PCR cloning and characterization of multiple ADP-glucose pyrophosphorylase cDNAs from tomato

ADP-glucose pyrophosphorylase genes are differentially regulated in sugar-dependent or -independent manners in tomato (Solanum lycopersicum L.) fruit

In early developing tomato (Solanum lycopersicum L.) fruit, starch accumulates at high levels and is used by various primary metabolites in ripening fruits. ADP-glucose pyrophosphorylase is responsible for the first key step of starch biosynthesis. Although it has been reported that AgpL1 and AgpS1 isoforms are mainly expressed in early developing fruit, their regulatory mechanism has not been elucidated. The present study investigated the transcriptional response of AgpL1 and AgpS1 to various metabolizable sugars, nonmetabolizable sugar analogues, hexokinase inhibitors and proline by an experimental system using half-cut fruits. AgpL1 was upregulated in response to sucrose and constituted hexoses such glucose, whereas the AgpS1 gene almost did not exhibit a prominent sugar response. Further analyses revealed that other disaccharides such maltose and trehalose did not show a remarkable effect on both AgpL1 and AgpS1 expressions. These results indicate that there are two distinct regulatory mechanisms, namely, sugar metabolism-dependent and -independent, for the regulation of AGPase gene expression. Interestingly, the ADP treatment, a hexokinase inhibitors, cancelled the sugar response of AgpL1, indicating that hexokinase-mediated sugar signaling should be involved in the sugar response of AgpL1. These results suggest that sugar-dependent (AgpL1) and sugar-independent (AgpS1) pathways coordinatively regulate starch biosynthesis in immature tomato fruit.

Promoter analysis of the sweet potato ADP-glucose pyrophosphorylase gene IbAGP1 in Nicotiana tabacum

KEY MESSAGE

The IbAGP1 gene of sweet potato ( Ipomoea batatas ) encodes the sucrose-inducible small subunit of ADP-glucose pyrophosphorylase. Through expression analysis of 5'-truncations and synthetic forms of the IbAGP1 promoter in transgenic tobacco, we show that SURE-Like elements and W-box elements of the promoter contribute to the sucrose inducibility of this gene. Sweet potato (Ipomoea batatas) contains two genes (IbAGP1 and IbAGP2) encoding the catalytically active small subunits of ADP-glucose pyrophosphorylase, an enzyme with an important role in regulating starch synthesis in higher plants. Previous studies have shown that IbAGP1 is expressed in the storage roots, leaves, and stem tissues of sweet potato, and its transcript is strongly induced by applying sucrose exogenously to detached leaves. To investigate the tissue-specific expression of the IbAGP1 promoter, a series of 5'-truncated promoters extending from bases -1913, -1598, -1298, -1053, -716, and -286 to base +75 were used to drive the expression of the β-glucuronidase reporter gene (GUS) in tobacco plants (Nicotiana tabacum). Histochemical and fluorometric GUS assays showed that (1) GUS expression driven by the longest fragment (1989 bp) of the IbAGP1 promoter was detected in vegetative tissues (roots, stems, leaves), (2) fragments extending to -1053 or beyond retained strong GUS expression in roots, stems, and leaves, whereas further 5'-deletions resulted in considerable reduction in GUS activity, and (3) the series of 5'-truncated promoters responded differently to exogenously applied sucrose. The 1989-bp IbAGP1 promoter contains five sequences (two AATAAAA, one AATAAAAAA, and two AATAAATAAA) that are similar to sucrose-responsive elements (SURE). These SURE-Like sequences are found at nucleotide positions -1273, -1239, -681, -610, and -189. Moreover, putative W-box elements are found at positions -1985, -1434, -750, and -578. Synthetic promoters containing tandem repeats of the 4X SURE-Like or 4X W-box upstream from a minimal CaMV35S promoter-GUS fusion showed significant expression in transgenic tobacco in response to exogenous sucrose. These results show that SURE-Like elements and W-box elements of the IbAGP1 promoter contribute to the sucrose inducibility of this gene.

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.

Salinity induces carbohydrate accumulation and sugar-regulated starch biosynthetic genes in tomato (Solanum lycopersicum L. cv. 'Micro-Tom') fruits in an ABA- and osmotic stress-independent manner

Salinity stress enhances sugar accumulation in tomato (Solanum lycopersicum) fruits. To elucidate the mechanisms underlying this phenomenon, the transport of carbohydrates into tomato fruits and the regulation of starch synthesis during fruit development in tomato plants cv. 'Micro-Tom' exposed to high levels of salinity stress were examined. Growth with 160 mM NaCl doubled starch accumulation in tomato fruits compared to control plants during the early stages of development, and soluble sugars increased as the fruit matured. Tracer analysis with (13)C confirmed that elevated carbohydrate accumulation in fruits exposed to salinity stress was confined to the early development stages and did not occur after ripening. Salinity stress also up-regulated sucrose transporter expression in source leaves and increased activity of ADP-glucose pyrophosphorylase (AGPase) in fruits during the early development stages. The results indicate that salinity stress enhanced carbohydrate accumulation as starch during the early development stages and it is responsible for the increase in soluble sugars in ripe fruit. Quantitative RT-PCR analyses of salinity-stressed plants showed that the AGPase-encoding genes, AgpL1 and AgpS1 were up-regulated in developing fruits, and AgpL1 was obviously up-regulated by sugar at the transcriptional level but not by abscisic acid and osmotic stress. These results indicate AgpL1 and AgpS1 are involved in the promotion of starch biosynthesis under the salinity stress in ABA- and osmotic stress-independent manners. These two genes are differentially regulated at the transcriptional level, and AgpL1 is suggested to play a regulatory role in this event.

A sepal-expressed ADP-glucose pyrophosphorylase gene (NtAGP) is required for petal expansion growth in 'Xanthi' tobacco

In this study, a tobacco (Nicotiana tabacum 'Xanthi') ADP-glucose pyrophosphorylase cDNA (NtAGP) was isolated from a flower bud cDNA library and the role of NtAGP in the growth of the floral organ was characterized. The expression of NtAGP was high in the sepal, moderate in the carpel and stamen, and low in the petal tissues. NtAGP-antisense plants produced flowers with abnormal petal limbs due to the early termination of the expansion growth of the petal limbs between the corolla lobes. Microscopic observation of the limb region revealed that cell expansion was limited in NtAGP-antisense plants but that cell numbers remained unchanged. mRNA levels of NtAGP, ADP-glucose pyrophosphorylase activity, and starch content in the sepal tissues of NtAGP-antisense plants were reduced, resulting in significantly lower levels of sugars (sucrose, glucose, and fructose) in the petal limbs. The feeding of these sugars to flower buds of the NtAGP-antisense plants restored the expansion growth in the limb area between the corolla lobes. Expansion growth of the petal limb between the corolla lobes was severely arrested in 'Xanthi' flowers from which sepals were removed, indicating that sepal carbohydrates are essential for petal limb expansion growth. These results demonstrate that NtAGP plays a crucial role in the morphogenesis of petal limbs in 'Xanthi' through the synthesis of starch, which is the main carbohydrate source for expansion growth of petal limbs, in sepal tissues.

A strong constitutive gene expression system derived from ibAGP1 promoter and its transit peptide

To develop a strong constitutive gene expression system, the activities of ibAGP1 promoter and its transit peptide were investigated using transgenic Arabidopsis and a GUS reporter gene. The ibAGP1 promoter directed GUS expression in almost entire tissues including rosette leaf, inflorescence stem, inflorescence, cauline leaf and root, suggesting that the ibAGP1 promoter is a constitutive promoter. GUS expression mediated by ibAGP1 promoter was weaker than that by CaMV35S promoter in all tissue types, but when GUS protein was targeted to plastids with the aid of the ibAGP1 transit peptide, GUS levels increased to higher levels in lamina, petiole and cauline leaf compared to those produced by CaMV35S promoter. The enhancing effect of ibAGP1 transit peptide on the accumulation of foreign protein was tissue-specific; accumulation was high in lamina and inflorescence, but low in root and primary inflorescence stem. The transit peptide effect in the leaves was maintained highly regardless of developmental stages of plants. The ibAGP1 promoter and its transit peptide also directed strong GUS gene expression in transiently expressed tobacco leaves. These results suggest that the ibAGP1 promoter and its transit peptide are a strong constitutive foreign gene expression system for transgenesis of dicot plants.

Temporally extended gene expression of the ADP-Glc pyrophosphorylase large subunit (AgpL1) leads to increased enzyme activity in developing tomato fruit

Tomato plants (Solanum lycopersicum) harboring the allele for the AGPase large subunit (AgpL1) derived from the wild species Solanum habrochaites (AgpL1 ( H )) are characterized by higher AGPase activity and increased starch content in the immature fruit, as well as higher soluble solids in the mature fruit following the breakdown of the transient starch, as compared to fruits from plants harboring the cultivated tomato allele (AgpL1 ( E )). Comparisons of AGPase subunit gene expression and protein levels during fruit development indicate that the increase in AGPase activity correlates with a prolonged expression of the AgpL1 gene in the AgpL1 ( H ) high starch line, leading to an extended presence of the L1 protein. The S1 (small subunit) protein also remained for an extended period of fruit development in the AgpL1 ( H ) fruit, linked to the presence of the L1 protein. There were no discernible differences between the kinetic characteristics of the partially purified AGPase-L1(E) and AGPase-L1(H) enzymes. The results indicate that the increased activity of AGPase in the AgpL1 ( H ) tomatoes is due to the extended expression of the regulatory L1 and to the subsequent stability of the heterotetramer in the presence of the L1 protein, implying a role for the large subunit not only in the allosteric control of AGPase activity but also in the stability of the AGPase L1-S1 heterotetramer. The introgression line of S. lycopersicum containing the wild species AgpL1 ( H ) allele is a novel example of transgressive heterosis in which the hybrid multimeric enzyme shows higher activity due to a modulated temporal expression of one of the subunits.

Two sweetpotato ADP-glucose pyrophosphorylase isoforms are regulated antagonistically in response to sucrose content in storage roots

The transcriptional regulation of ADP-glucose pyrophosphorylase (AGPase) genes in detached leaves in response to exogenous sucrose has been investigated earlier; however the effects of endogenous sucrose on AGPase gene transcription in leaves or starch-accumulating tissues have not yet been determined. We therefore have investigated the relationship between endogenous sucrose content in the storage tissues of sweetpotato (Ipomoea batatas cv. Yulmi) and the rate of transcription of the two sweetpotato AGPase isoforms, ibAGP1 and ibAGP2, by means of transient expression analysis of their promoters. Sequence analysis of the two promoters identified putative sucrose-responsive elements on the ibAGP1 promoter and, conversely, putative sucrose-starvation elements on the ibAGP2 promoter. Transient expression analyses on transverse storage root sections revealed that the ibAGP1 and ibAGP2 promoters directed strong expression in the sweetpotato storage roots (diameter: 1.5 cm). Sucrose contents of the sweetpotato storage roots were positively correlated with growth of the storage root. In the storage roots, ibAGP1 promoter activity became stronger with increasing endogenous sucrose levels, while ibAGP2 promoter activity became markedly weaker. Consequently, ibAGP2 was expressed primarily during the early stages of storage root development, whereas ibAGP1 was abundantly expressed in the later stages, during which a profound level of starch accumulation occurs. The antagonistic regulation of the two promoters in response to endogenous sucrose levels was also confirmed in carrot (Daucus carota L. cv. Hapa-ochon) taproots.

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.

Genomic organizations of two small subunit ADP-glucose pyrophosphorylase genes from sweetpotato

The genomic features of the small subunit ADP-glucose pyrophosphorylase (AGPase) isoforms are different in barley and maize. The two isoforms found in barley originated from one single gene through alternative splicing, while two independent genes encode the two isoforms in maize. To ascertain the genomic organizations of two small subunit AGPase isoforms in sweetpotato (ibAGP1 and ibAGP2), we isolated genomic DNAs containing the entire coding regions of two genes. Complete genomic structures of ibAGP1 and ibAGP2 were ascertained by the sequencing of these genomic regions. The transcribed regions of ibAGP1 and ibAGP2, comprising nine exons and eight introns, were distributed over 3.9 and 4.0 kb, respectively. The eight introns differed in length, from 76 to 946 bp in ibAGP1, and from 76 to 811 bp in ibAGP2, while the locations of introns in ibAGP1 and ibAGP2 were identical. There was 46-58% sequence identity between the intron sequences of the two genes. Intron sequence analyses suggested that either duplication in each intron, or gene conversion between introns of two isoforms, might cause major intron size differences between the two genes. Altogether, these results indicate that two small subunit AGPase isoforms in sweetpotato are encoded by two independent genes, in a fashion similar to that of maize small subunit AGPase genes.

The different large subunit isoforms of Arabidopsis thaliana ADP-glucose pyrophosphorylase confer distinct kinetic and regulatory properties to the heterotetrameric enzyme

ADP-glucose pyrophosphorylase catalyzes the first and limiting step in starch biosynthesis and is allosterically regulated by the levels of 3-phosphoglycerate and phosphate in plants. ADP-glucose pyrophosphorylases from plants are heterotetramers composed of two types of subunits (small and large). In this study, the six Arabidopsis thaliana genes coding for ADP-glucose pyrophosphorylase isoforms (two small and four large subunits) have been cloned and expressed in an Escherichia coli mutant deficient in ADP-glucose pyrophosphorylase activity. The co-expression of the small subunit APS1 with the different Arabidopsis large subunits (APL1, APL2, APL3, and APL4) resulted in heterotetramers with different regulatory and kinetic properties. Heterotetramers composed of APS1 and APL1 showed the highest sensitivity to the allosteric effectors as well as the highest apparent affinity for the substrates (glucose-1-phosphate and ATP), whereas heterotetramers formed by APS1 and APL2 showed the lower response to allosteric effectors and the lower affinity for the substrates. No activity was detected for the second gene coding for a small subunit isoform (APS2) annotated in the Arabidopsis genome. This lack of activity is possibly due to the absence of essential amino acids involved in catalysis and/or in the binding of glucose-1-phosphate and 3-phosphoglycerate. Kinetic and regulatory properties of the different heterotetramers, together with sequence analysis has allowed us to make a distinction between sink and source enzymes, because the combination of different large subunits would provide a high plasticity to ADP-glucose pyrophosphorylase activity and regulation. This is the first experimental data concerning the role that all the ADP-glucose pyrophosphorylase isoforms play in a single plant species. This phenomenon could have an important role in vivo, because different large subunits would confer distinct regulatory properties to ADP-glucose pyrophosphorylase according to the necessities for starch synthesis in a given tissue.

Characterization of a gene from chromosome 1B encoding the large subunit of ADPglucose pyrophosphorylase from wheat: evolutionary divergence and differential expression of Agp2 genes between leaves and developing endosperm

A full-length genomic clone containing the gene encoding the large subunit of the ADPglucose pyrophosphorylase (Agp2), was isolated from a genomic library prepared from etiolated shoots of hexaploid wheat (Triticum aestivum L., cv, Chinese Spring). The coding region of this gene is identical to one of the cDNA clones previously isolated from a developing wheat grain cDNA library and is therefore an actively transcribed gene. The sequence represented by the cDNA spans 4.8 kb of the genomic clone and contains 15 introns. 2852 bp of DNA flanking the transcription start site of the gene was cloned upstream of the GUS (beta-glucuronidase) reporter gene. This Agp2::GUS construct and promoter deletions were used to study the pattern of reporter gene expression in both transgenic tobacco and wheat plants. Histochemical analysis of GUS expression in transgenic tobacco demonstrated that the reporter gene was expressed in guard cells of leaves and throughout the seed. In transgenic wheat, reporter gene expression was confined to the endosperm and aleurone with no expression in leaves. The cloned Agp2 gene was located to chromosome 1B by gene-specific PCR with nullisomic-tetrasomic lines. Northern analysis demonstrated that the Agp2 genes are differentially expressed in leaves and developing endosperm; while all three classes of Agp2 genes are transcribed in developing wheat grain endosperm, only one is transcribed in leaves. The differences between the Agp2 genes are discussed in relation to the evolution of hexaploid wheat.

Sucrose regulation of ADP-glucose pyrophosphorylase subunit genes transcript levels in leaves and fruits

ADP-glucose pyrophosphorylase (AGPase, EC2.7.7.27) is a key regulatory enzyme in starch biosynthesis. The enzyme is a heterotetramer with two S and two B subunits. In tomato, there are three multiple forms of the S subunit gene. Agp S1, S2 and B are highly expressed in fruit from 10 to 25 days after anthesis. Agp S3 is only weakly expressed in fruit. Sucrose significantly elevates expression of Agp S1, S2 and B in both leaves and fruits. Agp S1 exhibits the highest degree of regulation by sucrose. In fact, sucrose may be required for Agp S1 expression. For excised leaves incubated in water, no transcripts for Agp S1 could be detected in the absence of sucrose, whereas it took up to 16 h in water before transcripts were no longer detectable for Agp S2 and B. Neither Agp S3 nor the tubulin gene is affected by sucrose, demonstrating that this response is specifically regulated by a carbohydrate metabolic signal, and is not due to a general increase in metabolism caused by sucrose treatment. Truncated versions of the promoter for Agp S1 indicate that a specific region 1.3-3.0 kb upstream from the transcription site is responsible for sucrose sensitivity. This region of the S1 promoter contains several cis-acting elements present in the promoters of other genes that are also regulated by sucrose.