Rice Caryopsis Research Articles

Sugar apoplastic transport in rice caryopsis during grain filling

Sugar apoplastic transport in rice caryopsis during grain filling

Cover Caption: Rice Caryopsis Development

As one of the most important food sources, rice caryopsis has a complex structure that composes of maternal tissues of pericarp and testa, and filial tissues of endosperm and embryo. In this issue, two papers, authored by Wu et al, illustrate in detail the development of maternal (pp. 772–785) and endosperm tissues (pp. 786–798). Results provided show that rice caryopsis undergoes dynamic changes in mitotic divisions, cellularization, cell differentiation and programmed cell death.

Distribution of iron and zinc in plant and grain of different rice genotypes grown under aerobic and wetland conditions

This paper examined the distribution of Fe and Zn in the plant and seed of different rice genotypes in different growing conditions. The Fe and Zn concentrations were determined in different plant tissues during the growth stages of 3 genotypes with high Fe and Zn check genotypes, and in different grain tissues of 15 genotypes grown in aerobic and wetland conditions. Iron and Zn were distributed differently in tissues of the rice plant, with the harvest index (panicle nutrient content as the % of the total above ground nutrients) at 3–4% for Fe and 54–74% for Zn. The concentrations of both Fe and Zn of the endosperm increased with the increasing proportion of the grain nutrient content allocated to the endosperm, but declined when the allocation to the bran fraction increased. The Fe concentrations of the de-husked caryopsis of rice grown in the aerobic soil and the Fe concentration of the de-husked caryopsis of rice grown in the wetland soil were closely related, but not in the endosperm Fe, while the grain Zn concentrations in the aerobic soil were found to correlate with the Zn concentrations in the wetland soil for both the de-husked caryopsis and the endosperm.

Observational studies on amyloplasts with single-starch granule in rice endosperm

Rice is one of the most important cereal crops worldwide. Grain chalkiness has been reported to decrease rice quality and to be associated with starch biosynthesis and accumulation. In order to explore mechanism of chalky starch formation, microscopic and submicroscopic techniques were adopted in this study to observe the amyloplast morphology of rice endosperm cells. These techniques used serial semi-thin sections of rice caryopsis endosperm cells and chalky rice kernels at different development stages after anthesis. Amyloplasts with single-starch granule existed in both developing and mature endosperm cells from their appearance to degradation. In addition, particle size analysis of the chalky and normal starch granules showed that the circularity of the chalky single-starch granule was greater than that of the normal starch granule. Given the loose arrangement of single-starch granule caused by undesirable development, its granule is different from that of the normal endosperm cell. Therefore, the defective development of starch might be related to the chalky formation.

Anomalous germination of dormant dehulled red rice seeds provides a new perspective to study the transition from dormancy to germination and to unravel the role of the caryopsis coat in seed dormancy

AbstractSeed dormancy is the temporary inability of an imbibed seed to germinate under otherwise favourable conditions. It is an important trait for seed persistence in many higher plants. Dormant dehulled red rice caryopses can have a strong dormancy: the studied population shows an almost complete dormancy; that is, these caryopses do not germinate (usually germination is <1–2%) when incubated in water for the time usually adopted for germination tests (i.e. 2 weeks). However, after several months of incubation in water, dormant red rice caryopses start germinating in an anomalous manner. Most notably, the piercing of the caryopsis coat is very slow, sometimes arrested, until the coat completely breaks down and embryo growth is resumed. There is, therefore, a time lag between the initial rupture of the caryopsis coat and the start of seedling growth. It is argued that embryo growth can be triggered by the failure of the caryopsis coat even if seed dormancy has not been previously relieved, and thus germination is started and dormancy is forcefully interrupted. Accordingly, the time course of the anomalous germination shows a Gompertz distribution of times to failure. It is concluded that: (1) if the seed rests with the coat ruptured without further growth, it is still dormant; if so, therefore, (2) the breaking of the coat is not necessarily a marker of germination in this context.

Variation of Zinc Concentration in Rice Caryopsis and Husk among Southern Rice Varieties Grown in Southern and Northern Thailand

This study evaluated the concentration of Zn in rice caryopsis (intact with pericarp and embryo) and husk (palea and lemma) among rice varieties from southern Thailand and whether the pattern in the Zn concentration of selected varieties was altered by production in a different genetic resource area in northern Thailand. Forty-eight southern rice varieties were grown in a paddy field at Phatthalung Rice Research Center, Thailand, designated as the southern location. Seeds of each variety were harvested and analyzed for Zn concentration in the caryopsis and husk and compared with the standard check varieties with low (RD21) and high (Namroo) Zn concentrations. Four varieties with different Zn concentrations in the caryopsis were selected from among the 48 above varieties and grown in a demonstration field at Chiang Mai University, Thailand, designated as the northern location, for examining the effect of planting location on Zn concentration in the caryopsis. Zn concentrations ranged widely, from 19.2 to 33.3 mg Zn/kg in the caryopsis and from 6.2 to 20.0 mg Zn/kg in the husk among the 48 southern varieties planted in the southern location. Zn concentrations in the caryopsis and husk planted at the southern location correlated significantly (r = 0.55*). The rice varieties Chaw Gam Preud, Leuang Hawm, Mai Yah, and Nahn Hak had the highest caryopsis Zn concentrations; all were higher than the high standard Zn check variety (29.0 mg Zn/kg). The wide range of Zn concentrations in the caryopsis among rice varieties from southern Thailand provides an opportunity for selective inclusion in breeding strategies to enhance dietary Zn uptake by rice consumers. The caryopsis Zn concentrations were reduced by 20-42% in three out of four southern cultivars grown in northern Thailand; this indicates the importance of considering the environment and agronomic practices when selecting cultivars with high Zn concentrations in pre-breeding programs.

Rice caryopsis development I: Dynamic changes in different cell layers.

Rice caryopsis as one of the most important food sources for humans has a complex structure that is composed of maternal tissues including the pericarp and testa and filial tissues including the endosperm and embryo. Although rice caryopsis studies have been conducted previously, a systematic characterization throughout the entire developmental process is still lacking. In this study, detailed morphological examinations of caryopses were made during the entire 30‐day developmental process. We observed some rapid changes in cell differentiation events and cataloged how cellular degeneration processes occurred in maternal tissues. The differentiations of tube cells and cross cells were achieved by 9 days after pollination (DAP). In the testa, the outer integument was degenerated by 3 DAP, while the outer layer of the inner integument degenerated by 7 DAP. In the nucellus, all tissues with the exception of the nucellar projection and the nucellar epidermis degenerated in the first 5 DAP. By 21 DAP, all maternal tissues, including vascular bundles, the nucellar projection and the nucellar epidermal cells were degenerated. In summary, this study provides a complete atlas of the dynamic changes in cell differentiation and degeneration for individual maternal cell layers of rice caryopsis.

Copper suppresses abscisic acid catabolism and catalase activity, and inhibits seed germination of rice.

Although copper (Cu) is an essential micronutrient for plants, a slight excess of Cu in soil can be harmful to plants. Unfortunately, Cu contamination is a growing problem all over the world due to human activities, and poses a soil stress to plant development. As one of the most important biological processes, seed germination is sensitive to Cu stress. However, little is known about the mechanism of Cu-induced inhibition of seed germination. In the present study, we investigated the relationship between Cu and ABA which is the predominant regulator of seed germination. Cu at a concentration of 30 µM effectively inhibited germination of rice caryopsis. ABA content in germinating seeds under copper stress was also higher than that under control conditions. Quantitative real-time PCR (qRT-PCR) revealed that Cu treatment reduced the expression of OsABA8ox2, a key gene of ABA catabolism in rice seeds. In addition, both malondialdehyde (MDA) and H2O2 contents were increased by Cu stress in the germinating seeds. Antioxidant enzyme assays revealed that only catalase activity was reduced by excess Cu, which was consistent with the mRNA profile of OsCATa during seed germination under Cu stress. Together, our results demonstrate that suppression of ABA catabolism and catalase (CAT) activity by excess Cu leads to the inhibition of seed germination of rice.

Structural and Histochemical Characterization of Developing Rice Caryopsis

The development of pericarp, seed coat, starchy endosperm and aleurone of the rice caryopsis was investigated, histochemically and structurally, from the time of flowering to maturity. The results showed that during its growth, the maximum length of the caryopsis was attained first, followed by width and then thickness. Histochemical examination of the caryopsis showed that starch was mainly accumulated in the endosperm, but the endosperm showed no metabolic activity, while embryo and pericarp contained a few starch grains, and embryo and aleurone were strongly active. Aleuronic cells contained many aleurone grains and spherosomes, and aleurone in the dorsal region developed earlier and contained more layers of cells. Amyloplasts in endosperm contained many starch granules and were spherical at early stages but polyhedric at late stages. The protein bodies appeared later than amyloplasts, and the number of protein bodies in subaleurone was greater than those in the starchy endosperm. The white-belly portion of endosperm might be relative to the status of amyloplast development.

A Simple and Rapid Single Kernel Screening Method to Estimate Amylose Content in Rice Grains

In rice breeding programmes large number of grain samples are routinely analysed for amylose content (AC) through a tedious spectrophotometric method that also involves high reagent costs. Here, we propose a rapid and economic screening technique for assessment of AC based on the amylose-iodine complex formation in the cut grains of rice, which we refer to as the cut grain dip (CGD) method. The CGD method involves cutting the rice kernels in the middle with a pair of scissors and dipping the cut end in an optimised iodide:iodine (KI-I) solution termed the rapid amylose detection solution (RADS). It was found that the time taken for deep blue colouration by the cut end of the grains after dipping in RADS was proportional to the AC. The CGD method was further validated in a large set of rice mutants with varied AC. The proposed method can be used to screen samples for AC rapidly, with a single rice caryopsis, without any costly equipment and can be especially suitable for screening of mutants and segregants with altered AC in large breeding populations.

Genetic analysis and gene cloning of a triangular hull 1 (tri1) mutant in rice (Oryza sativa L.)

Grain shape and size are two key factors that determine rice yield and quality. In the present study, a rice triangular hull mutant (tri1) was obtained from the progeny of japonica rice variety Taipei 309 treated with 60 Co γ-rays. Compared to the wild type, the tri1 mutant presents a triangular hull, and exhibits an increase in grain thickness and protein content, but with a slight decrease in plant height and grain weight. Genetic analysis indicated that the mutant phenotype was controlled by a recessive nuclear gene which is stably inherited. Using a map-based cloning strategy, we fine-mapped tri1 to a 47-kb region between the molecular markers CHR0122 and CHR0127 on the long arm of chromosome 1, and showed that it co-segregates with the molecular marker CHR0119. According to the rice genome sequence annotation there are six predicated genes within the mapped region. Sequencing analysis of the mutant and the wild type indicated that there was a deletion of an A nucleotide in exon 3 of the OsMADS32 gene, which could result in a downstream frameshift mutation and premature termination of the predicted polypeptide. Both semi-quantitative and real-time RT-PCR analyses showed that this gene expressed highly in young inflorescences, while expressed at very low levels in other tissues. These results implied that the OsMADS32 gene could be a candidate of TRI1. Taken together, the results of this study lay the foundation for further investigation into the molecular mechanisms regulating rice caryopsis development. Oryza sativa L., triangular hull 1 mutant (tri1), genetic analysis, gene cloning, OsMADS32

Identification of candidate genes related to rice grain weight under high-temperature stress

The rise of global warming presents a problem for all living organisms, including rice and other staple plants. High temperatures impair rice grain weight by inhibiting the filling of the caryopses during the milky stage. The molecular mechanism behind this process, however, is poorly understood. Identifying candidate genes involved in responses to high-temperature stress may provide a basis for the improvement of heat tolerance in rice. Using paired, genetically similar heat-tolerant and heat-sensitive rice lines as plant materials, cDNA-AFLP analysis revealed a total of 54 transcript derived fragments (TDFs), mainly from the heat-tolerant lines. This clearly indicated variations in gene expression between the two rice lines. BLAST results showed that 28 of the 54 TDFs were homologous sequences. These homologous genes were found to encode proteins involved in signal transduction, oxidation, transcriptional regulation, transport, and metabolism. The functions and differential expression patterns of some important genes are further discussed. High temperature stress may trigger a wide range of changes in gene expression in rice caryopses, in turn affecting functions ranging from signal transduction to cellular metabolism. Forty-five of the 54 TDFs were mapped to rice chromosomes. The genes identified in the present study would make good candidates for further study into the molecular mechanisms underlying rice adaptation to high-temperature stress.

Temporal expression profiling of lipase during germination and rice caryopsis development

Lipolytic enzymes play an important role in plant growth and development. In order to identify their functional roles, the temporal expression profiling of lipase was carried out during rice seed germination, growth and development of caryopsis. Changes in specific activities during germination revealed that the lipolytic activity increased significantly until the end of germination. As the lipase activity increased, two different lipase species were observed, which were designated as Lipase-I and Lipase-II based on their relative mobility. Lipase-II was active during germination. Lipase-I was responsible for lipid mobilization, a requirement for the growth of root and shoot. In comparison with the endosperm, the lipolytic activity in roots was three fold higher. During rice caryopsis development, the lipolytic activity increased gradually from initial panicle development and reached maximum as the grain dried to harvest maturity. Quantitative real-time PCR analysis revealed that the Lipase-II was a stage specific expressing gene during reproductive growth. The transcript level of Lipase-II reached maximum with completion of germination, then decreased and remained stable during post-germinative growth. During caryopsis development, Lipase-II is predominantly expressed in the developing seeds. The transcript abundance increased gradually during initial stages of development and reached a maximum until seed maturation. The results implicate that the dynamic changes in the enzyme activity of the two isoforms of lipase and gene expression patterns are associated with the energy reserve mobilization during seed germination and reproductive growth.

Effect of cooking on aleurone in the caryopsis of indica rice

Aleurone, the principal nourishing layer in the rice grain, consists of one to several layers of uniform, highly differentiated cells (Beteke et al 1997). In cooking, the starchy components in the caryopsis expand both laterally and vertically. This study was undertaken to determine the effect of cooking on aleurone in the rice caryopsis. Local varieties used in this study were long-, medium-, and short-grain types. Rice grains were dehusked manually and cooked in a domestic cooker with excess water, with and without aleurone. To attain a caryopsis without aleurone, the aleurone layer was manually removed. For the measurement, an average of 100 grains was used.

Localization of ATPase Activity on the Plasmalemma in the Dorsal Tissue of Rice Caryopsis and its Relation to Assimilate Transport

Localization of ATPase Activity on the Plasmalemma in the Dorsal Tissue of Rice Caryopsis and its Relation to Assimilate Transport

Evaluation of Protocols Used in 2-D Electrophoresis for Proteome Analysis of Young Rice Caryopsis

In order to obtain a high-resolution electrophorogram of rice young panicle proteome, we evaluated various protocols commonly used in two-dimensional (2D) polyacrylamide gel electrophoresis (PAGE) of proteins, including gel staining protocol, pH range of immobilized pH gradient (IPG) strips and sample loading quantity. Results showed that a silver staining protocol using sensitized solution containing glacial acetic acid, sodium acetate and sodium thiosulfate (reported by Heukeshoven and Dernick in 1988) and a Coomassie Brilliant Blue staining method using solution containing G-250, ammonium sulfate and phosphoric acid (reported by Pink et al. in 2010) demonstrated the superior staining effect. In addition, we also showed that higher resolution was achieved when IPG gel strip with pH range of 5-8 was used, compared to that with pH range of 4-7. Finally, the optimal loading quantity was determined as 130µg using the 17cm-long nonlinear IPG strip with pH 5-8 in combination with the silver nitrate staining protocol. The evaluated results would be helpful in proteome analysis of young rice caryopsis.

A study of biodiversity of flavonoid content in the rice caryopsis evidencing simultaneous accumulation of anthocyanins and proanthocyanidins in a black-grained genotype

Rice genotypes with pigmented caryopses have received increased attention because of their antioxidant properties. Previous works evidenced that the kernel of red rice is characterized by the presence of proanthocyanidins, whereas black rice is characterized by the presence of anthocyanins. In the present study, the total antioxidant capacity (TAC) and the polyphenol content of the caryopsis were evaluated on a set of Italian rice varieties (three white, two black and five red ones). The pigmented rices, on average, had a TAC four times higher than the white ones. As expected, red-grained genotypes contained no detectable anthocyanins and one black rice contained no detectable proanthocyanidins. However, the black-grained cv. Artemide had large amounts of both proanthocyanidins and anthocyanins. This genotype was also characterized by the highest TAC and polyphenol content: its TAC was about twice the TAC of the other pigmented rices, and it had a polyphenol content 2–3 times the content found in the other pigmented rices. Pigmented genotypes are confirmed to be very interesting to breed rice for high polyphenol content and TAC. Furthermore, the possibility to select for genotypes accumulating both anthocyanins and proanthocyanidins provides a way to substantially increase the polyphenol content and TAC of the rice caryopsis.

Rice caryopsis structure in relation to distribution of micronutrients (iron, zinc, β-carotene) of rice cultivars including transgenic indica rice

The transgenic indica rice lines of IR68144 and BR29, developed using endosperm-specific promoters were analyzed for their iron, zinc and β-carotene content in the endosperm. Biochemical analysis clearly revealed the presence of higher accumulation of iron, zinc and β-carotene in transgenic rice grains in comparison with control. Prussian blue staining reaction evidenced the presence of iron in the endosperm cells of transgenic rice grains in comparison with control where iron is restricted only to aleurone and embryo. The rice grain structure of IR64, IR72, IR68144, Swarna, BRRI Dhan 29 (BR29), BR28, Taipai 309 (T309) and New Plant Type-3 (NPT3) indicated that the number of aleurone layers, size of the embryo and size of the caryopsis determines the quantity of important micronutrients (iron, zinc) in the grains. Biochemical analysis revealed that iron and zinc content drastically varies in polished and unpolished rice and among the varieties examined. During the polishing process almost entire aleurone and most part of the embryo is removed which are the main storehouse for major micronutrients. It is estimated that more than 70% of micronutrients are lost during polishing process.

Effects of Nitrogen on Starch Accumulation and Activities of Enzymes Involved in Starch Synthesis in Rice Caryopsis

Starch accumulation is the main event in the developing rice grain, and the activities of key enzymes involved in starch synthesis are the important factors affecting the grain filling. N-fertilizer can regulate the plant growth, flower emergence, grain yield and quality, while the effects of N-fertilizer on starch accumulation and the activities of key enzymes in rice grains are rarely known. In this study, two rice cultivars, Yangdao 6 and Jingdao 941, were used as materials to study the effects of nitrogen on starch accumulation and the activities of related enzymes involved in starch synthesis. The results indicated that increased panicle N-fertilizer could significantly decrease the soluble sugar in the developing caryopsis, while the starch content reduced slightly. The amylose and amylopectin contents in the caryopsis increased synchronously with the raising of grain weight. When the pani-cle N-fertilizer was added, the amylose content in caryopsis as well as the content ratio of amylose to total starch decreased ob-servably. During the development of rice caryopses, the activities of ADP-glucose pyrophosphorylase (AGP), granule-bound starch synthase (GBSS), soluble starch synthase (SSS), and starch branching enzyme (SBE) in endosperm changed as single peak curve. That was, the enzyme activities increased at the early stage of grain filling, till their peak at 10–15 d after flowering, then depressed. Application of nitrogen fertilizer could improve the activities of AGP, SSS, and SBE in rice caryopsis at middle and late filling stages, while decrease GBSS activity at early and middle filling stages. And the effect of the N fertilized at the booting stage was superior to that at the tilling stage.

Study of the radioecological sensitivity of rice to radioactive contamination

This study focused on the radioecology of rice and explored how the French specificities of rice cultivation can be taken into account in risk assessment. The objective of the study was to characterize the radioecological sensitivity of rice with respect to an accidental release of radioactivity. We want to know if a uniform and specific deposit would involve the same contamination on representative paddy fields of Camargue. To model the transfer of the radionuclides in the rice caryopsis following an accidental atmospheric pollution, we used the classical modelling for the cereals which considers first the interception by the foliage (modelled by an interception ratio) then the translocation to the grain (modelled by a translocation factor). The values of the parameters (interception ratio, translocation factor and yield) were regionalized with the agronomical software ORIZA2000, developed by the IRRI (International Rice Research Institute). We partly calibrated ORIZA2000 for a French rice variety: Ariete, thanks to physiological data provided for International Cooperation Centre in Agronomical Research for Development (CIRAD). ORIZA2000 proposes a daily follow-up of the leaf area index which can be correlated with the interception ratio. Five simulations with various climates, irrigation managements, and technical uses have been inputted on ORIZA2000. The data inputted were extracted from the database AGROSYST from the French Institute for Agronomical Research (INRA) and from the CIRAD. We established two scenarios of contamination. The first one consisted in a single contamination of water. In this case, there is almost no difference between simulations. However, in each simulation (except for organic farming) it is relevant to notice that the fourth depletion of water leads to a significant variation of contamination. The second contamination scenario consisted in a double contamination of water and of air. In that case, technical practices are responsible of the highest source of variability of the grain contamination. The variability of the contamination is mainly due to the variability of the yield which has in the radioecological modelling a dilution effect. The rice produced by organic farming is more sensitive than the rice produced by conventional farming because of low yields.