fig. 6c, d) and less closely correlated with the remobilization (Table 2), when compared to -amylase. It has been proposed that -amylase may be a regulatory enzyme of starch breakdown since this enzyme, unlike ß-amylase, can attack and degrade intact starch granules (Dunn, 1974; Manners, 1985), while ß-amylase is thought to hydrolyse the oligosaccharide products of -amylase activity and play a secondary role in the regulation of starch hydrolysis (Gallagher et al., 1997). In this study, a high correlation of -amylase activity with soluble sugars and the remobilization of starch suggests that the fast hydrolysis of starch under water stress is mainly attributed to the enhanced -amylase activity in rice stems.
Water stress appeared to have little effect on other possible starch-breaking enzymes, such as -glucosidase and starch phosphorylase activities (Fig. 6e–h), No or negative correlations were observed between their activities and starch disappearance. A probable explanation is that -glucosidase activity in rice stems is too low to play a regulatory role in starch breakdown, while starch phosphorylase may be involved in starch synthesis instead of remobilization (Beck and Ziegler, 1989). A negative correlation between starch phosphorylase activity and starch remobilization (Table 2) may suggest that this enzyme is more closely associated with starch synthesis than degradation in the rice stem during grain filling.
Both SPS and SS may be involved in sucrose synthesis (Wardlaw and Willenbrink, 1994). These results showed that SS activity negatively correlated with sucrose content in the stems and was not affected by water stress (Fig. 8). This finding supports the speculation that SS may have a role in the degradation rather than synthesis of sucrose (Sowokinos and Varns, 1992). In contrast to SS, water stress enhanced SPS activity in both Vlimit and Vmax terms (Fig. 7a–d). An enhanced SPS activity was closely associated with an increase of sucrose accumulation in the stems (Fig. 4g, h). The result supports the proposal that SPS is a key enzyme in the synthesis of sucrose, the transported form of carbon assimilates in plants (Giaquinta, 1978; Huber, 1983; Huber and Huber, 1992). These results also showed that SPS activity at Vlimit was more enhanced than that at Vmax by water stress and more closely correlated with sucrose content (Fig. 4g, h) and the remobilization of starch in the stems (Table 2). Therefore, it is concluded that the increase in the Vlimit of SPS resulting from the high activation state of the enzyme is one of the regulating factors that enhances the remobilization of starch in rice stems when subjected to water deficit.
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Acknowledgments |
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We are grateful for grants from the FRG of Hong Kong Baptist University, RGC of Hong Kong University Grants Council, AOE Research Fund of the Chinese University of Hong Kong, the National Natural Science Foundation of China (Project No. 39870464) and the State Key Basic Research and Development Plan (G 1999011700).
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Notes |
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3 To whom correspondence should be addressed. Fax: +852 2339 5995. E-mail: jzhang{at}hkbu.edu.hk
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Abbreviations |
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C, carbon; DAA, days after anthesis; NSC, non-structural carbohydrate;
soil, soil water potential; SPS, sucrose-phosphate synthase; SS, sucrose synthase; WW, well-watered; WS, water-stressed.. |
References |
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