Starch biosynthesis in cassava: a genome-based pathway reconstruction and its exploitation in data integration

Editing of the soluble starch synthase gene MeSSIII-1 enhanced the amylose and resistant starch contents in cassava

Foods with high amylose and resistant starch (RS) contents have great potential to enhance human health. In this study, cassava soluble starch synthase MeSSIII-1 gene mutants were generated using CRISPR/Cas9 system. The results showed that the storage roots of messiii-1 mutants had higher contents of amylose, RS, and total starch than those in CK. The rates of small and large-sized starch granules were increased. Additionally, amylopectin starch in messiii-1 mutants had a higher proportion of medium- and long- chains, and a lower proportion of short-chains than those in CK. The onset, peak, and conclusion temperatures of starch gelatinization in messiii-1 mutants were significantly lower than those in CK, and the peak viscosity, trough viscosity and final viscosity all increased. MeSSIII-1 mutation could increase the contents of sucrose, glucose, and fructose in cassava storage roots. We hypothesize that these soluble sugars serve a dual role: they provide the necessary carbon source for starch synthesis and act as sugar signals to trigger the transcriptional reprogramming of genes involved in starch biosynthesis. This process results in a collective enhancement of amylose, RS, and total starch contents, accompanied by changes in starch granule morphology, fine structure, and physicochemical properties.

Genome-wide association analysis and transgenic characterization for amylose content regulating gene in tuber of Dioscorea zingiberensis

BACKGROUND

Amylose, a prebiotic found in yams is known to be beneficial for the gut microflora and is particularly advantageous for diabetic patients' diet. However, the genetic machinery underlying amylose production remains elusive. A comprehensive characterization of the genetic basis of amylose content in yam tubers is a prerequisite for accelerating the genetic engineering of yams with respect to amylose content variation.

RESULTS

To uncover the genetic variants underlying variation in amylose content, we evaluated amylose content in freshly harvested tubers from 150 accessions of Dioscorea zingibensis. With 30,000 high-quality single nucleotide polymorphisms (SNP), we performed a genome-wide association analysis (GWAS). The population structure analysis classified the D. zingiberensis accessions into three groups. A total of 115 significant loci were detected on four chromosomes. Of these, 112 significant SNPs (log10(p) = 5, q-value < 0.004) were clustered in a narrow window on the chromosome 6 (chr6). The peak SNP at the position 75,609,202 on chr6 could explain 63.15% of amylose variation in the population and fell into the first exon of the ADP-glucose pyrophosphorylase (AGPase) small subunit gene, causing a non-synonymous modification of the resulting protein sequence. Allele segregation analysis showed that accessions with the rare G allele had a higher amylose content than those harboring the common A allele. However, AGPase, a key enzyme precursor of amylose biosynthesis, was not expressed differentially between accessions with A and G alleles. Overexpression of the two variants of AGPase in Arabidopsis thaliana resulted in a significantly higher amylose content in lines transformed with the AGPase-G allele.

CONCLUSIONS

Overall, this study showed that a major genetic variant in AGPase probably enhances the enzyme activity leading to high amylose content in D. zingiberensis tuber. The results provide valuable insights for the development of amylose-enriched genotypes.

Reinvention of starch for oral drug delivery system design

Starch is a polysaccharide with varying amylose-to-amylopectin ratios as a function of its biological sources. It is characterized by low shear stress resistance, poor aqueous/organic solubility and gastrointestinal digestibility which limit its ease of processing and functionality display as an oral drug delivery vehicle. Modulation of starch composition through genetic engineering primarily alters amylose-to-amylopectin ratio. Greater molecular properties changes require chemical and enzymatic modifications of starch. Acetylation reduces water solubility and enzymatic digestibility of starch. Carboxymethylation turns starch acid-insoluble and aggregative at low pHs. The summative effects are sustaining drug release in the upper gut. Acid-insoluble carboxymethylated starch can be aminated to provide an ionic character essential for hydrogel formation which further reduces its drug release. Ionic starch can coacervate with oppositely charged starch, non-starch polyelectrolyte or drug into insoluble, controlled-release complexes. Enzymatically debranched and resistant starch has a small molecular size which confers chain aggregation into a helical hydrogel network that traps the drug molecules, protecting them from biodegradation. The modified starch has been used to modulate the intestinal/colon-specific or controlled systemic delivery of oral small molecule drugs and macromolecular therapeutics. This review highlights synthesis aspects of starch and starch derivatives, and their outcomes and challenges of applications in oral drug delivery.

Comparative Transcriptome Profiling of Cassava Tuberous Roots in Response to Postharvest Physiological Deterioration

Cassava is one of the most versatile tuberous-root crops on Earth. However, the postharvest storage properties of cassava tuberous root mean that it is perishable through a process known as postharvest physiological deterioration (PPD), which seriously affects its starch quality. Therefore, a comprehensive understanding of the transcriptional regulatory activity of cassava against the PPD response is necessary in order to extract key molecular mechanisms related to PPD tolerance. In this study, we found that RYG1 tuberous roots showed delayed PPD compared to those of SC8. In addition, RYG1 roots maintained a more stable cell wall structure after storage than those of SC8. The transcriptome changes in tuberous roots were analyzed for both RYG1 and SC8 after 21 days of storage (SR and SS) compared to fresh (FR and FS) by the RNA-Seq method. The total number of differentially expressed genes (DEGs) in the various comparisons of these four samples ranged from 68 to 3847. Of these, a total of 2008 co-DEGs in SR vs. SS were shared by either SR vs. FR or SS vs. FS. GO and KEGG enrichment analysis revealed that upregulated co-DEGs in SR vs. SS were mainly enriched in photosynthesis, protein processing, hormone and cutin, suberine and wax biosynthesis. By contrast, the downregulated co-DEGs were mainly related to cell wall organization, starch and sucrose metabolism, galactose metabolism, phenylpropanoid biosynthesis, diterpenoid biosynthesis, cysteine and methionine metabolism and flavonoid biosynthesis. The protein-protein interaction (PPI) networks of the co-DEGs showed a complex interaction of genes in different pathways, and 16 hub genes were characterized to have a degree in excess of 15, among which eight genes were associated with photosynthesis. These results provide new information for the study of cassava resistance to PPD and lay a foundation for the further molecular breeding of storage-tolerant cassava varieties.

Changes in sucrose metabolism patterns affect the early maturation of Cassava sexual tetraploid roots

BACKGROUND

Cassava (Manihot esculenta Crantz) is an important multiuse crop grown for economic and energy purposes. Its vegetative organs are storage roots, in which the main storage material is starch. The accumulation characteristics of starch in cassava roots can directly affect the yield, starch content and maturation of cassava storage roots. In this study, we used a cassava sexual tetraploid (ST), which showed early maturation heterosis in previous work, as the main test material. We analyzed the sucrose metabolism and starch accumulation characteristics of the ST and its parents from the leaf "source" to the storage root "sink" during different developmental stages and explored the regulatory mechanisms of ST storage root early maturation by combining the transcriptome data of the storage roots during the expansion period.

RESULTS

The results showed that the trends in sucrose, glucose and fructose contents in the ST leaves were similar to those of the two parents during different stages of development, but the trends in the ST storage roots were significantly different from those of their parents, which showed high sucrose utilization rates during the early stage of development and decreased utilization capacity in the late developmental stage. Transcriptome data showed that the genes that were expressed differentially between ST and its parents were mainly involved in the degradation and utilization of sucrose in the storage roots, and four key enzyme genes were significantly upregulated (Invertase MeNINV8/MeVINV3, Sucrose synthase MeSuSy2, Hexokinase MeHXK2), while the expressions of key enzyme genes involved in starch synthesis were not significantly different.

CONCLUSIONS

The results revealed that the pattern of sucrose degradation and utilization in the cassava ST was different from that of its parents and promoted early maturation in its tuberous roots. Starch accumulation in the ST from sucrose mainly occurred during the early expansion stage of the storage roots, and the starch content during this period was higher than that of both parents, mainly due to the regulation of invertase and hexokinase activities during sucrose metabolism. This study provides a basis for further genetic improvements to cassava traits and for breeding varieties that mature early and are adapted well to provide starch supply requirements.

Multi-Omics Approaches and Resources for Systems-Level Gene Function Prediction in the Plant Kingdom

In higher plants, the complexity of a system and the components within and among species are rapidly dissected by omics technologies. Multi-omics datasets are integrated to infer and enable a comprehensive understanding of the life processes of organisms of interest. Further, growing open-source datasets coupled with the emergence of high-performance computing and development of computational tools for biological sciences have assisted in silico functional prediction of unknown genes, proteins and metabolites, otherwise known as uncharacterized. The systems biology approach includes data collection and filtration, system modelling, experimentation and the establishment of new hypotheses for experimental validation. Informatics technologies add meaningful sense to the output generated by complex bioinformatics algorithms, which are now freely available in a user-friendly graphical user interface. These resources accentuate gene function prediction at a relatively minimal cost and effort. Herein, we present a comprehensive view of relevant approaches available for system-level gene function prediction in the plant kingdom. Together, the most recent applications and sought-after principles for gene mining are discussed to benefit the plant research community. A realistic tabulation of plant genomic resources is included for a less laborious and accurate candidate gene discovery in basic plant research and improvement strategies.

Profiling of transcriptional regulators associated with starch biosynthesis in sorghum (Sorghum bicolor L.)

Starch presents as the major component of grain endosperm of sorghum (Sorghum bicolor L.) and other cereals, serving as the main energy supplier for both plants and animals, as well as important industrial raw materials of human beings, and was intensively concerned world widely. However, few documents focused on the pathway and transcriptional regulations of starch biosynthesis in sorghum. Here we presented the RNA-sequencing profiles of 20 sorghum tissues at different developmental stages to dissect key genes associated with sorghum starch biosynthesis and potential transcriptional regulations. A total of 1,708 highly expressed genes were detected, namely, 416 in grains, 736 in inflorescence, 73 in the stalk, 215 in the root, and 268 genes in the leaf. Besides, 27 genes encoded key enzymes associated with starch biosynthesis in sorghum were identified, namely, six for ADP-glucose pyrophosphorylase (AGPase), 10 for starch synthases (SSs), four for both starch-branching enzymes (SBE) and starch-debranching enzymes (DBEs), two for starch phosphorylases (SPs), and one for Brittle-1 (BT1). In addition, 65 transcription factors (TFs) that are highly expressed in endosperm were detected to co-express with 16 out of 27 genes, and 90 cis-elements were possessed by all 27 identified genes. Four NAC TFs were cloned, and the further assay results showed that three of them could in vitro bind to the CACGCAA motif within the promoters of SbBt1 and SbGBSSI, two key genes associated with starch biosynthesis in sorghum, functioning in similar ways that reported in other cereals. These results confirmed that sorghum starch biosynthesis might share the same or similar transcriptional regulations documented in other cereals, and provided informative references for further regulatory mechanism dissection of TFs involved in starch biosynthesis in sorghum.

Cassava shrunken-2 homolog MeAPL3 determines storage root starch and dry matter content and modulates storage root postharvest physiological deterioration

Among the five cassava isoforms (MeAPL1-MeAPL5), MeAPL3 is responsible for determining storage root starch content. Degree of storage root postharvest physiological deterioration (PPD) is directly correlated with starch content. AGPase is heterotetramer composed of two small and two large subunits each coded by small gene families in higher plants. Studies in cassava (Manihot esculenta) identified and characterized five isoforms of Manihot esculenta ADP-glucose pyrophosphorylase large subunit (MeAPL1-MeAPL5) and employed virus induced gene silencing (VIGS) to show that MeAPL3 is the key isoform responsible for starch and dry matter accumulation in cassava storage roots. Silencing of MeAPL3 in cassava through stable transgenic lines resulted in plants displaying significant reduction in storage root starch and dry matter content (DMC) and induced a distinct phenotype associated with increased petiole/stem angle, resulting in a droopy leaf phenotype. Plants with reduced starch and DMC also displayed significantly reduced or no postharvest physiological deterioration (PPD) compared to controls and lines with high DMC and starch content. This provides strong evidence for direct relationships between starch/dry matter content and its role in PPD and canopy architecture traits in cassava.

Seasonal Variation in Transcriptomic Profiling of Tetrastigma hemsleyanum Fully Developed Tuberous Roots Enriches Candidate Genes in Essential Metabolic Pathways and Phytohormone Signaling

Tetrastigma hemsleyanum Diels et Gilg (Sanyeqing, SYQ) is a perennial climbing liana and an endemic plant to southern China. Its tuberous roots (TRs) are used in traditional Chinese medicine for treating some diseases such as high fever, pneumonia, asthma, hepatitis, and cancers. However, the mechanisms underlying the development of TR and the content of flavonoids and phenylpropanoids (FPs) are not well-understood. In this study, we performed a transcriptomic analysis of 12 fully developed TR (FD-TR) samples harvested in four seasons [spring (Sp), summer (Su), autumn (Au), and winter (Wi)] using the RNA-Sequencing (RNA-Seq). We obtained a total of 78.54 Gb raw data and 65,578 unigenes. Then, the unigenes were annotated by using six databases such as non-redundant protein database (NR), Pfam, eggNOG, SWISSProt, Kyoto Encyclopedia of Genes and Genomes (KEGG), and gene ontology (GO). The transcriptomic profiling showed closer relationships between the samples obtained in Su and Au than those obtained in Sp and Wi based on the results of both total unigenes and differentially expressed genes (DEGs). Three pathways, including the biosynthesis of FPs, metabolism of starch and sucrose, and signaling of phytohormones, were highly enriched, suggesting a gene-level seasonal variation. Based on the numbers of DEGs, brassinosteroid (BR) signal transduction factors appeared to play a key role in modulating the development of TRs while most of the auxin signaling genes were mainly activated in Wi and Sp FD-TRs. Most genes in the biosynthesis and biodegradation of starch and biodegradation of cellulose were activated in Wi FD-TRs. As determined by the high performance liquid chromatography (HPLC) and aluminum nitrate colorimetric method, the contents of total flavonoids and most detected FP components increased from Sp to Au but decreased in Wi. Enhanced expression levels of some genes in the biosynthetic pathways of FPs were detected in Su and Au samples, which corroborated well with metabolite content. Our findings provide the first transcriptomic and biochemical data on a seasonal variation in the composition of medically important metabolites in SYQ FD-TRs.

Unlocking conserved and diverged metabolic characteristics in cassava carbon assimilation via comparative genomics approach

Globally, cassava is an important source of starch, which is synthesized through carbon assimilation in cellular metabolism whereby harvested atmospheric carbon is assimilated into macromolecules. Although the carbon assimilation pathway is highly conserved across species, metabolic phenotypes could differ in composition, type, and quantity. To unravel the metabolic complexity and advantage of cassava over other starch crops, in terms of starch production, we investigated the carbon assimilation mechanisms in cassava through genome-based pathway reconstruction and comparative network analysis. First, MeRecon - the carbon assimilation pathway of cassava was reconstructed based upon six plant templates: Arabidopsis, rice, maize, castor bean, potato, and turnip. MeRecon, available at http://bml.sbi.kmutt.ac.th/MeRecon, comprises 259 reactions (199 EC numbers), 1,052 proteins (870 genes) and 259 metabolites in eight sub-metabolisms. Analysis of MeRecon and the carbon assimilation pathways of the plant templates revealed the overall topology is highly conserved, but variations at sub metabolism level were found in relation to complexity underlying each biochemical reaction, such as numbers of responsible enzymatic proteins and their evolved functions, which likely explain the distinct metabolic phenotype. Thus, this study provides insights into the network characteristics and mechanisms that regulate the synthesis of metabolic phenotypes of cassava.

Regional Heritability Mapping Provides Insights into Dry Matter Content in African White and Yellow Cassava Populations

The HarvestPlus program for cassava ( Crantz) fortifies cassava with β-carotene by breeding for carotene-rich tubers (yellow cassava). However, a negative correlation between yellowness and dry matter (DM) content has been identified. We investigated the genetic control of DM in white and yellow cassava. We used regional heritability mapping (RHM) to associate DM with genomic segments in both subpopulations. Significant segments were subjected to candidate gene analysis and candidates were validated with prediction accuracies. The RHM procedure was validated via a simulation approach and revealed significant hits for white cassava on chromosomes 1, 4, 5, 10, 17, and 18, whereas hits for the yellow were on chromosome 1. Candidate gene analysis revealed genes in the carbohydrate biosynthesis pathway including plant serine-threonine protein kinases (SnRKs), UDP (uridine diphosphate)-glycosyltransferases, UDP-sugar transporters, invertases, pectinases, and regulons. Validation using 1252 unique identifiers from the SnRK gene family genome-wide recovered 50% of the predictive accuracy of whole-genome single nucleotide polymorphisms for DM, whereas validation using 53 likely genes (extracted from the literature) from significant segments recovered 32%. Genes including an acid invertase, a neutral or alkaline invertase, and a glucose-6-phosphate isomerase were validated on the basis of an a priori list for the cassava starch pathway, and also a fructose-biphosphate aldolase from the Calvin cycle pathway. The power of the RHM procedure was estimated as 47% when the causal quantitative trait loci generated 10% of the phenotypic variance (sample size = 451). Cassava DM genetics are complex and RHM may be useful for complex traits.

The Discrepant and Similar Responses of Genome-Wide Transcriptional Profiles between Drought and Cold Stresses in Cassava

Background: Cassava, an important tropical crop, has remarkable drought tolerance, but is very sensitive to cold. The growth, development, and root productivity of cassava are all adversely affected under cold and drought. Methods: To profile the transcriptional response to cold and drought stresses, cassava seedlings were respectively subjected to 0, 6, 24, and 48 h of cold stress and 0, 4, 6, and 10 days of drought stress. Their folded leaves, fully extended leaves, and roots were respectively investigated using RNA-seq. Results: Many genes specifically and commonly responsive to cold and drought were revealed: genes related to basic cellular metabolism, tetrapyrrole synthesis, and brassinosteroid metabolism exclusively responded to cold; genes related to abiotic stress and ethylene metabolism exclusively responded to drought; and genes related to cell wall, photosynthesis, and carbohydrate metabolism, DNA synthesis/chromatic structure, abscisic acid and salicylic acid metabolism, and calcium signaling commonly responded to both cold and drought. Discussion: Combined with cold- and/or drought-responsive transcription factors, the regulatory networks responding to cold and drought in cassava were constructed. All these findings will improve our understanding of the specific and common responses to cold and drought in cassava, and shed light on genetic improvement of cold and drought tolerance in cassava.

Prediction of cassava protein interactome based on interolog method

Cassava is a starchy root crop whose role in food security becomes more significant nowadays. Together with the industrial uses for versatile purposes, demand for cassava starch is continuously growing. However, in-depth study to uncover the mystery of cellular regulation, especially the interaction between proteins, is lacking. To reduce the knowledge gap in protein-protein interaction (PPI), genome-scale PPI network of cassava was constructed using interolog-based method (MePPI-In, available at http://bml.sbi.kmutt.ac.th/ppi). The network was constructed from the information of seven template plants. The MePPI-In included 90,173 interactions from 7,209 proteins. At least, 39 percent of the total predictions were found with supports from gene/protein expression data, while further co-expression analysis yielded 16 highly promising PPIs. In addition, domain-domain interaction information was employed to increase reliability of the network and guide the search for more groups of promising PPIs. Moreover, the topology and functional content of MePPI-In was similar to the networks of Arabidopsis and rice. The potential contribution of MePPI-In for various applications, such as protein-complex formation and prediction of protein function, was discussed and exemplified. The insights provided by our MePPI-In would hopefully enable us to pursue precise trait improvement in cassava.

The industrial applications of cassava: current status, opportunities and prospects

Cassava (Manihot esculenta Crantz) is a drought-tolerant, staple food crop that is grown in tropical and subtropical areas. As an important raw material, cassava is a valuable food source in developing countries and is also extensively employed for producing starch, bioethanol and other bio-based products (e.g. feed, medicine, cosmetics and biopolymers). These cassava-based industries also generate large quantities of wastes/residues rich in organic matter and suspended solids, providing great potential for conversion into value-added products through biorefinery. However, the community of cassava researchers is relatively small and there is very limited information on cassava. Therefore this review summarizes current knowledge on the system biology, economic value, nutritional quality and industrial applications of cassava and its wastes in an attempt to accelerate understanding of the basic biology of cassava. The review also discusses future perspectives with respect to integrating and utilizing cassava information resources for increasing the economic and environmental sustainability of cassava industries. © 2017 Society of Chemical Industry.

Identification and expression analyses of new potential regulators of xylem development and cambium activity in cassava (Manihot esculenta)

We have identified new potential regulators of xylem cell-type determination and cellular proliferation in cassava and studied their expression in roots. Results are highly relevant for cassava biotechnology. Cassava's root system is composed of two types of root that coexist in every individual: the fibrous and the storage roots. Whether a root becomes fibrous or storage depends on the xylem cell types that it develops: fibrous roots develop xylem fibres and vessels while storage roots develop parenchyma xylem, the starch-storing tissue. A crucial question in cassava root development is how the specific xylem cell types differentiate and proliferate in the fibrous and storage roots. Using phylogenetic, protein sequence and synteny analyses we identified (1) MeVND6, MeVND7.1, MeVND7.2, MeNST3.1 and MeNST3.2 as the potential cassava orthologues of the Arabidopsis regulators of xylem cell type determination AtVND6, AtVND7 and AtNST3; and (2) MeWOX4.1 and MeWOX4.2 as the potential cassava orthologues of the Arabidopsis cambium regulator AtWOX4. Fibrous and storage roots were anatomically characterised and tested for the expression of the identified genes. Results revealed that (1) MeVND7.1 and MeVND7.2 are expressed in the fibrous but not in the storage roots; (2) MeVND6 shows low expression in both root types; (3) MeNST3.1 is not expressed in the fibrous or storage roots, while MeNST3.2 is highly expressed in both root-types and (4) MeWOX4.1 and, to a higher level, MeWOX4.2 are expressed in both the fibrous and storage roots. Results open new avenues for research in cassava root development and for food security-oriented biotechnology programmes.

Transcriptome-wide mining suggests conglomerate of genes associated with tuberous root growth and development in Aconitum heterophyllum Wall

Tuberous roots of Aconitum heterophyllum constitute storage organ for secondary metabolites, however, molecular components contributing to their formation are not known. The transcriptomes of A. heterophyllum were analyzed to identify possible genes associated with tuberous root development by taking clues from genes implicated in other plant species. Out of 18 genes, eight genes encoding GDP-mannose pyrophosphorylase (GMPase), SHAGGY, Expansin, RING-box protein 1 (RBX1), SRF receptor kinase (SRF), β-amylase, ADP-glucose pyrophosphorylase (AGPase) and Auxin responsive factor 2 (ARF2) showed higher transcript abundance in roots (13-171 folds) compared to shoots. Comparative expression analysis of those genes between tuberous root developmental stages showed 11-97 folds increase in transcripts in fully developed roots compared to young rootlets, thereby implying their association in biosynthesis, accumulation and storage of primary metabolites towards root biomass. Cluster analysis revealed a positive correlation with the gene expression data for different stages of tuberous root formation in A. heterophyllum. The outcome of this study can be useful in genetic improvement of A. heterophyllum for root biomass yield.

Proteomics Profiling Reveals Carbohydrate Metabolic Enzymes and 14-3-3 Proteins Play Important Roles for Starch Accumulation during Cassava Root Tuberization

Cassava is one of the most important root crops as a reliable source of food and carbohydrates. Carbohydrate metabolism and starch accumulation in cassava storage root is a cascade process that includes large amounts of proteins and cofactors. Here, comparative proteomics were conducted in cassava root at nine developmental stages. A total of 154 identified proteins were found to be differentially expressed during starch accumulation and root tuberization. Many enzymes involved in starch and sucrose metabolism were significantly up-regulated, and functional classification of the differentially expressed proteins demonstrated that the majority were binding-related enzymes. Many proteins were took part in carbohydrate metabolism to produce energy. Among them, three 14-3-3 isoforms were induced to be clearly phosphorylated during storage root enlargement. Overexpression of a cassava 14-3-3 gene in Arabidopsis thaliana confirmed that the older leaves of these transgenic plants contained higher sugar and starch contents than the wild-type leaves. The 14-3-3 proteins and their binding enzymes may play important roles in carbohydrate metabolism and starch accumulation during cassava root tuberization. These results not only deepened our understanding of the tuberous root proteome, but also uncovered new insights into carbohydrate metabolism and starch accumulation during cassava root enlargement.

Next-generation sequencing (NGS) transcriptomes reveal association of multiple genes and pathways contributing to secondary metabolites accumulation in tuberous roots of Aconitum heterophyllum Wall

The transcriptomes of Aconitum heterophyllum were assembled and characterized for the first time to decipher molecular components contributing to biosynthesis and accumulation of metabolites in tuberous roots. Aconitum heterophyllum Wall., popularly known as Atis, is a high-value medicinal herb of North-Western Himalayas. No information exists as of today on genetic factors contributing to the biosynthesis of secondary metabolites accumulating in tuberous roots, thereby, limiting genetic interventions towards genetic improvement of A. heterophyllum. Illumina paired-end sequencing followed by de novo assembly yielded 75,548 transcripts for root transcriptome and 39,100 transcripts for shoot transcriptome with minimum length of 200 bp. Biological role analysis of root versus shoot transcriptomes assigned 27,596 and 16,604 root transcripts; 12,340 and 9398 shoot transcripts into gene ontology and clusters of orthologous group, respectively. KEGG pathway mapping assigned 37 and 31 transcripts onto starch-sucrose metabolism while 329 and 341 KEGG orthologies associated with transcripts were found to be involved in biosynthesis of various secondary metabolites for root and shoot transcriptomes, respectively. In silico expression profiling of the mevalonate/2-C-methyl-D-erythritol 4-phosphate (non-mevalonate) pathway genes for aconites biosynthesis revealed 4 genes HMGR (3-hydroxy-3-methylglutaryl-CoA reductase), MVK (mevalonate kinase), MVDD (mevalonate diphosphate decarboxylase) and HDS (1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate synthase) with higher expression in root transcriptome compared to shoot transcriptome suggesting their key role in biosynthesis of aconite alkaloids. Five genes, GMPase (geranyl diphosphate mannose pyrophosphorylase), SHAGGY, RBX1 (RING-box protein 1), SRF receptor kinases and β-amylase, implicated in tuberous root formation in other plant species showed higher levels of expression in tuberous roots compared to shoots. A total of 15,487 transcription factors belonging to bHLH, MYB, bZIP families and 399 ABC transporters which regulate biosynthesis and accumulation of bioactive compounds were identified in root and shoot transcriptomes. The expression of 5 ABC transporters involved in tuberous root development was validated by quantitative PCR analysis. Network connectivity diagrams were drawn for starch-sucrose metabolism and isoquinoline alkaloid biosynthesis associated with tuberous root growth and secondary metabolism, respectively, in root transcriptome of A. heterophyllum. The current endeavor will be of practical importance in planning a suitable genetic intervention strategy for the improvement of A. heterophyllum.

EVALUACIÓN DE LA EXPRESIÓN DE GENES IMPLICADOS EN LA BIOSÍNTESIS DE ALMIDÓN EN DIFERENTES VARIEDADES DE YUCA

<p>Las raíces almacenadoras de yuca representan una fuente importante de almidón. La ruta metabólica del almidón ha sido reconstruida recientemente en yuca gracias a la liberación de la secuencia completa de su genoma. En este estudio se evaluó la expresión de los genes que codifican para las enzimas Pululanasa, Isoamilasa, α-amilasa, Enzima Desproporcionante, ADP-glucosa pirofoforilasa, Almidón sintasa unida al gránulo, Enzima ramificante del almidón y Sintasa soluble del almidón, en las raíces almacenadoras de plantas de 5 y 11 meses de edad, en un grupo de cinco variedades de yuca. Se evidenciaron diferencias importantes en la expresión de estos genes entre las variedades evaluadas y entre los dos tiempos. Las variedades CM523-7 y SM1219-2 presentaron uno de los niveles más altos de expresión para los genes ADP-glucosa pirofoforilasa y Almidón sintasa unida al gránulo mientras que el gen para α-amilasa fue el más bajo en estas dos variedades. Aunque  la variedad TMS60444 presentó niveles de expresión similares en genes implicados en la síntesis de almidón, fue la que presentó el mayor nivel de expresión de la α-amilasa. Estos datos se pueden correlacionar con el relativo bajo contenido de materia seca en esta variedad. Los datos de expresión génica presentados en este trabajo permitirán complementar información sobre actividad enzimática con miras a identificar los elementos más importantes en la acumulación diferencial de almidón entre variedades de yuca.</p><p><strong>ABSTRACT</strong></p><p>Cassava storage roots represent an important starch source. Recently, the starch metabolic pathway in cassava has been reconstructed thanks to the full release of its genome. In this study gene expression was evaluated for genes coding Pullulanase, Isoamylase, α-amylase, Deproportionating enzyme, ADP-glucose pyrophosphorylase, Granule bound starch synthase, Starch branching enzyme and Soluble starch synthase, in cassava storage roots 5 and 11 months old, in 5 cassava varieties. Important gene expression differences were detected both at the variety and time level. CM523-7 and SM1219-2 showed one of the highest expression levels for AGPase and GBSS genes, while α-amylase showed the lowest level in these two varieties. TMS60444 variety showed similar expression levels in starch biosynthesis-related genes, but conversely also showed the highest α-amylase expression. This correlates with the relative low dry-matter content in TMS60444. Gene expression data reported here will allow complementing actual information on enzymatic activity, in order to identify the most relevant factors in differential starch accumulation between cassava varieties.</p><br /><p> </p>

Nucleotide diversity of Maize ZmBT1 gene and association with starch physicochemical properties

Cereal Brittle1 protein has been demonstrated to be involved in the ADP-Glc transport into endosperm plastids, and plays vital roles in the biosynthesis of starch. In this study, the genomic sequences of the ZmBT1 gene in 80 elite maize inbred lines were obtained, and the nucleotide polymorphisms and haplotype diversity were detected. A total of 30 variants, including 22 SNPs and 8 indels, were detected from the full sequences of this gene. Among these polymorphic sites, 9 SNPs and 2 indels were found to be located in the coding region. The polymorphisms of CDS sequences classified the maize ZmBT1 gene into 6 haplotypes, which encode 6 different ZmBT1 proteins. Neutrality tests revealed a decrease in population size and/or balancing selection on the maize ZmBT1 locus. To detect the association between sequence variations of this gene and the starch physicochemical properties, 7 pasting and 4 gelatinization traits of starch were measured for the tested inbred lines using rapid visco analyzer (RVA) and differential scanning calorimeter (DSC), respectively. The result of association analysis revealed that an indel in the coding region was significantly associated with the phenotypic variation of starch gelatinization enthalpy.