Polyamine and ethylene interactions in grain filling of superior and inferior spikelets of rice

This study was to test the hypothesis that polyamines (PAs) and ethylene and their interactions may be involved in mediating the post-anthesis development of spikelets in rice (Oryza sativa L.). Six rice cultivars differing in grain filling rate were field-grown, and the changing patterns of PAs and ethylene levels in rice spikelets during the filling and their relations with grain filling rates were investigated. The results showed that inferior spikelets had much greater ethylene evolution rate and 1-aminocylopropane-1-carboxylic acid (ACC) concentration than superior spikelets. Opposite to ethylene production, superior spikelets showed much higher free-spermidine (Spd) and free-spermine (Spm) concentrations than inferior spikelets. Grain filling rate was very significantly and negatively correlated with ethylene evolution rate and ACC concentration, whereas positively correlated with free-Spd and free-Spm concentrations and with the ratio of free-Spd or free-Spm to ACC. Application of Spd, Spm, or aminoethoxyvinylglycine (an inhibitor of ethylene synthesis by inhibiting ACC synthesis) to panicles at the early grain filling stage significantly reduced ethylene evolution rate and ACC concentration, while significantly increased Spd and Spm concentrations, grain filling rate and grain weight of inferior spikelets. Application of ACC, ethephon (an ethylene-releasing agent), or methylglyoxal-bis (guanylhydrazone) (an inhibitor of Spd and Spm synthesis) showed the opposite effects. The results suggest that antagonistic interactions between PAs (Spd and Spm) and ethylene may be involved in mediating grain filling. A higher ratio of free-Spd or free-Spm to ethylene in rice spikelets could enhance grain filling.     

Hormones in the grains and roots in relation to post-anthesis development of inferior and superior spikelets in japonica/indica hybrid rice

Grain filling is usually not adequate in later-flowering inferior spikelets in japonica/indica (J/I) hybrid rice (Oryza sativa) although it shows stronger hybrid vigor than indica/indica (I/I) hybrid. This study investigated the potential causes by examining changes in zeatin (Z) + zeatin riboside (ZR), indole-3-acetic acid (IAA), gibberellins (GAs, GA₁ + GA₄), and abscisic acid (ABA) in spikelets and roots during the grain filling period. The inferior spikelets of J/I hybrid exhibited low rate of endosperm cell division and slow grain filling. During the early grain filing period, they had less Z + ZR, IAA, and ABA, but more GAs, than the earlier-flowering superior spikelets. If compared to the inferior spikelets of the I/I hybrid, the J/I inferior spikelets also had less Z + ZR, IAA, and ABA. Rates of endosperm cell division and grain filling were positively and significantly correlated with Z + ZR and ABA contents in both grains and roots or IAA in grains, whereas not significantly correlated with GAs either in grains or roots or IAA in roots. Applications of kinetin, IAA, or ABA to spikelets, or kinetin and ABA to roots, enhanced cell division and grain filling in the inferior spikelets. Results suggest that low contents of cytokinins and ABA in both grains and roots and low contents of IAA in grains may result in the poor filling of inferior spikelets in the J/I hybrid.     

Hormones in the grains in relation to sink strength and postanthesis development of spikelets in rice

Inferior spikelets usually exhibit a slower grain filling rate and lower grain weight than superior spikelets in a rice (Oryza sativa L.) panicle. This study investigated whether the variations in grain filling between the two kinds of spikelets were attributed to their sink strength and whether the sink strength was regulated by the hormonal levels in the grains. Using two field-grown rice genotypes, the division rate of endosperm cells, hormonal levels in the grains, and grain weight of both superior and inferior spikelets were determined during the grain filling period. The results showed that superior spikelets had dominance over inferior spikelets in endosperm cell division rate and cell number, grain filling and grain weight. Changes in zeatin (Z) and zeatin riboside (ZR) contents paralleled and were very significantly correlated with the cell division rate and cell number. Cell division rate and the content of indole-3-acetic acid (IAA) in the grains were also significantly correlated. Gibberellin (GAs; GA₁+ GA₄) content of the grains was high but ABA levels were low at the early grain filling stage. ABA increased substantially during the linear phase of grain growth and was very significantly correlated with grain dry weight during this period. Application of kinetin at 2 through 6 days post anthesis (DPA) significantly increased cell number, while spraying ABA at 11 through 15 DPA significantly increased the grain filling rate. The results suggest that differences in sink strength are responsible for variations in grain filling between superior and inferior spikelets. Both cytokinins and IAA in the grains may mediate cell division in rice endosperm at early grain filling stages, and therefore regulate the sink size of the grain, whereas ABA content correlates with sink activity during the linear period of grain growth.     

Involvement of polyamines in the post-anthesis development of inferior and superior spikelets in rice

Early-flowered superior spikelets usually exhibit a faster grain filling rate and heavier grain weight than late-flowered inferior spikelets in rice (Oryza sativa L.). But the intrinsic factors responsible for the variations between the two types of spikelets are unclear. This study investigated whether and how polyamines (PAs) are involved in regulating post-anthesis development of rice spikelets. Six rice genotypes differing in grain filling rate were field grown, and PA levels and activities of the enzymes involved in PA biosynthesis were measured in both superior and inferior spikelets. The results showed that superior spikelets exhibited higher levels of free spermidine (Spd) and free spermine (Spm) and higher activities of arginine decarboxylase (ADC, EC 4.1.1.19), S-adenosylmethionine decarboxylase (SAMDC, EC 4.1.1.50) and Spd synthase (EC 2.5.1.16) than inferior spikelets at the early endosperm cell division and grain filling stage. The maximum concentrations of free Spd and free Spm and the maximum activities of ADC, SAMDC and Spd synthase were significantly correlated with the maximum cell division and grain filling rates, maximum cell number and grain weight. Application of Spd and Spm to panicles resulted in significantly higher rates of endosperm cell division and grain filling in inferior spikelets along with the activities of sucrose synthase (EC 2.4.1.13), ADP glucose pyrophosphorylase (EC 2.7.7.27) and soluble starch synthase (EC 2.4.1.21), suggesting that these PAs are involved in the sucrose-starch metabolic pathway. The results indicate that the poor development of inferior spikelets is attributed, at least partly, to the low PA level and its low biosynthetic activity.     

Abscisic acid and ethylene interact in wheat grains in response to soil drying during grain filling

Grain filling is an intensive transportation process regulated by soil drying and plant hormones. This study investigated how the interaction between abscisic acid (ABA) and ethylene is involved in mediating the effects of soil drying on grain filling in wheat (Triticum aestivum). Two wheat cultivars, cv. Yangmai 6 and cv. Yangmai 11, were field-grown, and three irrigation treatments, well-watered, moderately soil-dried (MD) and severely soil-dried (SD), were imposed from 9 d post anthesis until maturity. A higher ABA concentration and lower concentrations of ethylene and 1-aminocylopropane-1-carboxylic acid (ACC) were found in superior grains (within a spike, those grains that were filled earlier and reached a greater size) than in inferior grains (within a spike, those grains that were filled later and were smaller), and were associated with a higher filling rate in the superior grains. An increase in ABA concentration and reductions in ethylene and ACC concentrations in grains under MD conditions increased the grain-filling rate, whereas much higher ethylene, ACC and ABA concentrations under SD conditions reduced the grain-filling rate. Application of chemical regulators gave similar results. The results did not differ between the two cultivars. The grain-filling rate in wheat is mediated by the balance between ABA and ethylene in the grains, and an increase in the ratio of ABA to ethylene increases the grain-filling rate.     

Growth characteristics and endosperm structure of superior and inferior spikelets of indica rice under high-temperature stress

Heat stress severely reduces rice yield and quality; however, differences between the superior, early-flowering and inferior, later-flowering spikelets of indica rice in response to high-temperature stress during grain filling remain unclear. This study investigated the effects of high temperature (HT, 33.6/20.7 °C day/night) on growth, endosperm structure, and hormone and polyamine content of superior and inferior spikelets of heat-sensitive (SG-1) and heat-tolerant (HHZ) indica cultivars. The HT decreased fertilization rate, caused earlier grain filling, and reduced duration of grain filling, thus resulting in decreased grain mass and a poor endosperm structure. In addition, soluble sugar and sucrose content increased, and starch synthesis decreased by HT at the early stage of grain filling. The HT increased polyamine [spermidine (Spd) and spermine (Spm)] and abscisic acid (ABA) content, but reduced zeatin (Z) + zeatin riboside (ZR) and indole-3-acetic acid (IAA) content in the grains. Such effects were more apparent in the inferior than superior spikelets; however, the inferior spikelets of SG-1 were more affected than those of HHZ. At the middle grain filling stage, HT produced little difference between the two cultivars. Our results suggest that the poor development of inferior spikelets of SG-1 under the HT could be attributed, at least in part, to the changed content and ratios of free polyamines [putrescine (Put), Spd, and Spm] and phytohormones (Z+ZR, IAA, and ABA) and the conversion efficiency of sucrose into starch.     

Post-anthesis alternate wetting and moderate soil drying enhances activities of key enzymes in sucrose-to-starch conversion in inferior spikelets of rice

This study tested the hypothesis that a post-anthesis moderate soil drying can improve grain filling through regulating the key enzymes in the sucrose-to-starch pathway in the grains of rice (Oryza sativa L.). Two rice cultivars were field grown and two irrigation regimes, alternate wetting and moderate soil drying (WMD) and conventional irrigation (CI, continuously flooded), were imposed during the grain-filling period. The grain-filling rate and activities of four key enzymes in sucrose-to-starch conversion, sucrose synthase (SuSase), adenosine diphosphate-glucose pyrophosphorylase (AGPase), starch synthase (StSase), and starch branching enzyme (SBE), showed no significant difference between WMD and CI regimes for the earlier flowering superior spikelets. However, they were significantly enhanced by the WMD for the later flowering inferior spikelets. The activities of both soluble and insoluble acid invertase in the grains were little affected by the WMD. The two cultivars showed the same tendencies. The activities of SuSase, AGPase, StSase, and SBE in grains were very significantly correlated with the grain-filling rate. The abscisic acid (ABA) concentration in inferior spikelets was remarkably increased in the WMD and very significantly correlated with activities of SuSase, AGPase, StSase, and SBE. Application of ABA on plants under CI produced similar results to those seen in plants receiving WMD. Applying fluridone, an indirect inhibitor of ABA synthesis, produced the opposite effect. The results suggest that post-anthesis WMD could enhance sink strength by regulating the key enzymes involved, and consequently, increase the grain-filling rate and grain weight of inferior spikelets. ABA plays an important role in this process.     

Apoplasmic assimilates and grain growth of contrasting rice cultivars differing in grain dry mass and size

Apical dominance in assimilate filling impacts grain growth in basal spikelets of rice panicle. In this study, organic materials of the pericarp, apoplasmic space and endosperm of the apical and basal caryopses, and photosynthesis of the flag leaf were measured during early part of grain development in three types of rice cultivars with similar phenology, but difference in grain weight and size in the dry and wet seasons of 2006 and 2007, respectively. Photosynthetic activity of the flag leaf was consistently low in small-seeded cultivars. Rates of grain filling and cell division of endosperm and concentration of assimilates, starch, proteins and chlorophylls of the caryopsis were lower, but spikelet ethylene production and peroxidase activity were higher in a small-seeded cultivar compared to a big-seeded cultivar. Similar disparities in grain filling and other attributes were noticed for the inferior basal spikelets of the panicle compared to the superior apical spikelets, except the assimilate concentration of the pericarp and endosperm. Temporal fluctuation in assimilate concentration of the organs were similar between the cultivars. Concentration of apoplasmic assimilates mostly exhibited negative correlation with that of pericarp and endosperm. Compared to the apical spikelets, correlation was more negative for the basal spikelets. Conversely, correlation was positive between the concentration of apoplasmic assimilates and endosperm cell number and grain weight of the cultivars. Ethylene released from the spikelets at anthesis affected growth and cell division rates of endosperm and enhanced protein and chlorophyll degradation and peroxidase activity of the caryopsis. It was concluded that variation in spikelet ethylene production may be responsible for differences in size or weight of grains among rice cultivars and spikelets at different locations of the panicle. The concentration of apoplasmic assimilates could be an indicator for grain filling capacity, and ethylene regulated the concentration by affecting pericarp activity for assimilate unloading.     

Involvement of abscisic acid and ethylene in the responses of rice grains to water stress during filling

This study was to test the hypothesis that the interaction between abscisic acid (ABA) and ethylene may be involved in mediating the effects of water stress on grain filling. Two high lodging‐resistant rice (Oryza sativa L.) cultivars were pot‐grown. Three treatments, well‐watered, moderate water‐stressed (MD), and severe water‐stressed (SD), were imposed from 9 d post‐anthesis until maturity. Grain filling rate and grain weight were significantly increased under MD but decreased under SD. The two cultivars behaved the same. ABA concentration in the grains was very low during the grain filling stage, reaching a maximum when the grain filling rate was highest. Both the grain filling rate and ABA concentration were substantially enhanced by water stress. In contrast to ABA, concentrations of ethylene and 1‐aminocylopropane ‐1‐carboxylic acid (ACC) in the grains were very high at early grain filling stage and sharply decreased during the linear period of grain growth. MD reduced, whereas SD remarkably increased, their accumulation. The ratio of ABA to ACC was increased in MD grains but decreased in SD grains, indicating that there was a greater enhancement of ABA concentration than ethylene production in the MD treatment only. Application of cobalt ion (inhibitor of ethylene synthesis) or ABA at the early grain filling stage significantly increased grain filling rate. Spraying with ethephon (ethylene‐releasing agent) or fluridone (inhibitor of ABA synthesis) had the opposite effect. The results suggest that antagonistic interactions between ABA and ethylene mediate the grain filling rate, and a high ratio of ABA to ethylene enhances grain filling rate.     

Ethylene control of seed coat development in low and high sterile semidwarf indica rice cultivars

Although basally positioned inferior spikelets of rice panicles emerge late from the flag leaf enclosure (boot), they mature early which precludes adequate grain filling. It is assumed that extended exposure to ethylene inside the boot restricts assimilate partitioning to the endosperm in basal spikelets by affecting the functions of seed coat. In the present study, ethylene concentration inside the boot was measured in two high yielding rice cultivars differing in percentage of spikelet sterility. To manipulate the concentration/action of ethylene, silver nitrate, aminoethoxyvinyl glycine and 2-chloroethylphosphonic acid were injected into the boot. The effect of these chemicals on the concentration of photosynthetic pigments, lipid peroxidation and peroxidase activity of the seed coat and lemma and palea were measured to monitor development. Ethylene reduced development during the juvenile phase but accelerated degradation of the photosynthetic tissues of the spikelets in the senescent phase. Boot ethylene correlated positively with number of barren spikelets in the panicle and negatively with concentration of photosynthetic pigments of the seed coat of inferior spikelets. The concentration of ethylene was higher in the high sterile cultivar Mahalaxmi than that of the low sterile Mahanadi. Inhibition of ethylene action or synthesis improved grain filling. The chemicals were most effective on the inferior spikelets. It was concluded that ethylene retarded seed coat development during the prestorage phase and reduced grain filling of basal spikelets.     

重穗型杂交水稻籽粒灌浆过程中强势和弱势颖花内源IAA、ABA和GA水平的动态状况

研究了重穗型杂交水稻培矮 6 4s/E3 2的灌浆过程和强、弱势颖花中内源IAA、ABA和GA1 GA3水平的动态状况。籽粒发育过程中不同内源激素水平高低依次为 :IAA >GA1 GA3>ABA。IAA和ABA水平在强势颖花中较高而GA1 GA3水平在弱势颖花中较高。 3种激素水平的变化与谷粒增重速率之间均存在正相关 ,两个最高的相关系数值分别存在于单位鲜重样本的IAA含量(ng/gFW ) 与籽粒鲜重的增重速率之间 (r =0 .82 1 8 )和单个籽粒IAA含量 (ng/grain)与籽粒干重的增重速率之间 (r =0 .8485 )。推测启动和维持籽粒灌浆过程可能需要较高的IAA水平 ;ABA可能具有促进籽粒中同化物的累积和种子成熟的作用 ;GA1 GA3可能具有保持弱势颖花活性的特殊作用     

Ethylene and ACC levels in developing grains are related to the poor appearance and milling quality of rice

The possible relationship between the levels of ethylene and 1-aminocylopropane-1-carboxylic acid (ACC) in the grains and the quality of rice (Oryza sativa L.) were investigated by using 12 rice cultivars. The results showed that both the ethylene evolution rate and ACC content in grains during the grain filling period correlated negatively with head rice production and positively with chalky kernels, chalky size, and chalkiness. The levels of ethylene and ACC were not significantly correlated with alkali spreading value and amylose content. Application of ethephon, an ethylene-releasing agent, or ACC to panicles at the early grain filling stage significantly reduced the rates of brown rice, milled rice, and head rice, and significantly increased the percentage of chalky kernels, chalky size, and chalkiness. Application of aminoethoxyvinylglycine, an inhibitor of ACC synthase, had the opposite effect. Chalkiness appears to be a senescence related phenomenon which is stimulated by ethylene. The results suggest that ethylene and ACC in grains play an important role in regulating rice quality, and that grain appearance and milling quality would be improved though the reduction of ethylene and ACC in grains during grain filling.     

重穗型杂交水稻籽粒灌浆过程中强势和强势颖花内源IAA、ABA和GA水平的动态状况

研究了重穗型杂交水稻培矮 6 4s/E3 2的灌浆过程和强、弱势颖花中内源IAA、ABA和GA1 GA3水平的动态状况。籽粒发育过程中不同内源激素水平高低依次为 :IAA >GA1 GA3>ABA。IAA和ABA水平在强势颖花中较高而GA1 GA3水平在弱势颖花中较高。 3种激素水平的变化与谷粒增重速率之间均存在正相关 ,两个最高的相关系数值分别存在于单位鲜重样本的IAA含量(ng/gFW ) 与籽粒鲜重的增重速率之间 (r =0 .82 1 8 )和单个籽粒IAA含量 (ng/grain)与籽粒干重的增重速率之间 (r =0 .8485 )。推测启动和维持籽粒灌浆过程可能需要较高的IAA水平 ;ABA可能具有促进籽粒中同化物的累积和种子成熟的作用 ;GA1 GA3可能具有保持弱势颖花活性的特殊作用     

High ethylene production slackens seed filling in compact panicled rice cultivar

Change of plant type in rice resulting in increased compactness of the panicle, allows space for accommodation of a larger number of spikelets, but grain yield does not increase proportionately because of limitations in grain filling. The objective of this study was to evaluate potential causes of poor filling of spikelets by comparing the physiological processes that influence source and sink activities between a compact- (OR-1920-7) and a loose-panicled (Lalat) rice cultivars growing in the open field conditions in the farm of Regional Research and Technology Transfer Station, Orissa University of Agriculture and Technology, Chiplima, India during dry season of 2007. Although grain number per unit length of the branches was higher in the compact-panicled cultivar than the loose-panicled cultivar, average grain weight was lower on the primary and secondary branches at top, middle and basal positions of the panicle in the former compared to the corresponding positions of the panicle in the latter. Compared to Lalat, ethylene production rate was considerably higher in the boot of the flag leaf sheath of OR-1920-7 during the pre-anthesis period. Ethylene evolution rate correlated negatively with growth and cell division rates and starch concentration of the juvenile endosperm. Because spikelet growth was slower in OR-1920-7 than in Lalat, unused carbohydrates accumulated in the endosperm. The stomatal conductance of the flag leaf during this period was also lower in the former than that of the latter and it correlated negatively with ethylene evolution rate of the boot. It is concluded that high ethylene production slackened grain filling of compact-panicled rice cultivar OR-1920-7 because of its adverse influence on both source and sink activities.     

Correlation of cytokinin levels in the endosperms and roots with cell number and cell division activity during endosperm development in rice

Cell number and cell division activity in rice (Oryza sativa) endosperms are possibly regulated by cytokinin levels in the endosperm and its source in the roots. This study tried to find the possible correlations among them. Six rice genotypes were grown in nutrient solution. Two patterns of endosperm cell division, synchronous and asynchronous, were observed among the genotypes based on the cell division rate of superior and inferior spikelets. Contents of zeatin (Z) + zeatin riboside (ZR) were much higher than those of N6-isopentenyladenine (iP) and N6-isopentenyladenosine (iPR) in both endosperms and roots. Changes in Z + ZR levels in endosperms were significantly correlated with those in roots, and both were very significantly correlated with the cell division rate. Changes in iP + iPR contents in the roots were not significantly correlated with those in the endosperms and the cell division rate. When roots were treated with kinetin, endosperm cell number and grain weight were increased. Such enhancement was more significantly achieved by the root kinetin treatment than by spraying kinetin on leaves and panicles. The results suggest that the cell number and cell division activity in rice endosperms are regulated by cytokinin levels in the endosperm and that root-derived Z + ZR play a pivotal role.     

Abscisic acid in developing grains of wheat and barley genotypes differing in grain weight

Two genetically related wheat lines growing in cabinets were given different temperatures during grain filling, and abscisic acid (ABA) was measured in whole grains by gas chromatography with an electron-capture detector. Three genetically related barley lines grown in the field were assayed for ABA content in endosperm and embryo fractions separately by radiommunoassay.Maximum grain growth rate and final weight per grain of the two wheat lines differed by 50–60% at low temperature and 30–40% at high temperature. During grain development two peaks in ABA level were observed at low temperature but only one at high temperature. At times when differences in grain growth rate between genotypes and between temperature treatments were large, the corresponding differences in ABA concentration were small. In barley, one line (Iabo 14) had 30% heavier grains than the other two (Onice and Opale). Endosperm ABA concentrations showed no clear differences between genotypes until grain filling was nearly complete. Embryo ABA levels were up to 10-times greater than those in the endosperm, with Opale having significantly less ABA in the embryo than the other two cultivars.Our experiments did not provide evidence for a causal relationship between ABA levels during grain filling and grain growth rate or final weight.Abbreviations ABA Abscisic acid - DAA days after anthesis - DW dry weight - FW fresh weight     

Abscisic Acid and Ethylene Interact in Rice Spikelets in Response to Water Stress During Meiosis

This study investigated whether and how the interaction between abscisic acid (ABA) and ethylene is involved in the regulation of rice (Oryza sativa L.) spikelet sterility when subjected to water stress during meiosis. Two rice cultivars, HA-3 (drought-resistant) and WY-7 (drought-susceptible), were used and subjected to well-watered and water-stressed (WS) treatments during meiosis (15–2 days before heading). Leaf water potentials of both cultivars markedly decreased during the day as a result of the WS treatment, but panicle water potentials remained constant. The percentage of sterile spikelets in WS plants was increased by 49.7% for WJ-7 but only 12.7% for HA-3. ABA, ethylene, and 1-aminocyclopropane-1-carboxylic acid were all enhanced in spikelets by the water stress, but ethylene was enhanced more than ABA in WY-7 when compared with that in HA-3. Spikelet sterility was significantly reduced when ABA or amino-ethoxyvinylglycine, an inhibitor of ethylene synthesis, was applied to the panicles of WS plants at the early meiosis stage. Application of ethephon, an ethylene-releasing agent, or fluridone, an inhibitor of ABA synthesis, had the opposite effect, and sterility was increased. The results suggest that antagonistic interactions between ABA and ethylene may be involved in mediating the effect of water stress on spikelet fertility. A higher ratio of ABA to ethylene would be a physiologic trait of rice adaptation to water stress.