Fangmin Cheng

Difference in Grain Yield and Quality among Tillers in Rice Genotypes Differing in Tillering Capacity

A japonica variety and its iso-allelic mutant with high tillering ability were used to investigate the differences in grain yield and quality among different tillers. There was a distinct difference in panicle weight among tillers during grain filling for both genotypes, with Xiushui 11 having a greater increase rate in panicle weight, and being earlier in reaching the maximum panicle weight than the mutant. There was a great significant difference between the two genotypes in grain yield and its components, with Xiushui 11 having higher grain yield, more grains per panicle, higher filled grain percentage and grain weight than the mutant for each type of tiller. Moreover, a significant difference was found in grain yield and yield components among different tillers for both genotypes, with grain number per panicle showing the greatest variation over tillers among all yield components. Compared with Xiushui 11, the mutant had higher brown rice rate and greater ratio of length to width of brown rice, and lower chalky rice rate, amylose content, and protein content of rice. Furthermore, there was a significant difference in grain quality among tillers within a plant for both genotypes, with later initiated tillers being lower in chalky rice rate, amylose and protein contents than early initiated ones. The variation of most quality parameters among tillers within a plant was markedly larger for the mutant than for Xiushui 11.

Temperature induced changes in the starch components and biosynthetic enzymes of two rice varieties

The effects of temperature on starch and amylose accumulation, fine structure of amylopectin and activities of some enzymes related to starch synthesis in developing rice endosperms was examined. Two early indica rice varieties were used, differing in amylose concentration (AC, %), namely Jia 935 (low AC) and Jia 353 (high AC). The results showed that the effects of high temperature on AC and amylopectin fine structure were variety-dependent. High temperature caused a reduction in amylose concentration and an increase in the short chain (CL<22) proportion of amylopectin for Jia 935; while opposite was true for Jia 353. High temperature also reduced and increased the activity of granule-bound starch synthase (GBSS) in Jia 935 and in Jia 353, respectively. This suggests that a change in the ratio of amylose/starch due to temperature was attributable to a change in GBSS activity. Moreover, obvious differences between the two rice varieties were detected in the activities of sucrose synthase (SuSy), ADP-glucose pyrophosphorylase (ADPG-Ppase), soluble starch synthase (SSS), starch branching enzyme (SBE), starch de-branching enzyme (SDBE) and starch phosphorylase (SPase) to high temperature. Accumulation rate of amylose was significantly and positively correlated with GBSS for Jia 935, but not for Jia 353. Amylose accumulation was also significantly and positively correlated with the activities of SDBE, SBE, ADPG-Ppase and SuSy for both varieties. The results suggest that the ratio of amylose to starch in rice endosperm is not only related to GBSS, but also affected by the activities of SDBE, SBE, ADPG-Ppase and SuSy.

The Relationship between Grain Filling and Hormone Content as Affected by Genotype and Source–sink Relation

The difference among florets within a panicle in grain filling and four hormone contents was compared by using two rice varieties differing in panicle type, i.e. compact-panicle variety Xiushui 994 and loose-panicle variety Xiushui 11. Meanwhile the effect of changing source (leaf cutting) and sink (floret removal) on grain filling and four hormone (indole-3-acetic acid, gibberellin, zeatin riboside and abscisic acid)level dynamics in developing grains was also examined. The results showed that Xiushui 994 had greater variation in grain filling rate (GFR), grain weight, filled grain percent (FGP) and four hormone contents during grain filling than Xiushui 11. Moreover, the former variety also showed the greater difference in the time of having the highest hormone contents between superior and inferior florets. The change in source and sink size had great effect on grain filling process and dynamics of four hormone contents, and the extent of the effect varied with variety and floret type. For Xiushui 994, floret-removal treatment had little effect on grain weight and FGP of the superior florets, but caused a marked decrease in GFR and four hormone contents, and a delay in the time when the highest GFR and hormone contents occurred. In the source-reducing treatment, GFR, grain weight and FGP for both superior and interior florets were all decreased. For Xiushui 11, both treatments had little effect on grain development in terms of grain filling properties and hormone content change in the two types of florets. The results indicated that the grain filling of compact-panicle variety is more source-limited than that of loose-panicle one, and relative level of hormone in both superior and inferior florets determined development of inferior florets.

Transcriptional profile of genes involved in ascorbate glutathione cycle in senescing leaves for an early senescence leaf (esl) rice mutant.

To clarify the complex relationship between ascorbate-glutathione (AsA-GSH) cycle and H2O2-induced leaf senescence, the genotype-dependent difference in some senescence-related physiological parameters and the transcript levels and the temporal patterns of genes involved in the AsA-GSH cycle during leaf senescence were investigated using two rice genotypes, namely, the early senescence leaf (esl) mutant and its wild type. Meanwhile, the triggering effect of exogenous H2O2 on the expression of OsAPX genes was examined using detached leaves. The results showed that the esl mutant had higher H2O2 level than its wild type at the initial stage of leaf senescence. At transcriptional level, the association of expression of various genes involved in the AsA-GSH cycle with leaf senescence was isoform dependent. For OsAPXs, the transcripts of two cytosolic OsAPX genes (OsAPX1 and OsAPX2), thylakoid-bound OsAPX8, chloroplastic OsAPX7 and peroxisomal OsAPX4 exhibited remarkable genotype-dependent variation in their expression levels and temporal patterns during leaf senescence, there were significantly increasing transcripts of OsAXP1 and OsAPX7, severely repressed transcripts of OsAPX4 and OsAPX8 for the esl rice at the initial leaf senescence. In contrast, the repressing transcript of OsAPX8 was highly sensitive to the increasing H2O2 level in the senescing rice leaves, while higher H2O2 concentration resulted in the enhancing transcripts of two cytosolic OsAPX genes, OsAPX7 transcript was greatly variable with different H2O2 concentrations and incubating duration, suggesting that the different OsAPXs isoforms played a complementary role in perceiving and scavenging H2O2 accumulation at various H2O2 concentrations during leaf senescence. Higher H2O2 level, increased AsA level, higher activities of APX and glutathione reductase (GR), and relatively stable GSH content during the entire sampling period in the leaves of esl mutant implied that a close interrelationship existed between AsA level and APX activity in the ongoing senescence of rice leaves. The GSH supply in rice leaves was not the limiting factor for the efficient maintenance of AsA-GSH cycle, despite the senescence-related change in GR activity between the two rice genotypes.

Senescence-specific change in ROS scavenging enzyme activities and regulation of various SOD isozymes to ROS levels in psf mutant rice leaves

To clarify the interaction between different antioxidant enzymes for monitoring oxidative stress and ROS burst in rice senescent leaves, we investigated the genotype-dependent alteration in temporal patterns of the O2•- production rate, H2O2 content, and ROS-scavenging enzyme activities during leaf senescence in two rice genotypes, namely, the premature senescence of flag leaf (psf) mutant and its wild type. Results showed that the psf mutant differed obviously from its wild type in leaf O2•- generation rate and H2O2 content accumulation, and the decreased activities of SOD, CAT, and APX in the psf leaves were strongly responsible for the increased ROS level and the accelerated leaf senescence. By contrast, the increase in POD activity was positively correlated with the senescence-related enhancement in O2•- generation in rice leaves. Among various SOD isoforms, Mn-SOD responded sensitively to the increasing O2•- generation rate, whereas Cu/Zn-SOD remained stable with the progression of leaf senescence. These findings suggest that the senescence-related decline in total SOD activity was mostly attributable to the downregulation of both the translation and transcription of Mn-SOD isoform. This occurrence finally resulted in the collapse of SOD defense system and accelerated leaf senescence for the psf mutant. Furthermore, we presented the possible contribution of several Cu/Zn-SOD expression patterns to the senescence-related O2•- detoxification in different cell compartments.

Positional variations in chalky occurrence within a rice panicle and its relation to grain nutritional quality

The effect of grain position within a panicle on chalky occurrence, and its relation to phytic acid and protein content, were studied using 6 Japonica rice cultivars with compact panicles and through leaf and floret removal treatments to alter source-sink ratio. There were significant differences in total chalky grain (TCG), white core grain (WCG), white belly grain (WBG), and WCG/WBG among grains within a panicle, with the top and primary rachides having higher WBG than the bottom and secondary rachides. Alteration of source-sink ratio had a remarkable influence on chalky grain occurrence and its positional distribution within a panicle, and the extent of the influence varied greatly with cultivars. In comparison with WCG, WBG occurrence was more susceptible to the alteration of source-sink ratio. No significant correlation was found between chalky percentage and phytic acid and protein contents for the grains in the different positions within a panicle. Chalky occurrence had little effect on the nutritional quality.

A single cytosine deletion in the OsPLS1 gene encoding vacuolar-type H+-ATPase subunit A1 leads to premature leaf senescence and seed dormancy in rice.

Highlight The OsPLS1 locus that encodes VHA-A1 in rice, was identified by map-based cloning. OsPLS1/VHA-A1 is involved in premature leaf senescence and seed dormancy.

Temperature Stress at Grain Filling Stage Mediates Expression of Three Isoform Genes Encoding Starch Branching Enzymes in Rice Endosperm

Abstract An early-maturity indica rice variety Zhefu 49, whose grain quality and starch structure are sensitive to environmental temperature, was subjected to different temperatures (32°C for high temperature and 22°C for optimum temperature) at the grain filling stage in plant growth chambers, and the different expressions of three isoform genes ( SBEI, SBEIII and SBEIV ) encoding starch branching enzyme (SBE) in the endosperms were studied by the real-time fluorescence quantitative PCR (FQ-PCR) method. Effects of high temperature on the SBE expression in developing rice endosperms were isoform-dependent. High temperature significantly down-regulated the expressions of SBEI and SBEIII , while up-regulated the expression of SBEIV . Compared with SBEIV and SBEIII , the expression of SBEI gene in Zhefu 49 rice endosperms was more sensitive to temperature variation at the grain filling stage. This study indicates that changes in weather/climate conditions especially temperature stress influence rice grain formation and its quality as evidenced by isoform expression.

Involvement of Abscisic Acid in PSII Photodamage and D1 Protein Turnover for Light-Induced Premature Senescence of Rice Flag Leaves

D1 protein in the PSII reaction center is the major target of photodamage, and it exhibits the highest turnover rate among all the thylakoid proteins. In this paper, rice psf (premature senescence of flag leaves) mutant and its wild type were used to investigate the genotype-dependent alteration in PSII photo-damage and D1 protein turnover during leaf senescence and its relation to ABA accumulation in senescent leaves. The symptom and extent of leaf senescence of the psf mutant appeared to be sunlight-dependent under natural field condition. The psf also displayed significantly higher levels of ABA accumulation in senescent leaves than the wild type. However, the premature senescence lesion of psf leaves could be alleviated by shaded treatment, concomitantly with the strikingly suppressed ABA level in the shaded areas of flag leaves. The change in ABA concentration contributed to the regulation of shade-delayed leaf senescence. The participation of ABA in the timing of senescence initiation and in the subsequent rate of leaf senescence was closely associated with PSII photodamage and D1 protein turnover during leaf senescence, in which the transcriptional expression of several key genes (psbA, psbB, psbC and OsFtsH2) involved in D1 protein biosynthesis and PSII repair cycle was seriously suppressed by the significantly increased ABA level. This response resulted in the low rate of D1 protein synthesis and impaired repair recovery in the presence of ABA. The psf showed evidently decreased D1 protein amount in the senescent leaves. Both the inhibition of de novo synthesized D1 protein and the slow rate of proteolytic removal for the photodamaged D1 protein was among the most crucial steps for the linkage between light-dependent leaf senescence and the varying ABA concentration in psf mutant leaves. OsFtsH2 transcriptional expression possibly played an important role in the regulation of D1 protein turnover and PSII repair cycle in relation to ABA mediated leaf senescence.

Expression Profiles of Rice Soluble Starch Synthase Isoform Genes in Response to High Temperature

Abstract Under mean daily temperature of 32°C (high temperature) and 22°C (normal temperature) controlled in growth chambers, the expression responses of 8 soluble starch synthase (SSS) isoform genes involving starch synthesis metabolism in rice ( Oryza sativa L.) endosperms were detected using real-time fluorescence quantitative PCR (FQ-PCR). The expression patterns of SSS genes under high temperature stress were isoform-dependent, in which some isoform genes, such as SSS IIb, SSS IIc, SSS IIIb, and SSS IVa, were up-regulated with relatively high expression levels, and other genes, such as SSS IIa and SSS IIIa, showed down-regulated patterns with relatively low expression levels. SSS I and SSS IIIa were highly expressed in rice endosperms during the whole filling period in both temperature treatments, implying that they are the major isoform genes of SSS. The expressions of SSS IIb, SSS IIIa, and SSS IVa genes were much more sensitive to high temperature stress than those of SSS I, SSS IIc, SSS IIIb, and SSS IVb.