Presence of multiple cDNAs encoding an isoform of ADP-glucose pyrophosphorylase large subunit from sweet potato and characterization of expression levels

Exploring the structural assembly of rice ADP-glucose pyrophosphorylase subunits using MD simulation

ADP-glucose pyrophosphorylase plays a pivotal role as an allosteric enzyme, essential for starch biosynthesis in plants. The higher plant AGPase comparises of a pair of large and a pair of small subunits to form a heterotetrameric complex. Growing evidence indicates that each subunit plays a distinct role in regulating the underlying mechanism of starch biosynthesis. In the rice genome, there are four large subunit genes (OsL1-L4) and three small subunit genes (OsS1, OsS2a, and OsS2b). While the structural assembly of cytosolic rice AGPase subunits (OsL2:OsS2b) has been elucidated, there is currently no such documented research available for plastidial rice AGPases (OsL1:OsS1). In this study, we employed protein modeling and MD simulation approaches to gain insights into the structural association of plastidial rice AGPase subunits. Our results demonstrate that the heterotetrameric association of OsL1:OsS1 is very similar to that of cytosolic OsL2:OsS2b and potato AGPase heterotetramer (StLS:StSS). Moreover, the yeast-two-hybrid results on OsL1:OsS1, which resemble StLS:StSS, suggest a differential protein assembly for OsL2:OsS2b. Thus, the regulatory and catalytic mechanisms for plastidial AGPases (OsL1:OsS1) could be different in rice culm and developing endosperm compared to those of OsL2:OsS2b, which are predominantly found in rice endosperm.

Sugar starvation activates the OsSnRK1a-OsbHLH111/OsSGI1-OsTPP7 module to mediate growth inhibition of rice

Sugar deficiency is the persistent challenge for plants during development. Trehalose-6-phosphate (T6P) is recognized as a key regulator in balancing plant sugar homeostasis. However, the underlying mechanisms by which sugar starvation limits plant development are unclear. Here, a basic helix-loop-helix (bHLH) transcription factor (OsbHLH111) was named starvation-associated growth inhibitor 1 (OsSGI1) and the focus is on the sugar shortage of rice. The transcript and protein levels of OsSGI1 were markedly increased during sugar starvation. The knockout mutants sgi1-1/2/3 exhibited increased grain size and promoted seed germination and vegetative growth, which were opposite to those of overexpression lines. The direct binding of OsSGI1 to sucrose non-fermenting-1 (SNF1)-related protein kinase 1a (OsSnRK1a) was enhanced during sugar shortage. Subsequently, OsSnRK1a-dependent phosphorylation of OsSGI1 enhanced the direct binding to the E-box of trehalose 6-phosphate phosphatase 7 (OsTPP7) promoter, thus rose the transcription inhibition on OsTPP7, then elevated trehalose 6-phosphate (Tre6P) content but decreased sucrose content. Meanwhile, OsSnRK1a degraded phosphorylated-OsSGI1 by proteasome pathway to prevent the cumulative toxicity of OsSGI1. Overall, we established the OsSGI1-OsTPP7-Tre6P loop with OsSnRK1a as center and OsSGI1 as forward, which is activated by sugar starvation to regulate sugar homeostasis and thus inhibits rice growth.

Sucrose signaling in higher plants

Sucrose controls various developmental and metabolic processes in plants. In this review, we evaluate whether sucrose could be a preferred signaling molecule that controls processes like carbohydrate metabolism, accumulation of storage proteins, sucrose transport, anthocyanin accumulation, and floral induction. We summarize putative sucrose-dependent signaling pathways. Sucrose, but not other sugars, stimulates the genes that encode ADP-glucose pyrophosphorylase (AGPase), granule-bound starch synthase I, and UDP-glucose pyrophosphorylase in several species. The class-1 patatin promoter is induced under high sucrose conditions in potato (Solanum tuberosum). Exogenous sucrose reduces the loading of sucrose to the phloem by inhibiting the expression of the sucrose transporter and its protein activity in sugar beet (Beta vulgaris). Sucrose also influences a wide range of growth processes, including cell division, ribosome synthesis, cotyledon development, far-red light signaling, and tuber development. Floral induction is promoted by sucrose in several species. The molecular mechanisms by which sucrose functions as a signal are largely unknown. Sucrose enhances the expression of transcription factors such as AtWRKY20 and MYB75, which function upstream of the sucrose-responsive genes. Sucrose controls the expression of AtbZIP11 at the post-transcriptional level by the peptide encoded by uORF2. Sucrose levels affect translation of a group of mRNAs in Arabidopsis. Sucrose increases the activity of AGPase by posttranslational redox-modification. Sucrose interrupts the interaction between sucrose transporter SUT4 and cytochrome b5. In addition, the SNF-related protein kinase-1 appears to be involved in sucrose-dependent pathways by controlling sucrose synthase (SUS4) expression.

The Role of Sugar Signaling in Regulating Plant Fatty Acid Synthesis

Photosynthates such as glucose, sucrose, and some of their derivatives play dual roles as metabolic intermediates and signaling molecules that influence plant cell metabolism. Such sugars provide substrates for de novo fatty acid (FA) biosynthesis. However, compared with the well-defined examples of sugar signaling in starch and anthocyanin synthesis, until recently relatively little was known about the role of signaling in regulating FA and lipid biosynthesis. Recent research progress shows that trehalose 6-phosphate and 2-oxoglutarate (2-OG) play direct signaling roles in the regulation of FA biosynthesis by modulating transcription factor stability and enzymatic activities involved in FA biosynthesis. Specifically, mechanistic links between sucrose non-fermenting-1-related protein kinase 1 (SnRK1)-mediated trehalose 6-phosphate (T6P) sensing and its regulation by phosphorylation of WRI1 stability, diacylglycerol acyltransferase 1 (DGAT1) enzyme activity, and of 2-OG-mediated relief of inhibition of acetyl-CoA carboxylase (ACCase) activity by protein PII are exemplified in detail in this review.

Fruit-localized phytochromes regulate plastid biogenesis, starch synthesis, and carotenoid metabolism in tomato

Light signaling has long been reported to influence fruit biology, although the regulatory impact of fruit-localized photoreceptors on fruit development and metabolism remains unclear. Studies performed in phytochrome (PHY)-deficient tomato (Solanum lycopersicum) mutants suggest that SlPHYA, SlPHYB2, and to a lesser extent SlPHYB1 influence fruit development and ripening. By employing fruit-specific RNAi-mediated silencing of SlPHY genes, we demonstrated that fruit-localized SlPHYA and SlPHYB2 play contrasting roles in regulating plastid biogenesis and maturation in tomato. Our data revealed that fruit-localized SlPHYA, rather than SlPHYB1 or SlPHYB2, positively influences tomato plastid differentiation and division machinery via changes in both light and cytokinin signaling-related gene expression. Fruit-localized SlPHYA and SlPHYB2 were also shown to modulate sugar metabolism in early developing fruits via overlapping, yet distinct, mechanisms involving the co-ordinated transcriptional regulation of genes related to sink strength and starch biosynthesis. Fruit-specific SlPHY silencing also drastically altered the transcriptional profile of genes encoding light-repressor proteins and carotenoid-biosynthesis regulators, leading to reduced carotenoid biosynthesis during fruit ripening. Together, our data reveal the existence of an intricate PHY-hormonal interplay during fruit development and ripening, and provide conclusive evidence on the regulation of tomato quality by fruit-localized phytochromes.

UDP-Glucose: A Potential Signaling Molecule in Plants?

This perspective paper focuses on the most recent results suggesting a potential role for UDP-Glucose as a signaling molecule in plants. In animals, UDP-Glucose is well-established as an extracellular signaling molecule that is sensed by G-protein coupled receptors, activating several downstream defense mechanisms. Recent studies have shown that abnormal growth occurred in both vegetative and reproductive tissue of plants with reduced UDP-Glucose levels, and this could be rescued by exogenous UDP-Glucose. In plants with increased biomass accumulation, the genes involved in UDP-Glucose production were up-regulated. However, excessive endogenous accumulation of UDP-Glucose induced programmed cell death (PCD), and this could also be obtained by exogenous UDP-Glucose application. Plants with decreased UDP-glucose were insensitive to pathogen induced PCD. We speculate that UDP-Glucose acts as an extracellular signaling molecule in plants, and that it may be perceived as a damage-associated molecular pattern.

Comparative Transcriptome Analysis Reveals Critical Function of Sucrose Metabolism Related-Enzymes in Starch Accumulation in the Storage Root of Sweet Potato

The starch properties of the storage root (SR) affect the quality of sweet potato (Ipomoea batatas (L.) Lam.). Although numerous studies have analyzed the accumulation and properties of starch in sweet potato SRs, the transcriptomic variation associated with starch properties in SR has not been quantified. In this study, we measured the starch and sugar contents and analyzed the transcriptome profiles of SRs harvested from sweet potatoes with high, medium, and extremely low starch contents, at five developmental stages [65, 80, 95, 110, and 125 days after transplanting (DAP)]. We found that differences in both water content and starch accumulation in the dry matter affect the starch content of SRs in different sweet potato genotypes. Based on transcriptome sequencing data, we assembled 112336 unigenes, and identified several differentially expressed genes (DEGs) involved in starch and sucrose metabolism, and revealed the transcriptional regulatory network controlling starch and sucrose metabolism in sweet potato SRs. Correlation analysis between expression patterns and starch and sugar contents suggested that the sugar-starch conversion steps catalyzed by sucrose synthase (SuSy) and UDP-glucose pyrophosphorylase (UGPase) may be essential for starch accumulation in the dry matter of SRs, and IbβFRUCT2, a vacuolar acid invertase, might also be a key regulator of starch content in the SRs. Our results provide valuable resources for future investigations aimed at deciphering the molecular mechanisms determining the starch properties of sweet potato SRs.

Activity and expression of ADP-glucose pyrophosphorylase during rhizome formation in lotus (Nelumbo nucifera Gaertn.)

BACKGROUND

Lotus root is a traditional and popular aquatic vegetable in China. Starch is an important component of the rhizome and directly affects the quality of processed products. ADP -glucose pyrophosphorylase (AGPase) is a rate-limiting enzyme associated with starch biosynthesis in plants. Therefore, in the present study, AGPase activity and NnAGP expression during rhizome development of lotus were analyzed.

RESULTS

Among 15 cultivars analyzed, the contents of amylose and total starch in the rhizome were highest in 'Mei Ren Hong'. 'Su Zhou' and 'Zhen Zhu' showed the lowest amylose, amylopectin and total starch contents. In the rhizome, activity of AGPase was highest at the middle swelling stage of development, and higher activity was observed in the 'Hou ba' leaf and terminational leaf at the same stage. Three AGPase genes, comprising two large subunit genes (NnAGPL1 and NnAGPL2) and one small subunit gene (NnAGPS), were isolated and identified. The deduced amino acid sequences showed 40.5 % similarity among the three genes. Full-length genomic DNA sequences of NnAGPL1, NnAGPL2, and NnAGPS were 4841, 11,346 and 4169 bp, respectively. Analysis of the temporal and spatial expression patterns revealed that the transcription levels of NnAGPL1 and NnAGPS were higher in the rhizome, followed by the 'Hou ba' leaf, whereas NnAGPL2 was significantly detected in the 'Hou ba' leaf and terminational leaf. The initial swelling stage of rhizome development was accompanied by the highest accumulation of mRNAs of NnAGPL1, whereas expression of NnAGPL2 was not detected during rhizome development. The transcript level of NnAGPS was highest at the initial swelling stage compared with the other rhizome developmental stages. Transcription of NnAGPL1, NnAGPL2, and NnAGPS was induced within 24 h after treatment with exogenous sucrose. The mRNA level of NnAGPL1 and NnAGPS was increased by exogenous ABA, whereas transcription of NnAGPL2 was not affected by ABA.

CONCLUSIONS

The three AGPase genes display marked differences in spatial and temporal expression patterns. Regulation of AGPase in relation to starch synthesis in lotus is indicated to be complex.

ADP-glucose pyrophosphorylase large subunit 2 is essential for storage substance accumulation and subunit interactions in rice endosperm

ADP-glucose pyrophosphorylase (AGPase) controls a rate-limiting step in the starch biosynthetic pathway in higher plants. Here we isolated a shrunken rice mutant w24. Map-based cloning identified OsAGPL2, a large subunit of the cytosolic AGPase in rice endosperm, as the gene responsible for the w24 mutation. In addition to severe inhibition of starch synthesis and significant accumulation of sugar, the w24 endosperm showed obvious defects in compound granule formation and storage protein synthesis. The defect in OsAGPL2 enhanced the expression levels of the AGPase family. Meanwhile, the elevated activities of starch phosphorylase 1 and sucrose synthase in the w24 endosperm might possibly partly account for the residual starch content in the mutant seeds. Moreover, the expression of OsAGPL2 and its counterpart, OsAGPS2b, was highly coordinated in rice endosperm. Yeast two-hybrid and BiFC assays verified direct interactions between OsAGPL2 and OsAGPS2b as well as OsAGPL1 and OsAGPS1, supporting the model for spatiotemporal complex formation of AGPase isoforms in rice endosperm. Besides, our data provided no evidence for the self-binding of OsAGPS2b, implying that OsAGPS2b might not interact to form higher molecular mass aggregates in the absence of OsAGPL2. Therefore, the molecular mechanism of rice AGPase assembly might differ from that of Arabidopsis.

Sucrose and ABA regulate starch biosynthesis in maize through a novel transcription factor, ZmEREB156

Sucrose is not only the carbon source for starch synthesis, but also a signal molecule. Alone or in coordination with ABA, it can regulate the expression of genes involved in starch synthesis. To investigate the molecular mechanisms underlying this effect, maize endosperms were collected from Zea mays L. B73 inbred line 10 d after pollination and treated with sucrose, ABA, or sucrose plus ABA at 28 °C in the dark for 24 h. RNA-sequence analysis of the maize endosperm transcriptome revealed 47 candidate transcription factors among the differentially expressed genes. We therefore speculate that starch synthetic gene expression is regulated by transcription factors induced by the combination of sucrose and ABA. ZmEREB156, a candidate transcription factor, is induced by sucrose plus ABA and is involved in starch biosynthesis. The ZmEREB156-GFP-fused protein was localized in the nuclei of onion epidermal cells, and ZmEREB156 protein possessed strong transcriptional activation activity. Promoter activity of the starch-related genes Zmsh2 and ZmSSIIIa increased after overexpression of ZmEREB156 in maize endosperm. ZmEREB156 could bind to the ZmSSIIIa promoter but not the Zmsh2 promoter in a yeast one-hybrid system. Thus, ZmEREB156 positively modulates starch biosynthetic gene ZmSSIIIa via the synergistic effect of sucrose and ABA.

ADP-glucose pyrophosphorylase gene plays a key role in the quality of corm and yield of cormels in gladiolus

Starch is the main storage compound in underground organs like corms. ADP-glucose pyrophosphorylase (AGPase) plays a key role in regulating starch biosynthesis in storage organs and is likely one of the most important determinant of sink strength. Here, we identify an AGPase gene (GhAGPS1) from gladiolus. The highest transcriptional levels of GhAGPS1 were observed in cormels and corms. Transformation of GhAGPS1 into Arabidopsis rescued the phenotype of aps1 mutant. Silencing GhAGPS1 in gladiolus corms by virus-induced gene silencing (VIGS) decreased the transcriptional levels of two genes and starch content. Transmission electron microscopy analyses of leaf and corm sections confirmed that starch biosynthesis was inhibited. Corm weight and cormel number reduced significantly in the silenced plants. Taken together, these results indicate that inhibiting the expression of AGPase gene could impair starch synthesis, which results in the lowered corm quality and cormel yield in gladiolus.

Isolation and characterization of cDNAs and genomic DNAs encoding ADP-glucose pyrophosphorylase large and small subunits from sweet potato

Sweet potato [Ipomoea batatas (L.) Lam.], the world's seventh most important food crop, is also a major industrial raw material for starch and ethanol production. In the plant starch biosynthesis pathway, ADP-glucose pyrophosphorylase (AGPase) catalyzes the first, rate-limiting step and plays a pivotal role in regulating this process. In spite of the importance of sweet potato as a starch source, only a few studies have focused on the molecular aspects of starch biosynthesis in sweet potato and almost no intensive research has been carried out on the AGPase gene family in this species. In this study, cDNAs encoding two small subunits (SSs) and four large subunits (LSs) of AGPase isoforms were cloned from sweet potato and the genomic organizations of the corresponding AGPase genes were elucidated. Expression pattern analysis revealed that the two SSs were constitutively expressed, whereas the four LSs displayed differential expression patterns in various tissues and at different developmental stages. Co-expression of SSs with different LSs in Escherichia coli yielded eight heterotetramers showing different catalytic activities. Interactions between different SSs and LSs were confirmed by a yeast two-hybrid experiment. Our findings provide comprehensive information about AGPase gene sequences, structures, expression profiles, and subunit interactions in sweet potato. The results can serve as a foundation for elucidation of molecular mechanisms of starch synthesis in tuberous roots, and should contribute to future regulation of starch biosynthesis to improve sweet potato starch yield.

Effect of nitrogen and phosphorus deficiency on transcriptional regulation of genes encoding key enzymes of starch metabolism in duckweed (Landoltia punctata)

The production of starch by plants influences their use as biofuels. Nitrogen (N) and phosphorus (P) regulate starch gene expression during plant growth and development, yet the role of key enzymes such as ADP-glucose pyrophosphorylase (E.C. 2.7.7.27 AGPase) in starch metabolism during N- and P-deficiency remains unknown. We investigated the effect of N- and P-deficiency on the expression of large (LeAPL1, LeAPL2, and LeAPL3) and small (LeAPS) subunits of AGPase in duckweed (Landoltia punctata) and their correlation with starch content. We first isolated the full-length cDNA encoding LeAPL1 (GenBank Accession No. KJ603244) and LeAPS (GenBank Accession No. KJ603243); they contained open reading frames of 1554 bp (57.7-kDa polypeptide of 517 amino acids) and 1578 bp (57.0 kDa polypeptide of 525 amino acids), respectively. Real-time PCR analysis revealed that LeAPL1 and LeAPL3 were highly expressed during early stages of N-deficiency, while LeAPL2 was only expressed during late stage. However, in response to P-deficiency, LeAPL1 and LeAPL2 were upregulated during early stages and LeAPL3 was primarily expressed in the late stage. Interestingly, LeAPS was highly expressed following N-deficiency during both stages, but was only upregulated in the early stage after P-deficiency. The activities of AGPase and soluble starch synthesis enzyme (SSS EC 2.4.1.21) were positively correlated with changes in starch content. Furthermore, LeAPL3 and LeSSS (SSS gene) were positively correlated with changes in starch content during N-deficiency, while LeAPS and LeSSS were correlated with starch content in response to P-deficiency. These results elevate current knowledge of the molecular mechanisms underlying starch synthesis.

Effects of light and temperature fluctuations on the growth of Myriophyllum spicatum in toxicity tests--a model-based analysis

Laboratory toxicity tests are a key component of the aquatic risk assessments of chemicals. Toxicity tests with Myriophyllum spicatum are conducted based on working procedures that provide detailed instructions on how to set up the experiment, e.g., which experimental design is necessary to get reproducible and thus comparable results. Approved working procedures are established by analyzing numerous toxicity tests to find a compromise between practical reasons (e.g., acceptable ranges of ambient conditions as they cannot be kept completely constant) and the ability for detecting growth alterations. However, the benefit of each step of a working procedure, e.g., the random repositioning of test beakers, cannot be exactly quantified, although this information might be useful to evaluate working procedures. In this paper, a growth model of M. spicatum was developed and used to assess the impact of temperature and light fluctuations within the standardized setup. It was analyzed how important it is to randomly reassign the location of each plant during laboratory tests to keep differences between the relative growth rates of individual plants low. Moreover, two examples are presented on how modeling can give insight into toxicity testing. Results showed that randomly repositioning of individual plants during an experiment can compensate for fluctuations of light and temperature. A method is presented on how models can be used to improve experimental designs and to quantify their benefits by predicting growth responses.

Sucrose signaling in plants: a world yet to be explored

The role of sucrose as a signaling molecule in plants was originally proposed several decades ago. However, recognition of sucrose as a true signal has been largely debated and only recently this role has been fully accepted. The best-studied cases of sucrose signaling involve metabolic processes, such as the induction of fructan or anthocyanin synthesis, but a large volume of scattered information suggests that sucrose signals may control a vast array of developmental processes along the whole life cycle of the plant. Also, wide gaps exist in our current understanding of the intracellular steps that mediate sucrose action. Sucrose concentration in plant tissues tends to be directly related to light intensity, and inversely related to temperature, and accordingly, exogenous sucrose supply often mimics the effect of high light and cold. However, many exceptions to this rule seem to occur due to interactions with other signaling pathways. In conclusion, the sucrose role as a signal molecule in plants is starting to be unveiled and much research is still needed to have a complete map of its significance in plant function.

Use of TILLING and robotised enzyme assays to generate an allelic series of Arabidopsis thaliana mutants with altered ADP-glucose pyrophosphorylase activity

ADP-glucose pyrophosphorylase (AGPase) catalyses the synthesis of ADP-glucose, and is a highly regulated enzyme in the pathway of starch synthesis. In Arabidopsis thaliana, the enzyme is a heterotetramer, containing two small subunits encoded by the APS1 gene and two large subunits encoded by the APL1-4 genes. TILLING (Targeting Induced Local Lesions IN Genomes) of a chemically mutagenised population of A. thaliana plants identified 33 novel mutations in the APS1 gene, including 21 missense mutations in the protein coding region. High throughput measurements using a robotised cycling assay showed that maximal AGPase activity in the aps1 mutants varied from <15 to 117% of wild type (WT), and that the kinetic properties of the enzyme were altered in several lines, indicating a role for the substituted amino acid residues in catalysis or substrate binding. These results validate the concept of using such a platform for efficient high-throughput screening of very large populations of mutants, natural accessions or introgression lines. AGPase was estimated to have a flux control coefficient of 0.20, indicating that the enzyme exerted only modest control over the rate of starch synthesis in plants grown under short day conditions (8 h light/16 h dark) with an irradiance of 150 μmol quanta m(-2)s(-1). Redox activation of the enzyme, via reduction of the intermolecular disulphide bridge between the two small subunits, was increased in several lines. This was sometimes, but not always, associated with a decrease in the abundance of the APS1 protein. In conclusion, the TILLING technique was used to generate an allelic series of aps1 mutants in A. thaliana that revealed new insights into the multi-layered regulation of AGPase. These mutants offer some advantages over the available loss-of-function mutants, e.g. adg1, for investigating the effects of subtle changes in the enzyme's activity on the rate of starch synthesis.

Two negative cis-regulatory regions involved in fruit-specific promoter activity from watermelon (Citrullus vulgaris S.)

A 1.8 kb 5'-flanking region of the large subunit of ADP-glucose pyrophosphorylase, isolated from watermelon (Citrullus vulgaris S.), has fruit-specific promoter activity in transgenic tomato plants. Two negative regulatory regions, from -986 to -959 and from -472 to -424, were identified in this promoter region by fine deletion analyses. Removal of both regions led to constitutive expression in epidermal cells. Gain-of-function experiments showed that these two regions were sufficient to inhibit RFP (red fluorescent protein) expression in transformed epidermal cells when fused to the cauliflower mosaic virus (CaMV) 35S minimal promoter. Gel mobility shift experiments demonstrated the presence of leaf nuclear factors that interact with these two elements. A TCCAAAA motif was identified in these two regions, as well as one in the reverse orientation, which was confirmed to be a novel specific cis-element. A quantitative beta-glucuronidase (GUS) activity assay of stable transgenic tomato plants showed that the activities of chimeric promoters harbouring only one of the two cis-elements, or both, were approximately 10-fold higher in fruits than in leaves. These data confirm that the TCCAAAA motif functions as a fruit-specific element by inhibiting gene expression in leaves.

Gene expression of ADP-glucose pyrophosphorylase and starch contents in rice cultured cells are cooperatively regulated by sucrose and ABA

Six cDNA clones encoding two small subunits and four large subunits of ADP-glucose pyrophosphorylase (AGPase) were mined from the database of rice full-length cDNAs, cloned and subsequently named: OsAPS1, OsAPS2, OsAPL1, OsAPL2, OsAPL3 and OsAPL4. Expression patterns of the six genes were examined by Northern blot analysis with gene-specific probes. OsAPL3 was predominantly expressed in the middle phases of seed development, and OsAPS1, OsAPL1 and OsAPL2 were expressed later in seed development. OsAPS2 and OsAPL4 were constitutively expressed and these isoforms were coordinated with starch accumulation in the developing rice seed. In order to clarify the effect of sugars and plant hormones on AGPase gene expression more precisely, a rice cell culture system was used. OsAPL3 transcript significantly accumulated in response to increased levels of sucrose and abscisic acid (ABA) concentration in the medium; however, the transcripts of other AGPase genes did not show significant accumulation. Under identical conditions, starch contents in the cultured cells also increased. Interestingly, ABA alone did not affect the gene expression of OsAPL3 and starch content. Collectively, these results indicated that the expression level of OsAPL3 and starch content in the cultured cells were cooperatively controlled by alterations in the concentration of both sucrose and ABA.

Isolation and characterization of cDNA clones encoding ADP-glucose pyrophosphorylase (AGPase) large and small subunits from chickpea (Cicer arietinum L.)

Four cDNA clones encoding two large subunits and two small subunits of the starch regulatory enzyme ADP-glucose pyrophosphorylase (AGPase) were isolated from a chickpea (Cicer arietinum L.) stem cDNA library. DNA sequence and Southern blot analyses of these clones, designated CagpL1, CagpL2 (large subunits) and CagpS1 and CagpS2 (small subunits), revealed that these isoforms represented different AGPase large and small subunits. RNA expression analysis indicated that CagpL1 was expressed strongly in leaves with reduced expression in the stem. No detectable expression was observed in seeds and roots. CagpL2 was expressed moderately in seeds followed by weak expression in leaves, stems and roots. Similar analysis showed that CagpS1 and CagpS2 displayed a spatial expression pattern similar to that observed for CagpL2 with the exception that CagpS1 showed a much higher expression in seeds than CagpS2. The spatial expression patterns of these different AGPase subunit sequences indicate that different AGPase isoforms are used to control starch biosynthesis in different organs during chickpea development.