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

Functional characterization of a starch synthesis-related gene AmAGP in Amorphophallus muelleri

has attracted tremendous interest because of its high contents of glucomannan and starch. Very few genes regulating glucomannan and starch were reported in Amorphophallus. In this study, an ADP-glucose pyrophosphorylase (AGP) gene that plays a significant role in plant starch synthesis was cloned from Amorphophallus muelleri. It was shown that it encoded a predicted protein containing a conserved plant ADP-Glucose-PP repeat domain and seven potential ligand-binding sites. The real-time quantitative PCR showed that AmAGP was most abundant in tubers, and it was positively correlated with starch content. Additionally, its influencers about temperature and exogenous plant hormone were also discussed, showing that AmAGP expressed highly in tubers under treatments using 25°C and IAA. Furthermore, starch content was closely related to AmAGP expression level, suggesting that AmAGP was involved in the regulation of starch synthesis in A. muelleri. Therefore, identifying the sequence of AmAGP and its expression pattern during tuber enlarging and the changes of its transcript levels in response to temperature and plant hormones would contribute to a better understanding of starch synthesis, and also providing a reference information for future preferable breeding for obtaining more starch or more glucomannan in Amorphophallus.

Differential effects of synthetic media on long-term growth, starch accumulation and transcription of ADP-glucosepyrophosphorylase subunit genes in Landoltia punctata

Murashige & Skoog (MS) and Hoagland's media were previously used for in vitro culture of Landoltia punctata. During subsequent ex vitro culture, the use of MS medium resulted in a higher growth rate, compared to Hoagland's medium. Thus, a higher starch content of L. punctata in MS medium was previously hypothesized. Here, L. punctata strain 5632 was isolated and characterized using morphological characteristics and the atpF-atpH intergenic region. During early cultivation stage, fresh weight and relative growth rate in MS medium were lower than Hoagland's medium. Conversely, starch content in MS medium was considerably higher than in Hoagland's medium. Medium effects on expression of genes coding for starch-biosynthesis ADP-glucosepyrophosphorylase (AGPase) were determined. Genomic fragments of small (LeAPS) and large (LeAPL1) AGPase subunits were characterized. Differential expression between each AGPase subunit genes was observed in both media. Additionally, in MS medium, the highest correlation coefficients between starch content and gene expression was found with LeAPS (0.81) and followed by LeAPL3 (0.67), LeAPL2 (0.65) and LeAPL1 (0.28). In Hoagland's medium, the coefficients of LeAPL3 (0.83) and LeAPL2 (0.62) were higher than LeAPS (0.18) and LeAPL1 (-0.62). This suggested different levels of contributions of these genes in starch biosynthesis in both media.

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.

The role of the large subunit in redox regulation of the rice endosperm ADP-glucose pyrophosphorylase

The starch regulatory enzyme ADP-glucose pyrophosphorylase is activated by 3-phosphoglyceric acid (3-PGA) and inhibited by inorganic phosphate (Pi ). The activity of the plastid-localized enzyme is also subject to fine regulation by redox control in response to changing light and sugar levels. The less active oxidized form of the enzyme contains an inter-subunit disulfide bond formed between the pair of small subunit's Cys12 residues of the heterotetrameric enzyme. Although this cysteine residue is not conserved in the small subunits of cereal endosperm cytosolic AGPases, biochemical studies of the major rice endosperm enzyme indicate that the cytosolic isoform, like the plastidial enzymes, is subject to redox control. Kinetic analysis revealed that the reduced forms of the partially purified native and purified recombinant AGPases have 6- and 3.4-fold, respectively, more affinity to 3-PGA, rendering the enzymes more active at lower 3-PGA concentration than the non-reduced enzyme. Truncation of the large subunit by removal of N-terminal peptide resulted in a decrease in 3-PGA affinity and loss of redox response of the enzyme. Site-directed mutagenesis of the conserved cysteine residues at the N-terminal of the large subunit showed that C47 and C58, but not C12, are essential for proper redox response of the enzyme. Overall, our results show that the major rice endosperm AGPase activity is controlled by a combination of allosteric regulation and redox control, the latter through modification of the large subunit instead of the small subunit as evident in the plastid-localized enzyme.

Transcriptional regulation of the ADP-glucose pyrophosphorylase isoforms in the leaf and the stem under long and short photoperiod in lentil

ADP-glucose pyrophosphorylase (AGPase) is a key enzyme in plant starch biosynthesis. It contains large (LS) and small (SS) subunits encoded by two different genes. In this study, we explored the transcriptional regulation of both the LS and SS subunits of AGPase in stem and leaf under different photoperiods length in lentil. To this end, we first isolated and characterized different isoforms of the LS and SS of lentil AGPase and then we performed quantitative real time PCR (qPCR) to see the effect of photoperiod length on the transcription of the AGPase isforms under the different photoperiod regimes in lentil. Analysis of the qPCR results revealed that the transcription of different isoforms of the LSs and the SSs of lentil AGPase are differentially regulated when photoperiod shifted from long-day to short-day in stem and leaves. While transcript levels of LS1 and SS2 in leaf significantly decreased, overall transcript levels of SS1 increased in short-day regime. Our results indicated that day length affects the transcription of lentil AGPase isoforms differentially in stems and leaves most likely to supply carbon from the stem to other tissues to regulate carbon metabolism under short-day conditions.

Characterization of the AGPase large subunit isoforms from tomato indicates that the recombinant L3 subunit is active as a monomer

The enzyme AGPase [ADP-Glc (glucose) pyrophosphorylase] catalyses a rate-limiting step in starch synthesis in tomato (Solanum lycopersicon) fruit, which undergoes a transient period of starch accumulation. It has been a generally accepted paradigm in starch metabolism that the enzyme naturally functions primarily as a heterotetramer comprised of two large subunits (L) and two small subunits (S). The tomato genome harbours a single gene encoding S and three genes for L proteins, which are expressed in both a tissue- and time-specific manner. In the present study the allosteric contributions of the different L subunits were compared by expressing each one in Escherichia coli, in conjunction with S and individually, and characterizing the resulting enzyme activity. Our results indicate different kinetic characteristics of the tomato L1/S and L3/S heterotetramers. Surprisingly, the recombinant L3 protein was also active when expressed alone and size-exclusion and immunoblotting showed that it functioned as a monomer. Subunit interaction modelling pointed to two amino acids potentially affecting subunit interactions. However, directed mutations did not have an impact on subunit tetramerization. These results indicate a hitherto unknown active role for the L subunit in the synthesis of ADP-Glc.

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.

Identification of the ADP-glucose pyrophosphorylase isoforms essential for starch synthesis in the leaf and seed endosperm of rice (Oryza sativa L.)

ADP-glucose pyrophosphorylase (AGP) catalyzes the first committed step of starch biosynthesis in higher plants. To identify AGP isoforms essential for this biosynthetic process in sink and source tissues of rice plants, we analyzed the rice AGP gene family which consists of two genes, OsAGPS1 and OsAGPS2, encoding small subunits (SSU) and four genes, OsAGPL1, OsAGPL2, OsAGPL3 and OsAGPL4, encoding large subunits (LSU) of this enzyme heterotetrameric complex. Subcellular localization studies using green fluorescent protein (GFP) fusion constructs indicate that OsAGPS2a, the product of the leaf-preferential transcript of OsAGPS2, and OsAGPS1, OsAGPL1, OsAGPL3, and OsAGPL4 are plastid-targeted isoforms. In contrast, two isoforms, SSU OsAGPS2b which is a product of a seed-specific transcript of OsAGPS2, and LSU OsAGPL2, are localized in the cytosol. Analysis of osagps2 and osagpl2 mutants revealed that a lesion of one of the two cytosolic isoforms, OsAGPL2 and OsAGPS2b, causes a shrunken endosperm due to a remarkable reduction in starch synthesis. In leaves, however, only the osagps2 mutant appears to severely reduce the transitory starch content. Interestingly, the osagps2 mutant was indistinguishable from wild type during vegetative plant growth. Western blot analysis of the osagp mutants and wild type plants demonstrated that OsAGPS2a is an SSU isoform mainly present in leaves, and that OsAGPS2b and OsAGPL2 are the major SSU and LSU isoforms, respectively, in the endosperm. Finally, we propose a spatiotemporal complex model of OsAGP SSU and LSU isoforms in leaves and in developing endosperm of rice plants.

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.