JXB Advance Access originally published online on December 5, 2005
Journal of Experimental Botany 2006 57(1):149-160; doi:10.1093/jxb/erj018
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
RESEARCH PAPER |
Post-anthesis development of inferior and superior spikelets in rice in relation to abscisic acid and ethylene
1Key Laboratory of Crop Genetics and Physiology of Jiangsu Province, Yangzhou University, Yangzhou, Jiangsu, China
2Department of Biology, Hong Kong Baptist University, Hong Kong, China
* To whom correspondence should be addressed. E-mail: jzhang{at}hkbu.edu.hk
Received 4 May 2005; Accepted 18 October 2005
 |
Abstract |
|---|
 Top Abstract IntroductionMaterials and methodsResultsDiscussionReferences |
|---|
The purpose of this study was to test the hypothesis that the interaction between abscisic acid (ABA) and ethylene may be involved in mediating the post-anthesis development of spikelets in rice (Oryza sativa L.). Two rice genotypes were field-grown, and the changes of ABA, ethylene, and 1-aminocylopropane-1-carboxylic acid (ACC) levels in spikelets during grain filling and their relationships with endosperm-division and grain-filling rates were investigated. The results showed that earlier-flowering superior spikelets exerted dominance over later-flowering inferior spikelets in endosperm cell-division and grain-filling rates. The two genotypes behaved the same. Later-flowering spikelets had higher levels of ethylene and ACC than earlier-flowering spikelets. The ethylene evolution rate was significantly and negatively correlated with the cell division and grain filling rates. By contrast to ethylene, later-flowering spikelets contained a lower ABA content/concentration and showed a low content ratio of ABA to ACC than earlier-flowering ones. The cell-division and grain-filling rates were significantly and positively correlated with both ABA contents and the ratio of ABA to ACC. Application of cobalt ion (inhibitor of ethylene synthesis) or ABA at an early grain-filling stage significantly increased endosperm cell division rate and cell number, grain-filling rate, and grain weight of inferior spikelets. Application of ethephon (an ethylene-releasing agent) or fluridone (an inhibitor of carotenoid synthesis) had the opposite effect. The results suggest that antagonistic interactions between ABA and ethylene mediate endosperm cell-division and grain-filling in rice. A higher ratio of ABA to ethylene in rice spikelets is required to maintain a faster grain-filling rate.
Key words: Abscisic acid, 1-aminocylopropane-1-carboxylic acid, endosperm cell, ethylene, grain filling, inferior spikelets, rice, superior spikelets
|
Introduction |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
A rice (Oryza sativa L.) panicle is composed of a large number of spikelets, and each spikelet is considered an individual unit in the complex inflorescence (Mohapatra and Sahu, 1991
; Cao et al., 1992). During ontogeny, the development of primary branches on the panicle axis, opposite to their initiation, is in basipetal succession from the top to base, and spikelet development on a primary or secondary branch is acropetal, with the exception of the one at the tip of the branch which flowers first (Xu and Vergara, 1986). Lack of synchronization in spikelet development causes variations in kernel quality and weight (Mohapatra and Sahu, 1991; Yang et al., 2000). In general, earlier-flowering superior spikelets (grains), usually located on apical primary branches, fill fast and produce larger and heavier grains. While later-flowering inferior spikelets (grains), usually located on proximal secondary branches, are either sterile or fill slowly and poorly to produce grains unsuitable for human consumption (Mohapatra et al., 1993; Naik and Mohapatra, 1999). The slow grain-filling rate and low grain weight of inferior spikelets have often been attributed to a limitation in carbohydrate supply (Sikder and Gupta, 1976; Wang, 1981; Murty and Murty, 1982; Zhu et al., 1988). More recent work has shown, however, that there is no clear causative relationship between assimilate concentration and spikelet development in rice (Mohapatra and Sahu, 1991; Mohapatra et al., 1993, 2000). So far, the intrinsic factors responsible for variations in grain filling between the superior and inferior spikelets remain elusive.
It has been proposed that spikelet development may be mediated through endogenous hormones (Naik and Mohapatra, 1999; Yang et al., 2000, 2001), and a low ratio between promotive and inhibitory hormones in inferior spikelets may lead to their poor development (Naik and Mohapatra, 1999). Among phytohormones, both ethylene and abscisic acid (ABA) are generally regarded as inhibitory growth regulators (Walton, 1980; Trewavas and Jones, 1991; Chen and Lur, 1996; Mohapatra et al., 2000). A high ethylene evolution rate has frequently been related to abortion in maize (Zea mays) (Chen and Lur, 1996) and reduction in grain weight in wheat (Triticum aestivum) (Xu et al., 1995; Beltrano et al., 1999). Application of ethylene inhibitors improves dry matter partitioning and development of later-flowering spikelets on rice panicles (Mohapatra et al., 2000). However, the cause-and-effect relationship between ethylene emission from rice spikelets and the development of the spikelets has not been investigated.
There is a report that the anthers located on the distal floret positions of a wheat spikelet contain higher ABA levels than those on the proximal positions, and that the high ABA concentration is linked to reduced grain set (Lee et al., 1988). Under water stress, the reduction in grain set and kernel growth in wheat (Morgan, 1980; Saini and Aspinall, 1982; Ahmadi and Baker, 1999) and a decreased rate of endosperm cell division rate in maize (Myers et al., 1990; Ober et al., 1991) have been observed to be associated with elevated levels of ABA. There are many observations, however, that ABA can promote dry matter accumulation in the sink organ and its level is correlated with the growth rate of fruits or seeds (Eeuwens and Schwabe, 1975; Browning, 1980; Berüter, 1983; Schussler et al., 1984, 1991; Wang et al., 1987; Ross and MacWha, 1990; Kato et al., 1993; Wang et al., 1998; Yang et al., 2001).
The endosperm of rice represents >90% of the final weight of a kernel (Murata and Matsushima, 1975; Cao et al., 1992). The process of grain filling is actually the increase in both cell number and cell filling in the endosperm. There are generally positive relationships between endosperm cell division rate, endosperm cell number, and grain weight (Cao et al., 1992; Yang et al., 2002a). It is highly possible that both ABA and ethylene may exert their effects on cell division at the endosperm development stage, but information linking ABA and ethylene actions to cell division is lacking in the literature.
The purpose of this study was to test the hypothesis that ABA and ethylene and their interactions may be involved in mediating the post-anthesis development of inferior and superior spikelets on a rice panicle. The changing patterns of ABA and ethylene concentrations in rice grains during grain filling and their relationships with endosperm-division and grain-filling rates were investigated. The effects of chemical regulators on the levels of ABA and ethylene in the grains were also studied to verify the roles of the two hormones.
|
Materials and methods |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
Plant materials
The study was conducted at a farm belonging to Yangzhou University, Jiangsu Province, China (32° 30' N, 119° 25' E) during the rice growing season (May to October) of 2002, and repeated in 2003. Two rice genotypes, YD-4 (Yangdao 4, an indica inbred cultivar) and LY-9 (Pei-ai 64S/Yangdao 7, an indica/indica F1 hybrid), were grown in the paddy field. Seedlings were raised in the field and were sown on 10 May and transplanted on 11 June at a hill spacing of 0.2x0.2 m with two seedlings per hill. The plot dimension was 6x8 m. Each of the genotypes had four plots as repetitions in a complete randomized block design. The soil in the field was sandy loam (Typic fluvaquents, Entisols; US classification) that contained organic matter at 2.45% and available N-P-K at 108, 34.2, and 66.9 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 (50 kg ha–1). Both genotypes (50% of plants) headed on 21–23 August, and were harvested on 20 October. Except for drainage at the end of tillering (11–15 July), the water level in the field was kept at 1–2 cm during the whole growth period. The temperatures, averaged per 10 d from anthesis (21–23 August) to harvest, were 27.4, 26.5, 25.3, 24.1, 23.5, and 21.8 °C.
Sampling
Eight hundred panicles that headed on the same day were chosen and tagged for each plot, and for 50 of them the flowering date and the position of each spikelet on the panicle were recorded. The duration of anthesis from the first spikelet to the last on a panicle was 7 d for both genotypes. The spikelets that flowered on the same day were sampled as the same group and, accordingly, seven groups of spikelets on a panicle were identified. The spikelets that flowered on the first day on a panicle were considered as the first group (D1), and those that flowered on the second day as the second group (D2), and so on. Each group of spikelets, sampled at 2-d intervals from anthesis to 30 d post-anthesis (DPA) and at 6-d intervals from 30 DPA to maturity, consisted of 280–300 spikelets (grains) at each sampling time. Two-thirds of sampled grains were used for ABA, ethylene, and 1-aminocyclopropane-1-carboxylic acid (ACC) assays. Eighty to ninety sampled grains were used for measurements of grain dry weight and soluble carbohydrate. These were dried at 70 °C to constant weight, dehulled, and weighed. Soluble carbohydrate in the grains was determined as described by Yoshida et al. (1976). Ten to twelve grains, with a small hole cut on the edge of the hull, were fixed in Carnoy's solution (absolute ethanol:glacial acetic acid:chloroform; 9:3:1; v/v/v) for 48 h, and then kept in 70% (v/v) ethanol pending examination of endosperm cell number.
Nuclear/cell counting
The method for isolation and counting of endosperm cells was modified from Singh and Jenner (1982). Briefly, fixed grains were dehulled and transferred into 50% (v/v) and 25% (v/v) ethanol, respectively, and finally into distilled water for 5–7 h prior to dissection of the endosperm. The endosperm was isolated under a dissecting microscope and dyed with a Delafied's haematoxylin solution for 24–30 h, washed several times with distilled water, and then hydrolysed in 0.1% (w/v) cellulase (No. c-2415; Sigma Chemical Co., St Louis, MO, USA) solution (pH 5.0) at 40 °C for 4–6 h and oscillated. The isolated endosperm cells were diluted to 2–10 ml according to the development stage of the endosperm, from which 8–10 subsamples (20 µl for each subsample) were taken to a counting chamber (1 cm2 area). Using a light microscope, the endosperm cell number of 10 fields of view for each counting chamber was noted. Within 2–4 DPA, the number of nuclei was counted as the endosperm cell number. The total cell number per endosperm was calculated according to Liang et al. (2001). Eight grains (endosperms) were examined for each genotype at each measurement.
The division processes of endosperm cells, as well as the processes of grain filling, were fitted by Richards' growth equation (Richards, 1959) as described by Zhu et al. (1988):
 | (1) |
Endosperm cell division rate or grain-filling rate (R) was calculated as the derivative of equation (1)
 | (2) |
ABA extraction, purification, and quantification
The methods for extraction and purification of abscisic acid [(+)-ABA] were modified from those described by Bollmark et al. (1988) and He (1993). Samples of 20–30 grains were ground in a mortar (at 0 °C) in 10 ml 80% (v/v) methanol extraction medium containing 1 mM butylated hydroxytoluence as an antioxidant. The extract was incubated at 4 °C for 4 h and centrifuged at 4800 g for 15 min at the same temperature. The supernatants were passed through Chromosep C18 columns (C18 Sep-Park Cartridge, Waters Corp, Millford, MA, USA), pre-washed with 10 ml 100% and 5 ml 80% methanol, respectively. The hormone fractions were dried under N2, and dissolved in 2 ml phosphate-buffered saline, containing 0.1% (v/v) Tween 20 and 0.1% (w/v) gelatin (pH 7.5), for analysis by an enzyme-linked immunosorbent assay (ELISA).
The mouse monoclonal antigen and antibody against ABA, and Ig G-HRP (immunoglobulin G–horseradish peroxidase) used in ELISA were produced at the Phytohormones Research Institute, China Agricultural University, China (He, 1993). The methods for quantification of ABA by ELISA and recovery test were as described previously (Yang et al., 2001, 2002b). The recovery percentage of ABA in grains was 82.5±5.6. The specificity of the monoclonal antibody and other possible non-specific immunoreactive interference had been checked previously and proved reliable (Wu et al., 1988; Yang et al., 2002b).
Ethylene and ACC analysis
Ethylene evolved from grains was determined according to Beltrano et al. (1994) with modifications. Briefly, sampled grains were placed between two sheets of moist paper for 1 h at 27 °C in darkness to allow wound ethylene to subside. Each sample contained 60–80 grains. Grains were then transferred into 15 ml glass vials containing moist filter paper and immediately sealed with airtight subaseal stoppers, and incubated in the dark for 24 h at 27 °C. A 1 ml gas sample was withdrawn through the subaseal with a gas-tight syringe, and ethylene was assayed using a gas chromatograph (HP5890 Series II; Hewlett Packard Com, Palo Alto, CA, USA) equipped with a Porapak Q column (0.3x200 cm, 50–80 mesh) and flame ionization detector. Temperatures for the injection port, column, and detector were kept constant at 140, 100, and 200 °C, respectively. Nitrogen was used as the carrier at a flow rate of 30 ml min–1, and hydrogen and air were used for flame ionization detection at the rate of 30 ml min–1 and 300 ml min–1, respectively. The rate of ethylene evolution was expressed as a function of per grain.
ACC in the grains was determined according to Chen and Lur (1996). Ethylene evolved from ACC was assayed by using gas chromatography as described above. The transformation rates as a percentage from ACC to ethylene were 86±4.2, 92±5.4, and 78±5.1, respectively, at early, mid- and late grain-filling stages.
Chemical applications
Both the genotypes were used for chemical application. Plants were grown in eight cement tanks in open-field conditions. Each tank (0.3 cm high, 1.6 m wide, 8.8 m long) was filled with sandy loam soil with the same nutrient content as the field experiment. Thirty-day-old seedlings raised in the field were transplanted on 12 June into the tanks at a hill spacing of 0.15x0.20 m with one seedling per hill. N (6 g m–2 as urea), P (3 g m–2 as single superphosphate), and K (3 g m–2 as KCl) were applied and incorporated before transplanting. N as urea was also applied at mid-tillering (3 g m–2) and at panicle initiation (3 g m–2). The water level in the tank was kept at 1–2 cm during the whole growth period.
Synthetic (±)-ABA, ethephon (an ethylene-releasing agent), cobaltous nitrate [Co(NO3)2, which inhibits ethylene synthesis by inhibiting ACC oxidase] (all from Sigma, St Louis, MO, USA) and fluridone (Fluka, Riedel-de Haën, Germany), an inhibitor of carotenoid biosynthesis, which may indirectly also reduce ABA synthesis, were applied to the plants. Preparation of the chemical solutions was as described elsewhere (Ober and Sharp, 1994; Chen and Lur, 1996). Starting at initiation of heading (panicles begin to appear out the sheath of the flag leaf), 20x10–6 M (±)-ABA, 20x10–6 M fluridone, 50x10–3 M ethephon, or 5x10–5 M Co(NO3)2 was applied to panicles by using a writing brush which had been dipped in the solutions. The chemicals were applied daily for 8 d at the rate of 2–5 ml per panicle at each application. All the solutions contained ethanol and Tween 20 at final concentrations of 0.1% (v/v) and 0.01 (v/v), respectively. The same volume of deionized water containing the same concentrations of ethanol and Tween 20 was applied to the control plants. Each chemical treatment was on an area of 2.4 m2 with four replications.
For all the chemical treatments, spikelets on a panicle were divided into two groups, i.e. the superior and the inferior. Superior spikelets were those that flowered on the first two days and inferior ones were those that flowered on the last two days on a panicle. Levels of ABA and ethylene in the grains were determined 9 and 16 DPA, respectively. Endosperm cell number was measured at 2-d intervals from anthesis to 30 DPA, and grain weight at 6-d intervals from anthesis to maturity. Measurement methods were the same as described above. Twenty plants (170–176 panicles) for each treatment were harvested at maturity to determine final grain weight.
Statistical analysis
The results were analysed for variance using SAS/STAT statistical analysis package (version 6.12, SAS Institute, Cary, NC, USA). Data from each sampling date were analysed separately. Means were tested by least significant difference at the P ±0.05 level (LSD0.05). Linear regression was used to evaluate the relationships between concentrations of ABA and ethylene in the grains and the endosperm cell-division rate or grain-filling rate. As the data from both years were very similar they were averaged.
|
Results |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
Cell division and grain filling
Figure 1 illustrates the division progression and division rate of endosperm cells for the spikelets that flowered on different dates. The earlier the spikelets flowered, the earlier the maximum cell division rate was reached, the shorter the active cell division period, and the greater the cell division rate and final cell number. There were no significant differences in the cell numbers and cell division periods between the spikelets that flowered on the first day (D1) and those on the second day (D2).The two genotypes behaved the same (Fig. 1).
|
Very similar to the changing pattern of endosperm cell number, grain dry weight increased much faster for the earlier-flowering superior spikelets than for later-flowering inferior ones (Fig. 2). The earlier the spikelets flowered, the greater the grain-filling rate and the heavier the grain. As shown in Figs 1 and 2, the grain weight was closely related to the cell number, i.e. the more endosperm cells in spikelets, the higher the grain weight, suggesting that endosperm cell number plays a dominant role in the determination of grain weight in rice.
|
By contrast to the cell-division rate and grain-filling rate, later-flowering spikelets contained a higher soluble carbohydrate concentration than earlier-flowering ones at the early grain-filling stage (Fig. 3a, b), indicating that assimilate supply is not a factor limiting endosperm cell division and grain filling in inferior spikelets. The changes in soluble carbohydrate were closely associated with those of sucrose in the spikelets (Fig. 3c, d), and they were significantly correlated (r=0.98**, P=0.01), suggesting that the high concentration of soluble carbohydrate was mainly attributed to a high concentration of sucrose in the inferior spikelets.
|
Levels of ABA, ethylene, and ACC in spikelets
ABA levels were much higher, while ethylene and ACC levels were much lower, in superior spikelets than in inferior spikelets (expressed either as per unit weight or as per grain during the grain-filling period; Table 1). Correlation of hormone content (per grain) and hormone concentration (per unit weight) during grain filling was significant with r=0.91**, 0.93**, and 0.96** (n=30, P=0.01), respectively, for ABA, ethylene, and ACC. As endosperm cell-division and grain-filling rates were expressed on the basis of per grain, data presented as hormone content would be more comparable in the analysis of the relationship between hormones and grain development.
|
At the initial stage of grain filling, the ABA content of spikelets was low, but it increased with the grain-filling process, declining after reaching a maximum (Fig. 3a, b). The concentrations and the peak values of ABA varied with the flowering dates of spikelets. The earlier the spikelets flowered, the earlier they reached a maximum and the greater the peak value of ABA.
In sharp contrast to ABA, ethylene evolution from spikelets was very high at the early grain-filling stage, but rapidly decreased with the grain-filling processes (Fig. 4c, d). The later the spikelets flowered, the greater the ethylene evolution rate at early and mid-grain-filling stages. A very similar pattern was observed for ACC content in the spikelets (Fig. 4e, f). There was a significant correlation between ACC content and the ethylene evolution rate (r=0.99**, P=0.001), suggesting that the high ethylene evolution rate is attributed to the high ACC content in inferior spikelets.
|
As shown in Fig. 5, earlier-flowering spikelets also showed a higher ratio of ABA to ACC than later-flowering spikelets. The earlier the spikelets flowered, the greater the ratio (Fig. 5). The peak values of both ABA and the ratio of ABA to ACC were associated with the maximum cell-division rates. Regression analysis demonstrated that there was a significant and positive correlation between the maximum rate of endosperm cell division and the average ABA content and the ratio of ABA to ACC (r=0.89** and r=0.94**, respectively, P=0.01), whereas the correlation between the maximum rate of endosperm cell division, during the active cell division period or the phase of linear endosperm cell increase, and the ethylene evolution rate was significant and negative (r=–0.95**, P=0.01) (Fig. 6a–c).
|
|
During active grain filling or linear growth period, changes in ABA content and the ratio of ABA to ACC in grains (Figs 4a, b, 5a, b) paralleled the grain-filling rate (Fig. 2a, b). Figure 7 shows that there was a significant and positive correlation between ABA content and ABA to ACC ratios, whereas the correlation between ethylene evolution rates and grain-filling rates was significant and inverse (r=0.94**, r=0.82**, and r= –0.55**, respectively, P=0.01) (Fig. 7a–c).
|
Effects of chemical applications
Application of Co(NO3)2, an inhibitor of ethylene synthesis, significantly reduced the ethylene evolution rate and concentration of soluble sugars in inferior spikelets, and increased the cell-division rate, maximum cell number, grain-filling rate and grain weight (Tables 2, 3). Application of ethephon, an ethylene-releasing substance, exhibited the opposite effects. Application of fluridone, an indirect inhibitor of ABA synthesis, reduced ABA content but increased the ethylene evolution rate of both superior and inferior spikelets (Table 2). It significantly reduced the cell-division rate, maximum cell number, grain-filling rate, and grain weight of both types of spikelets (Table 3). By contrast to fluridone, application of ABA significantly increased ABA and reduced ethylene levels in inferior grains, and the cell division rate, maximum cell number, grain filling rate, and grain weight of these grains increased significantly. All the chemicals affected the superior spikelets less than the inferior ones (Tables 2, 3).
|
|
|
Discussion |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
It is generally presumed that constraints in assimilate availability for inferior spikelets results in their poor grain filling (Sikder and Gupta, 1976
; Wang, 1981; Murty and Murty, 1982; Zhu et al., 1988; Cao et al., 1992). The present results showed, however, that later-flowering inferior spikelets contained much higher levels of soluble sugars than earlier-flowering superior ones at the early grain-filling stage (Fig. 3a, b). The results demonstrate that carbohydrates are not the limiting factor, at least at the initial grain-filling stage, to the development of inferior spikelets.
The phenomenon that earlier-flowering superior spikelets exert dominance over later-flowering inferior spikelets is often explained as apical dominance or primigenic dominance (Bangerth, 1989). In the hypothesis of primigenic dominance, it is suggested that indole-3-acetic acid (IAA) export of the earlier developed sink inhibits the IAA export of later developed sinks, and this depressed IAA export of the subordinated fruit/sink acts as the signal that leads to inhibited development (Bangerth, 1989). Earlier work (Yang et al., 2000) has shown, however, that IAA concentration in later-flowering inferior grains is not necessarily lower than that in the earlier-flowering superior grains in rice, and the grain-filling rate is not significantly correlated with the IAA concentration. Duan et al. (1999) reported a similar observation. The data on the effect of exogenous IAA on the grain-filling rate are rather controversial (Labrana et al., 1991; Patel and Mohapatra, 1992; Zhou et al., 1999; Wang et al., 2001; Bhatia and Singh, 2002).The present data, for the first time, have shown a close association between changes of ethylene production and ABA content in grains and the rates of endosperm cell division and grain filling (Figs 1, 2, 4). Later-flowering inferior spikelets had a much greater ethylene evolution rate and higher ACC content than earlier-flowering superior spikelets (Fig. 4c–f). There was significant and negative correlation between ethylene evolution rate and the cell-division and grain-filling rates (Figs 6b, 7b). Application of the inhibitor of ethylene synthesis (cobalt ion) increased, and while applying ethylene-releasing agent (ethephon) reduced, the cell division-rate, grain-filling rate, and grain weight (Tables 2, 3). The data suggest that ethylene plays a role in inhibiting endosperm cell division and grain filling, and high ethylene production in inferior spikelets contributes to their poor development.
Little is known how ethylene affects cell division and grain growth. It is suggested that ethylene may enhance the breakdown of cytokinins (Bollmark and Eliasson, 1990), which play an important role in maintaining cell division in the endosperm (Morris et al., 1993; Yang et al., 2002a). Naik and Mohapatra (2000) have reported that ethylene inhibitors applied to rice panicles at booting stage could significantly enhance the activities of sucrose synthase, which is generally regarded as a biochemical marker of sink strength (Wang et al., 1993), in the grains and promote grain filling. Their work may suggest that ethylene limits sink strength by inhibiting the key enzymes involved.
It has been proposed that ethylene produced from the dominant basal spikelets of maize at the time of pollination inhibits growth of the poorly developed kernels at the distal end of the inflorescence (Reed and Singletary, 1989). It was observed that ethylene and ACC levels in later-flowering inferior spikelets were much higher than those in earlier-flowering superior spikelets at the initial stage of grain filling (Fig. 4c–f). Such a result implies that high ethylene and ACC levels in inferior spikelets might be due to synthesis per se. One explanation is that the upper leaves of rice produce a large amount of ethylene during the period of grain filling (Debata and Murty, 1983; Khan and Choudhury, 1992), and ethylene may exert more of an inhibitory action on the inferior spikelets than on the superior ones, because the former are usually located on the lower part of a panicle and remain confined to the enclosure formed by the flag leaf for a longer time than the latter (Mohapatra et al., 2000). However, the causal role of leaf-released ethylene in inhibiting inferior spikelets is doubtful because, in wheat, leaves also evolve a large amount ethylene during the grain filling period (Labrana et al., 1991; Beltrano et al., 1994), but proximal spikelets usually produce heavier grains than the apical ones (Peng et al., 1992; Jiang et al., 2003).
It was observed that the higher levels of ethylene and ACC were closely associated with the lower ABA content in inferior grains (Fig. 4a, b). The change in ABA concentration in the grains followed a similar pattern to the grain filling rate (Figs 2c, d, 4a, b). The cell-division rate and grain-filling rate were significantly and positively correlated with the contents of ABA (Figs 6a, 7a). Application of ABA to panicles significantly increased the rates of cell division and grain filling and grain weight of inferior spikelets, whereas application of fluridone, an indirect inhibitor of ABA synthesis, had the opposite effect (Tables 2, 3). From the above therefore it is speculated that, contrary to ethylene, ABA plays a role in enhancing grain filling. It is possible that insufficient ABA to suppress ethylene emission leads, at least partly, to the poor grain filling of inferior spikelets.
The mechanism by which ABA facilitates endosperm cell division and grain filling is not understood. It has been observed that ABA regulates epidermal cell type-specific gene expression in the meristematic zone of Arabidopsis (van Hengel et al., 2004). Studies of several species indicate that an important role of endogenous ABA is to limit ethylene production, and thereby functions to maintain plant growth (Sharp et al., 2000; LeNoble et al., 2004). It is also proposed that ABA plays an important role in relation to sugar-signalling pathways and enhances the ability of plant tissues to respond to subsequent sugar signals (Rook et al., 2001). There are many reports that ABA enhances the movement of photosynthetic assimilates towards developing seeds (Dewdney and McWha, 1979; Ackerson, 1985; Brenner and Cheikh, 1995; Yang et al., 2004a). Earlier work (Yang et al., 2004b) has shown that the activities of key enzymes involved in sucrose-to-starch conversion, such as sucrose synthase and starch synthase, are enhanced in wheat grains by a mild water stress imposed during grain filling, and are highly correlated with elevated ABA levels there. Furthermore, the activities of the enzymes and remobilization of assimilates from the stem to the grains are significantly increased when ABA was applied to plants at an early grain-filling stage. These results suggest, by contrast to ethylene, that ABA may promote grain filling through regulating sink strength.
The present results showed that the changes in the ratio of ABA to ACC in grains were also associated with the endosperm cell-division rate and the grain-filling rate (Figs 1b, d, 2b, d, 5a, b). The rates of cell division and grain filling not only correlated with the levels of ABA and ethylene, but they also correlated with the ratio of ABA to ACC (Figs 6c, 7c). Such results imply that antagonistic interactions between ABA and ethylene may mediate cell division and grain filling in rice.
In conclusion, earlier-flowering superior spikelets exhibit dominance over later-flowering inferior ones. Carbohydrates may not be the major limiting factor to the development of inferior spikelets. High ethylene production and low ABA content in inferior spikelets result in slow endosperm cell division and poor grain filling, which lead to low grain weight. On the other hand, a high ABA concentration in superior spikelets contributes to a fast grain-filling rate. The rates of cell division and grain filling are not only correlated with the levels of ABA and ethylene, but are also correlated with the ratio of ABA to ACC in grains. Antagonistic interactions between ABA and ethylene may be involved in mediating cell division and grain filling. A higher ratio of ABA and ethylene in rice spikelets would be required to enhance the grain-filling rate. Further investigation is needed to establish a possible cause-and-effect relationship between ABA/ethylene and grain development in rice using mutants or transgenic plants with an attenuated capacity to respond to, or synthesize, the hormones.
|
Acknowledgements |
|---|
We are grateful for grants from the National Natural Science Foundation of China (Project No. 30270778, 30370828, BK2003041), the Research Grant Council of Hong Kong (RGC 2149/04M, 2165/05M), and the Area of Excellence for Plant and Fungal Biotechnology in the Chinese University of Hong Kong.
|
Footnotes |
|---|
Abbreviations: ABA, abscisic acid; ACC, 1-aminocylopropane-1-carboxylic acid; DPA, days post-anthesis; IAA, indole-3-acetic acid.
|
References |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
Ackerson RC. 1985. Invertase activity and abscisic acid in relation to carbohydrate status in developing soybean reproductive structures. Crop Science 25, 615–618.
Ahmadi A, Baker DA. 1999. Effects of abscisic acid (ABA) on grain filling processes in wheat. Plant Growth Regulation 28, 187–197.
Bangerth F. 1989. Dominance among fruits/sinks and the search for a correlative signal. Physiologia Plantarum 76, 608–614.[CrossRef]
Beltrano J, Carbone A, Montaldi ER, Guiamet JJ. 1994. Ethylene as promoter of wheat grain maturation and ear senescence. Plant Growth Regulation 15, 107–112.
Beltrano J, Ronco MG, Montaldi ER. 1999. Drought stress syndrome in wheat is provoked ethylene evolution imbalance and reversed by rewatering, aminoethoxyvinylglycine, or sodium benzoate. Journal of Plant Growth Regulation 18, 59–64.[Medline]
Berüter J. 1983. Effect of abscisic acid on sorbitol uptake in growing apple fruits. Journal of Experimental Botany 34, 737–743.
Bhatia S, Singh R. 2002. Phytohormone-mediated transformation of sugars to starch in relation to the activities of amylases, sucrose-metabolizing enzymes in sorghum grain. Plant Growth Regulation 36, 97–104.[CrossRef]
Bollmark M, Eliasson L. 1990. Ethylene accelerates the breakdown of cytokinin and thereby stimulates rooting in Norway spruce hypocotyl cuttings. Physiologia Plantarum 80, 534–540.[CrossRef]
Bollmark M, Kubat B, Eliasson L. 1988. Variations in endogenous cytokinin content during adventitious root formation in pea cuttings. Journal of Plant Physiology 132, 262–265.
Brenner ML, Cheikh N. 1995. The role of hormones in photosynthate partitioning and seed filling. In: Davies PJ, ed. Plant hormones, physiology, biochemistry and molecular biology. Dordrecht: Kluwer Academic Publishers, 649–670.
Browning G. 1980. Endogenous cis, trans-abscisic acid and pea seed development: evidence for a role in seed growth from changes induced by temperature. Journal of Experimental Botany 31, 185–197.
Cao X, Zhu Q, Yang J. 1992. Classification of source-sink types in rice varieties with corresponding cultivated ways. In: Xiong Z, Min S, eds. Prospects of rice farming for 2000. Hangzhou: Zhejiang Science & Technology Press, 361–372.
Chen CY, Lur HS. 1996. Ethylene may be involved in abortion of the maize caryopsis. Physiologia Plantarum 98, 245–252.[CrossRef]
Debata A, Murty KS. 1983. Endogenous ethylene content in rice leaves during senescence. Indian Journal of Plant Physiology 26, 425–427.
Dewdney SJ, McWha JA. 1979. Abscisic acid and the movement of photosynthetic assimilates towards developing wheat (Triticum aestivum L.) grains. Zeitschrift für Pflanzenphysiologie 92, 186–193.
Duan J, Tian C, Liang C, Huang Y, Liu H. 1999. Dynamic changes of endogenous plant hormones in rice grains in different parts of panicle at grain filling stage. Acta Botanica Sinica 41, 75–79.
Eeuwens CJ, Schwabe WW. 1975. Seed and pod wall development in Pisum sativum L. in relation to exacted and applied hormones. Journal of Experimental Botany 26, 1–14.
He Z. 1993. Method for an indirect enzyme-linked immunosorbent asssay. In: He ZP, ed. Guidance on experimental chemical control in crop plants. Beijing: Beijing Agricultural University Publishers, 60–68.
Jiang D, Cao W, Dai T, Jing Q. 2003. Activities of key enzymes for starch synthesis in relation to growth of superior and inferior grains on winter wheat (Triticum aestivum L.) spike. Plant Growth Regulation 41, 247–257.[CrossRef]
Kato T, Sakurai N, Kuraishi S. 1993. The changes of endogenous abscisic acid in developing grains of two rice cultivars with different grain size. Japanese Journal of Crop Science 62, 456–461.
Khan RI, Choudhury MA. 1992. Role of endogenous hormones in the regulation of whole plant senescence in rice. Indian Journal of Experimental Biology 30, 131–134.
Labrana X, Vendrell M, Araus JL. 1991. Ethylene production in wheat flag leaves and ears during grain filling. Plant Physiology and Biochemistry 29, 349–354.
Lee BT, Martin P, Bangerth F. 1988. Phytohormone levels in the florets of a single wheat spikelet during pre-anthesis development and relationships to grain set. Journal of Experimental Botany 39, 927–933.
LeNoble ME, Spollen WG, Sharp RE. 2004. Maintenance of shoot growth by endogenous ABA: genetic assessment of the involvement of ethylene suppression. Journal of Experimental Botany 55, 237–245.
Liang J, Zhang J, Cao X. 2001. Grain sink strength may be related to the poor grain filling of indica-japonica rice (Oryza sativa) hybrids. Physiologia Plantarum 112, 470–477.[CrossRef][Medline]
Mohapatra PK, Naik PK, Patel R. 2000. Ethylene inhibitors improve dry matter partitioning and development of late flowering spikelets on rice panicles. Australian Journal of Plant Physiology 27, 311–323.
Mohapatra PK, Patel R, Sahu SK. 1993. Time of flowering affects grain quality and spikelet partitioning within the rice panicle. Australian Journal of Plant Physiology 20, 231–242.
Mohapatra PK, Sahu SK. 1991. Heterogeneity of primary branch development and spikelet survival in rice in relation to assimilates of primary branches. Journal of Experimental Botany 42, 871–879.
Morgan JM. 1980. Possible role of abscisic acid in reducing seed set in water-stressed wheat plants. Nature 285, 655–657.[CrossRef]
Morris RD, Blevins DG, Dietrich JT, et al. 1993. Cytokinins in plant pathogenic bacteria and developing cereal grains. Australian Journal of Plant Physiology 20, 621–637.
Murata Y, Matsushima S. 1975. Rice. In: Evans LT, ed. Crop physiology. Cambridge: Cambridge University Press, 75–99.
Murty PSS, Murty KS. 1982. Spikelet sterility in relation to nitrogen and carbohydrate contents in rice. Indian Journal of Plant Physiology 25, 40–48.
Myers PN, Setter TL, Madson JT, Thompson JF. 1990. Abscisic acid inhibition of endosperm cell division in cultured maize kernels. Plant Physiology 94, 1330–1336.
Naik PK, Mohapatra PK. 1999. Ethylene inhibitors promote male gametophyte survival in rice. Plant Growth Regulation 28, 29–39.[CrossRef]
Naik PK, Mohapatra PK. 2000. Ethylene inhibitors enhanced sucrose synthase activity and promoted grain filling of basal rice kernels. Australian Journal of Plant Physiology 27, 997–1008.
Ober ES, Setter TL, Madison JT, Thompson JF, Shapiro PS. 1991. Influence of water deficit on maize endosperm development: enzyme activities and RNA transcripts of starch and zein synthesis, abscisic acid, and cell division. Plant Physiology 97, 154–164.
Ober ES, Sharp RE. 1994. Proline accumulation in maize (Zea mays L.) primary roots at low water potentials. I. Requirement for increased levels of abscisic acid. Plant Physiology 105, 981–987.[Abstract]
Patel R, Mohapatra PK. 1992. Regulation of spikelet development in rice by hormones. Journal of Experimental Botany 43, 257–262.
Peng Y, Guo W, Yan L. 1992. Source–sink relationship in wheat and its regulation. In: Peng Y, ed. Wheat production and physiology. Nanjing: Dongnan University Publishers, 20–62.
Reed AJ, Singletary GW. 1989. Roles of carbohydrate supply and phytohormones in maize kernel abortion. Plant Physiology 91, 986–992.
Richards FJ. 1959. A flexible growth function for empirical use. Journal of Experimental Botany 10, 290–300.
Rook F, Corke F, Card R, Munz G, Smith C, Bevan MW. 2001. Impaired sucrose-induction mutants reveal the modulation of sugar-induced starch biosynthetic gene expression by abscisic acid signaling. The Plant Journal 26, 421–433.[CrossRef][Web of Science][Medline]
Ross GS, McWha JA. 1990. The distribution of abscisic acid in Pisum sativum plants during seed development. Journal of Plant Physiology 136, 137–142.
Saini HS, Aspinall DA. 1982. Sterility in wheat (Triticum aestivum L.) induced by water deficit or high temperature: possible mediation by abscisic acid. Australian Journal of Plant Physiology 9, 529–537.[Web of Science]
Schussler JR, Brenner ML, Brun WA. 1984. Abscisic acid and its relationship to seed filling in soybeans. Plant Physiology 76, 301–306.
Schussler JR, Brenner ML, Brun WA. 1991. Relationship of endogenous abscisic acid to sucrose level and seed growth rate of soybeans. Plant Physiology 96, 1308–1313.
Sharp RE, LeNoble ME, Else MA, Thorne ET, Gherardi F. 2000. Endogenous ABA maintains shoot growth in tomato independently of effects on plant water balance: evidence for an interaction with ethylene. Journal of Experimental Botany 51, 1575–1584.
Sikder HP, Gupta DKD. 1976. Physiology of grain in rice. Indian Agriculture 20, 133–141.
Singh BM, Jenner CF. 1982. A modified method for the determination of cell number in wheat endosperm. Plant Science Letters 26, 273–278.[CrossRef]
Trewavas AJ, Jones HG. 1991. An assessment of the role of ABA in plant development. In: Davies WJ, Jones HG, eds. Abscisic acid: physiology and biochemistry. Oxford: Bios Scientific Publishers, 169–188.
van Hengel AJ, Barber C, Roberts K. 2004. The expression patterns of arabinogalactan-protein AtAGP30 and GLABRA2 reveal a role for abscisic acid in the early stages of root epidermal patterning. The Plant Journal 39, 70–83.[CrossRef][Web of Science][Medline]
Walton DC. 1980. Biochemistry and physiology of abscisic acid. Annual Review of Plant Physiology 31, 453–489.[Web of Science]
Wang F, Sanz A, Brenner ML, Smith A. 1993. Sucrose synthase, starch accumulation, and tomato fruit sink strength. Plant Physiology 101, 321–327.[Abstract]
Wang TL, Cook SK, Francis RJ, Ambrose MJ, Hedley CL. 1987. An analysis of seed development in Pisum sativum. VI. Abscisic acid accumulation. Journal of Experimental Botany 38, 1921–1932.
Wang Y. 1981. Effectiveness of supplied nitrogen at the primordial panicle stage on rice characteristics and yields. International Rice Research News Letter 6, 23–24.
Wang Z, Cao W, Dai T, Zhu Q. 2001. Effects of exogenous hormones on floret development and grain set in wheat. Plant Growth Regulation 35, 225–231.[CrossRef]
Wang Z, Yang J, Zhu Q, Zhang Z, Lang Y, Wang X. 1998. Reasons for poor grain filling in intersubspecific hybrid rice. Acta Agronomica Sinica 24, 782–787.
Wu S, Chen W, Zhou X. 1988. Enzyme linked immunosorbent assay for endogenous plant hormones. Plant Physiology Communication 5, 53–57.
Xu X, Vergara BS. 1986. Morphological changes in rice panicle development: a review of literature. International Rice Research Institute Research Paper Series 117, 1–16.
Xu ZZ, Yu ZW, Qi XH, Yu SL. 1995. Effect of soil drought on ethylene evolution, polyamine accumulation and cell membrane in flag leaf of winter wheat. Acta Physiologia Sinica 21, 295–301.
Yang J, Peng S, Visperas RM, Sanico AL, Zhu Q, Gu S. 2000. Grain filling pattern and cytokinin content in the grains and roots of rice plants. Plant Growth Regulation 30, 261–270.[CrossRef]
Yang J, Zhang J, Huang Z, Wang Z, Zhu Q, Liu L. 2002a. Correlation of cytokinin levels in the endosperm and roots with cell number division activity during endosperm development in rice. Annals of Botany 90, 369–377.
Yang J, Zhang J, Huang Z, Wang Z, Zhu Q, Liu L. 2002b. Abscisic acid and cytokinins in the root exudates and leaves and their relationship to senescence and remobilization of carbon reserves in rice subjected to water stress during grain filling. Planta 215, 645–652.[CrossRef][Web of Science][Medline]
Yang J, Zhang J, Wang Z, Xu G, Zhu Q. 2004a. Activities of key enzymes in sucrose-to-starch conversion in wheat grains subjected to water deficit during grain filling. Plant Physiology 135, 1621–1629.
Yang J, Zhang J, Wang Z, Zhu Q, Liu L. 2004b. Activities of fructan and sucrose-metabolizing enzymes in wheat stems subjected to water stress during grain filling. Planta 220, 331–343.[CrossRef][Web of Science][Medline]
Yang J, Zhang J, Wang Z, Zhu Q, Wang W. 2001. Hormonal changes in the grains of rice subjected to water stress during grain filling. Plant Physiology 127, 315–323.
Yoshida S, Forno D, Cock J, Gomez K. 1976. Determination of sugar and starch in plant tissue. In: Yoshida S, ed. Laboratory manual for physiological studies of rice. Los Baños, The Philippines: International Rice Research Institute, 46–49.
Zhou XM, MacKenzie AF, Madramootoo CA, Smith DL. 1999. Effects of stem-injected plant growth regulators, with or without sucrose, on grain production, biomass and photosynthetic activity of field-grown corn plants. Journal of Agronomy and Crop Science 183, 103–110.[CrossRef]
Zhu Q, Cao X, Luo Y. 1988. Growth analysis in the process of grain filling in rice [in Chinese with English abstract]. Acta Agronomica Sinica 14, 182–192.
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  T. Tang, H. Xie, Y. Wang, B. Lu, and J. Liang The effect of sucrose and abscisic acid interaction on sucrose synthase and its relationship to grain filling of rice (Oryza sativa L.) J. Exp. Bot., July 1, 2009; 60(9): 2641 - 2652. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. C. Day, R. P. Herridge, B. A. Ambrose, and R. C. Macknight Transcriptome Analysis of Proliferating Arabidopsis Endosperm Reveals Biological Implications for the Control of Syncytial Division, Cytokinin Signaling, and Gene Expression Regulation Plant Physiology, December 1, 2008; 148(4): 1964 - 1984. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||