A mutant of rice lacking the leaf large subunit of ADP-glucose pyrophosphorylase has drastically reduced leaf starch content but grows normally

Plant Productivity and Leaf Starch During Grain Fill Is Linked to QTL Containing Flowering Locus T1 (FT1) in Wheat (Triticum aestivum L.)

Shifts in the environment due to climate change necessitate breeding efforts aimed at adapting wheat to longer, warmer growing seasons. In this study, 21 modern wheat (Triticum aestivum L.) cultivars and 29 landraces were screened for flag leaf starch levels, with the goal of identifying a genetic marker for targeted breeding. The landrace PI 61693 was identified as having exceptionally high flag leaf starch values. Yield trials were carried out in a Berkut × PI 61693 recombinant inbred line (RIL) population and a negative correlation was observed between leaf starch, flowering time, and yield. Genetic mapping identified a Quantitative Trait Loci (QTL) explaining 22-34% variation for leaf starch, flowering time, biomass, and seed yield. The starch synthase TraesCS7D02G117800 (wSsI-1) is located in this region, which possibly accounts for leaf starch variation in this population; also within this QTL is TraesCS7D02G111600 (FT-D1). Sequencing of FT-D1 identified a single base pair deletion in the 3rd exon of the Berkut allele. This indel has recently been shown to significantly impact flowering time and productivity, and likely led to significant variation in flowering date and yield in this population. Here, we illustrate how allelic selection of FT-D1 within breeding programs may aid in adapting wheat to changing environments.

Simultaneous knockout of cytosolic and plastidial disproportionating enzymes disrupts grain setting and filling in rice

Rice (Oryza sativa) plants contain plastidial and cytosolic disproportionating enzymes (DPE1 and DPE2). Our previous studies showed that DPE2 acts on maltose, the major product of starch degradation in pollens, releasing one glucose to fuel pollen tube growth and fertilization, whereas DPE1 participates in endosperm starch synthesis by transferring maltooligosyl groups from amylose to amylopectin, and removing excess short maltooligosaccharides. However, little is known about their integrated function. Here, we report that the coordinated actions of DPE1 and DPE2 contribute to grain setting and filling in rice. The dpe1dpe2 mutants could not be isolated from the progeny of heterozygous parental plants but were obtained via anther culture. Unlike that reported in Arabidopsis (Arabidopsis thaliana) and potato (Solanum tuberosum), the dpe1dpe2 rice plants grew normally but only yielded a small number of empty, unfilled seeds. In the dpe1dpe2 seeds, nutrient accumulation was substantially reduced, and dorsal vascular bundles were also severely malnourished. Zymogram analyses showed that changes in the activities of the major starch-synthesizing enzymes matched well with various endosperm phenotypes of mutant seeds. Mechanistically, DPE1 deficiency allowed normal starch mobilization in leaves and pollens but affected starch synthesis in endosperm, while DPE2 deficiency blocked starch degradation, resulting in substantially decreased levels of the sugars available for pollen tube growth and grain filling. Overall, our results demonstrate the great potential of DPE1-DPE2 as an important regulatory module to realize higher crop yields and present a promising target for regulating nutrient accumulation in cereal crop endosperm.

Genome-Wide Identification and Analysis of SUS and AGPase Family Members in Sweet Potato: Response to Excessive Nitrogen Stress during Storage Root Formation

(1) The development of sweet potato storage roots is impacted by nitrogen (N) levels, with excessive nitrogen often impeding development. Starch synthesis enzymes such as sucrose synthase (SUS) and ADP-glucose pyrophosphorylase (AGPase) are pivotal in this context. Although the effects of excessive nitrogen on the formation of sweet potato storage roots are well documented, the specific responses of IbSUSs and IbAGPases have not been extensively reported on. (2) Pot experiments were conducted using the sweet potato cultivar "Pushu 32" at moderate (MN, 120 kg N ha-1) and excessive nitrogen levels (EN, 240 kg N ha-1). (3) Nine IbSUS and nine IbAGPase genes were categorized into three and two distinct subgroups based on phylogenetic analysis. Excessive nitrogen significantly (p < 0.05) suppressed the expression of IbAGPL1, IbAGPL2, IbAGPL4, IbAGPL5, IbAGPL6, IbAGPS1, and IbAGPS2 in fibrous roots and IbSUS2, IbSUS6, IbSUS7, IbSUS8, IbSUS9, IbAGPL2, and IbAGPL4 in storage roots, and then significantly (p < 0.05) decreased the SUS and AGPase activities and starch content of fibrous root and storage root, ultimately reducing the storage root formation of sweet potato. Excessive nitrogen extremely significantly (p < 0.01) enhanced the expression of IbAGPL3, which was strongly negatively correlated with the number and weight of storage roots per plant. (4) IbAGPL3 may be a key gene in the response to excessive nitrogen stress and modifying starch synthesis in sweet potato.

BPB1 regulates rice (Oryza sative L.) panicle length and panicle branch development by promoting lignin and inhibiting cellulose accumulation

Panicle structure is one of the most important agronomic traits directly related to rice yield. This study identified a rice mutant basal primary branch 1 (bpb1), which exhibited a phenotype of reduced panicle length and arrested basal primary branch development. In addition, lignin content was found to be increased while cellulose content was decreased in bpb1 young panicles. Map-based cloning methods characterized the gene BPB1, which encodes a peptide transporter (PTR) family transporter. Phylogenetic tree analysis showed that the BPB1 family is highly conserved in plants, especially the PTR2 domain. It is worth noting that BPB1 is divided into two categories based on monocotyledonous and dicotyledonous plants. Transcriptome analysis showed that BPB1 mutation can promote lignin synthesis and inhibit cellulose synthesis, starch and sucrose metabolism, cell cycle, expression of various plant hormones, and some star genes, thereby inhibiting rice panicle length, resulting in basal primary branch development stagnant phenotypes. In this study, BPB1 provides new insights into the molecular mechanism of rice panicle structure regulation by BPB1 by regulating lignin and cellulose content and several transcriptional metabolic pathways.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-023-01389-x.

Dynamic Change in Starch Biosynthetic Enzymes Complexes during Grain-Filling Stages in BEIIb Active and Deficient Rice

Starch is the predominant reserve in rice (Oryza sativa L.) endosperm, which is synthesized by the coordinated efforts of a series of starch biosynthetic-related enzymes in the form of a multiple enzyme complex. Whether the enzyme complex changes during seed development is not fully understood. Here, we investigated the dynamic change in multi-protein complexes in an indica rice variety IR36 (wild type, WT) and its BEIIb-deficient mutant (be2b) at different developmental stages. Gel permeation chromatography (GPC) and Western blotting analysis of soluble protein fractions revealed most of the enzymes except for SSIVb were eluted in smaller molecular weight fractions at the early developing stage and were transferred to higher molecular weight fractions at the later stage in both WT and be2b. Accordingly, protein interactions were enhanced during seed development as demonstrated by co-immunoprecipitation analysis, suggesting that the enzymes were recruited to form larger protein complexes during starch biosynthesis. The converse elution pattern from GPC of SSIVb may be attributed to its vital role in the initiation step of starch synthesis. The number of protein complexes was markedly decreased in be2b at all development stages. Although SSIVb could partially compensate for the role of BEIIb in protein complex formation, it was hard to form a larger protein complex containing over five proteins in be2b. In addition, other proteins such as PPDKA and PPDKB were possibly present in the multi-enzyme complexes by proteomic analyses of high molecular weight fractions separated from GPC. Two putative protein kinases were found to be potentially associated with starch biosynthetic enzymes. Collectively, our findings unraveled a dynamic change in the protein complex during seed development, and potential roles of BEIIb in starch biosynthesis via various protein complex formations, which enables a deeper understanding of the complex mechanism of starch biosynthesis in rice.

VaAPL1 Promotes Starch Synthesis to Constantly Contribute to Soluble Sugar Accumulation, Improving Low Temperature Tolerance in Arabidopsis and Tomato

ADP-glucose pyrophosphorylase (AGPase) is a key rate-limiting enzyme involved in starch synthesis. APL1, an AGPase large subunit, plays an important role in the growth and development of grapes; however, its function in withstanding low temperature (LT) remains elusive. Hence, VaAPL1 was cloned from Vitis amurensis (Zuoshan I), and its function was characterized. The gene was highly expressed in the phloem of V. amurensis during winter dormancy (0, -5, and - 10°C). Phylogenetic relationships demonstrated that VaAPL1 was closely genetic related to SlAPL1 (from Solanum lycopersicum), and clustered into I group. Further, VaAPL1 was ectopically expressed in Arabidopsis thaliana (ecotype Columbia, Col) and tomato ("Micro-Tom" tomato) to characterize its function under LT. Compared with Col, the average survival rate of VaAPL1-overexpressing A. thaliana exceeded 75.47% after freezing treatment. Moreover, reactive oxygen species (ROS) content decreased in VaAPL1-overexpressing A. thaliana and tomato plants under LT stress. The activities of AGPase, and starch contents in VaAPL1-overexpressing A. thaliana were higher than in Col after LT stress. The contents of sucrose and glucose were accumulated in overexpressing plants compared with wild-type at 0 h and 24 h after LT stress. Transcriptome sequencing of overexpressing tomato plants revealed involvement in sugar metabolism and the hormone signal pathway, and Ca²⁺ signaling pathway-related genes were up-regulated. Hence, these results suggest that overexpression of VaAPL1 not only ensured sufficient starch converting into soluble sugars to maintain cell osmotic potential and provided energy, but also indirectly activated signal pathways involved in LT to enhance plant tolerance.

Proteomic analysis reveals a role of ADP-glucose pyrophosphorylase in the asynchronous filling of rice superior and inferior spikelets

The poor grain filling of inferior spikelets (IS) situated on the lower secondary rachis branch leads to a remarkable decrease in rice yield and quality. The AGPase small subunit 2 (AGPS2) encodes a small subunit of adenosine diphosphate-glucose pyrophosphorylase (AGPase) enzyme, which plays an important role in sucrose-starch conversion and starch biosynthesis in the grain filling of rice. In the present study, qPCR analysis showed low expression abundance of AGPS2 in IS, compared to the superior spikelets (SS), which was consistent with the lower grain weight of IS. To evaluate the molecular mechanism of AGPS2, we first identified the AGPS2 interaction network through Co-immunoprecipitation (Co-IP). In total, 29 proteins of AGPS2 interaction network were characterized by LC-MS/MS. Bioinformatics analysis revealed that, the characterized proteins in the interaction network are likely to be related to starch synthesis, sugar conversion, energy pathway, and folding/modification, and most of them were involved in the grain filling of rice. The sequent Co-IP analysis showed that AGPS2 can bind to starch branching enzyme (SBE), pullulanase (PUL) and starch debranching enzyme (DBE) and assemble into starch synthesizing protein complex (SSPC). In addition, the 14-3-3 protein GF14e was also found to interact with AGPS2. Further analysis by qPCR showed that the expression of GF14e was much higher on IS than on SS. The qPCR results also showed that the expression of GF14e was relatively stable in SS, but changed significantly in IS under alternate wetting and moderate soil drying (WMD), which is consistent with the AGPS2 expression pattern. Our present work provides direct molecular evidence for the different expression patterns of AGPS2 in SS and IS, which could be greatly helpful for the molecular amelioration of the poor grain filling of IS in rice.

Two ADP-glucose pyrophosphorylase subunits, OsAGPL1 and OsAGPS1, modulate phosphorus homeostasis in rice

Plant acclimatory responses to phosphate (Pi) starvation stress include the accumulation of carbohydrates, namely sugar and starch. However, whether altered endogenous carbohydrate profile could in turn affect plant Pi starvation responses remains widely unexplored. Here, two genes encoding the large and small subunits of an ADP-glucose pyrophosphorylase (AGP) in rice (Oryza sativa), AGP Large Subunit 1 (AGPL1) and AGP Small Subunit 1 (AGPS1), were functionally characterized with regard to maintenance of phosphorus (P) homeostasis and regulation of Pi starvation signaling. AGPL1 and AGPS1 were both positively responsive to nitrogen (N) or Pi deprivation, and expressed in almost all the tissues except in the meristem and mature zones of root. AGPL1 and AGPS1 physically interacted in chloroplast, and catalyzed the rate-limiting step of starch biosynthesis. Low-N- (LN) and low-Pi (LP)-triggered starch accumulation in leaves was impaired in agpl1, agps1 and apgl1 agps1 mutants compared with the wild-type plants. By contrast, mutation of AGPL1 and/or AGPS1 led to an increase in the content of the major sugar, sucrose, in leaf sheath and root under control and LN conditions. Moreover, the Pi accumulation was enhanced in the mutants under control and LN conditions, but not LP conditions. Notably, the LN-induced suppression of Pi accumulation was compromised attributed to the mutation of AGPL1 and/or AGPS1. Furthermore, the increased Pi accumulation was accompanied by the specific suppression of OsSPX2 and activation of several Pi transporter genes. These results indicate that a balanced level of carbohydrates is vital for maintaining plant P homeostasis.

Genetic engineering of transitory starch accumulation by knockdown of OsSEX4 in rice plants for enhanced bioethanol production

Rice straw, a common agricultural waste, is used as a potential feedstock for bioethanol production. Currently, bioethanol is made mostly from the microbial fermentation of starch-containing raw materials. Therefore, genetically engineered starch-excess rice straw through interference of starch degradation as a potential strategy to enhance bioethanol production was evaluated in this study. Arabidopsis Starch Excess 4 (SEX4) encodes a chloroplast-localized glucan phosphatase and plays a role in transitory starch degradation. Despite the identification of a SEX4 homolog in rice, OsSEX4, its biological function remains uncertain. Ectopic expression of OsSEX4 complementary DNA complemented the leaf starch-excess phenotype of the Arabidopsis sex4-4 mutant. OsSEX4-knockdown transgenic rice plants were generated using the RNA interference approach. Starch accumulation was higher in OsSEX4-knockdown suspension-cultured cells, leaves, and rice straw compared with the wild type, suggesting that OsSEX4 plays an important role in degradation of transitory starch. The OsSEX4-knockdown rice plants showed normal plant growth and no yield penalty. Starch-excess OsSEX4-knockdown rice straw used as feedstock for fermentation resulted in improved bioethanol yield, with a 50% increase in ethanol production in a vertical mass-flow type bioreactor, compared with that of the wild-type straw.

Cassava AGPase genes and their encoded proteins are different from those of other plants

Cassava AGPase and AGPase genes have some unique characteristics. ADP-glucose pyrophosphorylase (AGPase) is a rate-limiting enzyme for starch synthesis. In this study, cassava AGPase genes (MeAGP) were analyzed based on six cultivars and one wild species. A total of seven MeAGPs was identified, including four encoding AGPase large subunits (MeAGPLs 1, 2, 3 and 4) and three encoding AGPase small subunits (MeAGPSs 1, 2 and 3). The copy number of MeAGPs varied in cassava germplasm materials. There were 14 introns for MeAGPLs 1, 2 and 3, 13 introns for MeAGPL4, and 8 introns for other three MeAGPSs. Multiple conservative amino acid sequence motifs were found in the MeAGPs. There were differences in amino acids at binding sites of substrates and regulators among different MeAGP subunits and between MeAGPs and a potato AGPase small subunit (1YP2:B). MeAGPs were all located in chloroplasts. MeAGP expression was not only associated with gene copy number and types/combinations, regions and levels of the DNA methylation but was also affected by environmental factors with the involvement of various transcription factors in multiple regulation networks and in various cis-elements in the gene promoter regions. The MeAGP activity also changed with environmental conditions and had potential differences among the subunits. Taken together, MeAGPs differ in number from those of Arabidopsis, potato, maize, banana, sweet potato, and tomato.

CRISPR/Cas9 mutations in the rice Waxy/GBSSI gene induce allele-specific and zygosity-dependent feedback effects on endosperm starch biosynthesis

Induced mutations in the waxy locus in rice endosperm did not abolish GBSS activity completely. Compensatory mechanisms in endosperm and leaves caused a major reprogramming of the starch biosynthetic machinery. The mutation of genes in the starch biosynthesis pathway has a profound effect on starch quality and quantity and is an important target for plant breeders. Mutations in endosperm starch biosynthetic genes may impact starch metabolism in vegetative tissues such as leaves in unexpected ways due to the complex feedback mechanisms regulating the pathway. Surprisingly this aspect of global starch metabolism has received little attention. We used CRISPR/Cas9 to introduce mutations affecting the Waxy (Wx) locus encoding granule-bound starch synthase I (GBSSI) in rice endosperm. Our specific objective was to develop a mechanistic understanding of how the endogenous starch biosynthetic machinery might be affected at the transcriptional level following the targeted knock out of GBSSI in the endosperm. We found that the mutations reduced but did not abolish GBSS activity in seeds due to partial compensation caused by the upregulation of GBSSII. The GBSS activity in the mutants was 61-71% of wild-type levels, similarly to two irradiation mutants, but the amylose content declined to 8-12% in heterozygous seeds and to as low as 5% in homozygous seeds, accompanied by abnormal cellular organization in the aleurone layer and amorphous starch grain structures. Expression of many other starch biosynthetic genes was modulated in seeds and leaves. This modulation of gene expression resulted in changes in AGPase and sucrose synthase activity that explained the corresponding levels of starch and soluble sugars.

Dynamic changes in the starch-sugar interconversion within plant source and sink tissues promote a better abiotic stress response

Starch is a significant store of sugars, and the starch-sugar interconversion in source and sink tissues plays a profound physiological role in all plants. In this review, we discuss how changes in starch metabolism can facilitate adaptive changes in source-sink carbon allocation for protection against environmental stresses. The stress-related roles of starch are described, and published mechanisms by which starch metabolism responds to short- or long-term water deficit, salinity, or extreme temperatures are discussed. Numerous examples of starch metabolism as a stress response are also provided, focusing on studies where carbohydrates and cognate enzymes were assayed in source, sink, or both. We develop a model that integrates these findings with the theoretical and known roles of sugars and starch in various species, tissues, and developmental stages. In this model, localized starch degradation into sugars is vital to the plant cold stress response, with the sugars produced providing osmoprotection. In contrast, high starch accumulation is prominent under salinity stress, and is associated with higher assimilate allocation from source to sink. Our model explains how starch-sugar interconversion can be a convergent point for regulating carbon use in stress tolerance at the whole-plant level.

Starch formation inside plastids of higher plants

Starch is a water-insoluble polyglucan synthesized inside the plastid stroma within plant cells, serving a crucial role in the carbon budget of the whole plant by acting as a short-term and long-term store of energy. The highly complex, hierarchical structure of the starch granule arises from the actions of a large suite of enzyme activities, in addition to physicochemical self-assembly mechanisms. This review outlines current knowledge of the starch biosynthetic pathway operating in plant cells in relation to the micro- and macro-structures of the starch granule. We highlight the gaps in our knowledge, in particular, the relationship between enzyme function and operation at the molecular level and the formation of the final, macroscopic architecture of the granule.

OsPKpα1 encodes a plastidic pyruvate kinase that affects starch biosynthesis in the rice endosperm

Pyruvate kinase (PK) is a key enzyme in glycolysis and carbon metabolism. Here, we isolated a rice (Oryza sativa) mutant, w59, with a white-core floury endosperm. Map-based cloning of w59 identified a mutation in OsPKpα1, which encodes a plastidic isoform of PK (PKp). OsPKpα1 localizes to the amyloplast stroma in the developing endosperm, and the mutation of OsPKpα1 in w59 decreases the plastidic PK activity, resulting in dramatic changes to the lipid biosynthesis in seeds. The w59 grains were also characterized by a marked decrease in starch content. Consistent with a decrease in number and size of the w59 amyloplasts, large empty spaces were observed in the central region of the w59 endosperm, at the early grain-filling stage. Moreover, a phylogenetic analysis revealed four potential rice isoforms of OsPKp. We validated the in vitro PK activity of these OsPKps through reconstituting active PKp complexes derived from inactive individual OsPKps, revealing the heteromeric structure of rice PKps, which was further confirmed using a protein-protein interaction analysis. These findings suggest a functional connection between lipid and starch synthesis in rice endosperm amyloplasts.

Barley lys3 mutants are unique amongst shrunken-endosperm mutants in having abnormally large embryos

Many shrunken endosperm mutants of barley (Hordeum vulgare L.) have been described and several of these are known to have lesions in starch biosynthesis genes. Here we confirm that one type of shrunken endosperm mutant, lys3 (so called because it was first identified as a high-lysine mutant) has an additional phenotype: as well as shrunken endosperm it also has enlarged embryos. The lys3 embryos have a dry weight that is 50-150% larger than normal. Observations of developing lys3 embryos suggest that they undergo a form of premature germination and the mature lys3 grains show reduced dormancy. In many respects, the phenotype of barley lys3 is similar to that of rice GIANT EMBRYO mutants (affected in the OsGE gene). However, the barley orthologue of OsGE is located on a different chromosome from Lys3. Together these results suggest that the gene underlying Lys3 is unlikely to encode a starch biosynthesis protein but rather a protein influencing grain development.

OsHAK1 controls the vegetative growth and panicle fertility of rice by its effect on potassium-mediated sugar metabolism

Plant growth and reproduction are both energy-requiring processes; the necessary energy is supplied by the products of photosynthesis. Both the vegetative growth and reproductive success of rice are compromised by the absence of a functional copy of the gene OsHAK1. Here, a comparison between wild type rice and OsHAK1 knockout mutants not only confirmed the known detrimental effect of the absence of OsHAK1 on root growth, pollen viability and fertility, but also showed that sucrose phosphate synthase activity was lowered, and the sucrose content of the leaves was markedly increased, due to a partial block on the up-loading of sucrose into the phloem. The impaired allocation of sugar to the roots and spikelets caused by the knocking out of OsHAK1 was accompanied by a down-regulation in the leaf sheaths and panicle axes of genes encoding sucrose transporters (SUT genes), which are active in the phloem, as well as in the roots and spikelets of those encoding monosaccharide transporters (MST genes), which transport hexose sugars across the plant plasma membrane. The activity of sucrose synthase, acid invertase and neutral invertase in the roots of mutant plants assayed at the tillering stage, and in their spikelets, assayed during grain-filling, was significantly lower than in the equivalent organs of wild type plants. As a result, the supply of total soluble sugar, glucose and fructose to sink organs was reduced, consistent with the effect of the mutation on root growth and panicle fertility. Compared to wild type plants, the mutants accumulated less potassium (K) throughout the plant. The conclusion was that the failure to fully supply the demand of the mutant's sink organs for assimilate was responsible for its compromised phenotype, and that the deficiency in K uptake induced by the loss of OsHAK1 functionality was responsible for the disruption of sugar metabolism.

CRISPR/Cas9-induced monoallelic mutations in the cytosolic AGPase large subunit gene APL2 induce the ectopic expression of APL2 and the corresponding small subunit gene APS2b in rice leaves

The first committed step in the endosperm starch biosynthetic pathway is catalyzed by the cytosolic glucose-1-phosphate adenylyl transferase (AGPase) comprising large and small subunits encoded by the OsAPL2 and OsAPS2b genes, respectively. OsAPL2 is expressed solely in the endosperm so we hypothesized that mutating this gene would block starch biosynthesis in the endosperm without affecting the leaves. We used CRISPR/Cas9 to create two heterozygous mutants, one with a severely truncated and nonfunctional AGPase and the other with a C-terminal structural modification causing a partial loss of activity. Unexpectedly, we observed starch depletion in the leaves of both mutants and a corresponding increase in the level of soluble sugars. This reflected the unanticipated expression of both OsAPL2 and OsAPS2b in the leaves, generating a complete ectopic AGPase in the leaf cytosol, and a corresponding decrease in the expression of the plastidial small subunit OsAPS2a that was only partially complemented by an increase in the expression of OsAPS1. The new cytosolic AGPase was not sufficient to compensate for the loss of plastidial AGPase, most likely because there is no wider starch biosynthesis pathway in the leaf cytosol and because pathway intermediates are not shuttled between the two compartments.

Mutation of the OsSAC1 Gene, which Encodes an Endoplasmic Reticulum Protein with an Unknown Function, Causes Sugar Accumulation in Rice Leaves

Sugars are the most abundant organic compounds produced by plants, and can be used to build carbon skeletons and generate energy. The sugar accumulation 1 (OsSAC1) gene encodes a protein with an unknown function that exhibits four N-terminal transmembrane regions and two conserved domains of unknown function, DUF4220 and DUF594. OsSAC1 was found to be poorly and specifically expressed at the bottoms of young leaves and in the developing leaf sheaths. Subcellular location results showed that OsSAC1 was co-localized with ER:mCherry and targeted the endoplasmic reticulum (ER). OsSAC1 has been found to affect sugar partitioning in rice (Oryza sativa). I2/KI starch staining, ultrastructure observations and starch content measurements indicated that more and larger starch granules accumulated in ossac1 source leaves than in wild-type (WT) source leaves. Additionally, higher sucrose and glucose concentrations accumulated in the ossac1 source leaves than in WT source leaves, whereas lower sucrose and glucose concentrations were observed in the ossac1 young leaves and developing leaf sheaths than in those of the WT. Much greater expression of OsAGPL1 and OsAGPS1 (responsible for starch synthesis) and significantly less expression of OscFBP1, OscFBP2, OsSPS1 and OsSPS11 (responsible for sucrose synthesis) and OsSWEET11, OsSWEET14 and OsSUT1 (responsible for sucrose loading) occurred in ossac1 source leaves than in WT source leaves. A greater amount of the rice plasmodesmatal negative regulator OsGSD1 was detected in ossac1 young leaves and developing leaf sheaths than in those of the WT. These results suggest that ER-targeted OsSAC1 may indirectly regulate sugar partitioning in carbon-demanding young leaves and developing leaf sheaths.

Starch as a source, starch as a sink: the bifunctional role of starch in carbon allocation

Starch commands a central role in the carbon budget of the majority of plants on earth, and its biological role changes during development and in response to the environment. Throughout the life of a plant, starch plays a dual role in carbon allocation, acting as both a source, releasing carbon reserves in leaves for growth and development, and as a sink, either as a dedicated starch store in its own right (in seeds and tubers), or as a temporary reserve of carbon contributing to sink strength, in organs such as flowers, fruits, and developing non-starchy seeds. The presence of starch in tissues and organs thus has a profound impact on the physiology of the growing plant as its synthesis and degradation governs the availability of free sugars, which in turn control various growth and developmental processes. This review attempts to summarize the large body of information currently available on starch metabolism and its relationship to wider aspects of carbon metabolism and plant nutrition. It highlights gaps in our knowledge and points to research areas that show promise for bioengineering and manipulation of starch metabolism in order to achieve more desirable phenotypes such as increased yield or plant biomass.

Overexpression of ADP-glucose pyrophosphorylase in both leaf and seed tissue synergistically increase biomass and seed number in rice (Oryza sativa ssp. japonica)

Increased expression of leaf or seed ADPglucose pyrophosphorylase activity (AGPase) has been shown to increase plant growth. However, no study has directly compared AGPase overexpression in leaves and/or seeds. In the present study, transgenic rice overexpressing AGPase in leaves or in seeds were crossed, resulting in four F2:3 homozygous genotypes with AGPase overexpression in leaves, seeds, both leaves and seeds, or neither tissue. The impact of AGPase overexpression in these genotypic groups was examined at the metabolic, transcriptomic, and plant growth levels. Leaf-specific AGPase overexpression increased flag leaf starch up to five times that of the wild type (WT) whereas overexpression of AGPase in both leaves and seeds conferred the greatest productivity advantages. Relative to the WT, AGPase overexpression in both leaves and seeds increased plant biomass and panicle number by 61% and 51%, respectively while leaf-specific AGPase overexpression alone only increased plant biomass and panicle number by 24 and 32% respectively. Extraction and analysis of RNA and leaf-specific metabolites demonstrated that carbon metabolism was broadly increased by AGPase overexpression in seeds and leaves. These findings indicate that stimulation of whole-plant growth and productivity can be best achieved by upregulation of starch biosynthesis in both leaves and seeds.

A reduction of sucrose phosphate synthase (SPS) activity affects sucrose/starch ratio in leaves but does not inhibit normal plant growth in rice

Sucrose phosphate synthase (SPS) has been shown to mediate sucrose/starch ratio in plant leaves through studies of 'starch leaf' species that mainly accumulate starch in their leaves. However, the contribution of SPS to sucrose/starch ratio in rice leaves, which mainly accumulate sucrose (i.e., 'sugar leaf'), has not been confirmed due to inconsistencies in the results of previous studies. In this study, we analyzed mutant lines with reduced SPS activity, which were generated using Tos17 insertion, RNAi, and the CRISPR/Cas9 system. The knockdown and knockout mutants of OsSPS1 showed a 29-46% reduction in SPS activity in the leaves, but the carbohydrate content in the leaves and plant growth were not significantly different from those of wild-type plants. In a double knockout mutant of OsSPS1 and OsSPS11 (sps1/sps11), an 84% reduction in leaf SPS activity resulted in higher starch accumulation in the leaves than in the wild-type leaves. However, the sps1/sps11 plants grew normally, which is in contrast to the inhibited growth of SPS mutants of Arabidopsis thaliana, a typical starch leaf plant. These results suggest that SPS has a smaller effect on the sucrose/starch ratio in leaves and growth of rice than on starch leaf species.

Plastidic phosphoglucomutase and ADP-glucose pyrophosphorylase mutants impair starch synthesis in rice pollen grains and cause male sterility

To elucidate the starch synthesis pathway and the role of this reserve in rice pollen, we characterized mutations in the plastidic phosphoglucomutase, OspPGM, and the plastidic large subunit of ADP-glucose (ADP-Glc) pyrophosphorylase, OsAGPL4 Both genes were up-regulated in maturing pollen, a stage when starch begins to accumulate. Progeny analysis of self-pollinated heterozygous lines carrying the OspPGM mutant alleles, osppgm-1 and osppgm-2, or the OsAGPL4 mutant allele, osagpl4-1, as well as reciprocal crosses between the wild type (WT) and heterozygotes revealed that loss of OspPGM or OsAGPL4 caused male sterility, with the former condition rescued by the introduction of the WT OspPGM gene. While iodine staining and transmission electron microscopy analyses of pollen grains from homozygous osppgm-1 lines produced by anther culture confirmed the starch null phenotype, pollen from homozygous osagpl4 mutant lines, osagpl4-2 and osagpl4-3, generated by the CRISPR/Cas system, accumulated small amounts of starch which were sufficient to produce viable seed. Such osagpl4 mutant pollen, however, was unable to compete against WT pollen successfully, validating the important role of this reserve in fertilization. Our results demonstrate that starch is mainly polymerized from ADP-Glc synthesized from plastidic hexose phosphates in rice pollen and that starch is an essential requirement for successful fertilization in rice.

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.

AGPase: its role in crop productivity with emphasis on heat tolerance in cereals

AGPase, a key enzyme of starch biosynthetic pathway, has a significant role in crop productivity. Thermotolerant variants of AGPase in cereals may be used for developing cultivars, which may enhance productivity under heat stress. Improvement of crop productivity has always been the major goal of plant breeders to meet the global demand for food. However, crop productivity itself is influenced in a large measure by a number of abiotic stresses including heat, which causes major losses in crop productivity. In cereals, crop productivity in terms of grain yield mainly depends upon the seed starch content so that starch biosynthesis and the enzymes involved in this process have been a major area of investigation for plant physiologists and plant breeders alike. Considerable work has been done on AGPase and its role in crop productivity, particularly under heat stress, because this enzyme is one of the major enzymes, which catalyses the rate-limiting first committed key enzymatic step of starch biosynthesis. Keeping the above in view, this review focuses on the basic features of AGPase including its structure, regulatory mechanisms involving allosteric regulators, its sub-cellular localization and its genetics. Major emphasis, however, has been laid on the genetics of AGPases and its manipulation for developing high yielding cultivars that will have comparable productivity under heat stress. Some important thermotolerant variants of AGPase, which mainly involve specific amino acid substitutions, have been highlighted, and the prospects of using these thermotolerant variants of AGPase in developing cultivars for heat prone areas have been discussed. The review also includes a brief account on transgenics for AGPase, which have been developed for basic studies and crop improvement.

Phenotypic and genetic dissection of component traits for early vigour in rice using plant growth modelling, sugar content analyses and association mapping

Early vigour of rice, defined as seedling capacity to accumulate shoot dry weight (SDW) rapidly, is a complex trait. It depends on a genotype propensity to assimilate, store, and/or use non-structural carbohydrates (NSC) for producing large and/or numerous leaves, involving physiological trade-offs in the expression of component traits and, possibly, physiological and genetic linkages. This study explores a plant-model-assisted phenotyping approach to dissect the genetic architecture of rice early vigour, applying the Genome Wide Association Study (GWAS) to morphological and NSC measurements, as well as fitted parameters for the functional-structural plant model, Ecomeristem. Leaf size, number, SDW, and source-leaf NSC concentration were measured on a panel of 123 japonica accessions. The data were used to estimate Ecomeristem genotypic parameters driving organ appearance rate, size, and carbon dynamics. GWAS was performed based on 12 221 single-nucleotide polymorphisms (SNP). Twenty-three associations were detected at P <1×10(-4) and 64 at P <5×10(-4). Associations for NSC and model parameters revealed new regions related to early vigour that had greater significance than morphological traits, providing additional information on the genetic control of early vigour. Plant model parameters were used to characterize physiological and genetic trade-offs among component traits. Twelve associations were related to loci for cloned genes, with nine related to organogenesis, plant height, cell size or cell number. The potential use of these associations as markers for breeding is discussed.

Suppression of starch synthesis in rice stems splays tiller angle due to gravitropic insensitivity but does not affect yield

In rice (Oryza sativa L.), tiller angle - defined as the angle between the main culm and its side tillers - is one of the important factors involved in light use efficiency. To clarify the relationship between tiller angle, gravitropism and stem-starch accumulation, we investigated the shoot gravitropic response of a low stem-starch rice mutant which lacks a large subunit of ADP-glucose pyrophosphorylase (AGP), called OsAGPL1 and exhibits relatively spread tiller angle. The insensitive gravitropic response exhibited by the mutant led us to the conclusion that insensitivity of gravitropism caused by stem-starch reduction splayed the tiller angle. Furthermore, since another AGP gene called OsAGPL3 was expressed at considerable levels in graviresponding sites, we generated a double mutant lacking both OsAGPL1 and OsAGPL3. The double mutant exhibited still lower stem-starch content, less sensitive gravitropic response and greater tiller angle spread than the single mutants. This indicated that the expansion of the tiller angle caused by the reduction in starch level was intense according to the extent of the reduction. We found there were no significant differences between the double mutant and wild-type plants in terms of dry matter production. These results provided new insight into the importance of stem-starch accumulation and ideal plant architecture.

Grain development in Brachypodium and other grasses: possible interactions between cell expansion, starch deposition, and cell-wall synthesis

To explain the low levels of starch, high levels of (1,3;1,4)-β-glucan, and thick cell walls in grains of Brachypodium distachyon L. relative to those in other Pooideae, aspects of grain development were compared between B. distachyon and barley (Hordeum vulgare L.). Cell proliferation, cell expansion, and endoreduplication were reduced in B. distachyon relative to barley and, consistent with these changes, transcriptional downregulation of the cell-cycle genes CDKB1 and cyclin A3 was observed. Similarly, reduced transcription of starch synthase I and starch-branching enzyme I was observed as well as reduced activity of starch synthase and ADP-glucose pyrophosphorylase, which are consistent with the lowered starch content in B. distachyon grains. No change was detected in transcription of the major gene involved in (1,3;1,4)-β-glucan synthesis, cellulose synthase-like F6. These results suggest that, while low starch content results from a reduced capacity for starch synthesis, the unusually thick cell walls in B. distachyon endosperm probably result from continuing (1,3;1,4)-β-glucan deposition in endosperm cells that fail to expand. This raises the possibility that endosperm expansion is linked to starch deposition.

Starch reduction in rice stems due to a lack of OsAGPL1 or OsAPL3 decreases grain yield under low irradiance during ripening and modifies plant architecture

Starch accumulated in rice (Oryza sativa L.) stems before heading as nonstructural carbohydrates (NSCs) is reported to be important for improving and stabilising grain yield. To evaluate the importance of stem starch, we investigated a retrotransposon (Tos17) insertion rice mutant lacking a gene encoding a large subunit of ADP-glucose pyrophosphorylase (AGP) called OsAGPL1 or OsAPL3. The AGP activity and starch contents of the mutant were drastically reduced in the stem (i.e. leaf sheath and culm) but not in the leaf blade or endosperm. This starch reduction in the leaf sheaths of the mutant was complemented by the introduction of wild-type OsAGPL1. These results strongly suggest that OsAGPL1 plays a principal role in stem starch accumulation. Field experimentations spanning 2 years revealed that the mutant plants were shorter than the wild-type plants. Moreover, the tiller number and angle were larger in the mutant plants than the wild-type plants, but the dry weight at heading stage was not different. The grain yield was slightly lower in control plots without shading treatment. However, this difference increased substantially with shading. Therefore, stem starch is indispensable for normal ripening under low irradiance conditions and probably contributes to the maintenance of appropriate plant architecture.

Improving starch yield in cereals by over-expression of ADPglucose pyrophosphorylase: expectations and unanticipated outcomes

Significant improvements in crop productivity are required to meet the nutritional requirements of a growing world population. This challenge is magnified by an increased demand for bioenergy as a means to mitigate carbon inputs into the environment. Starch is a major component of the harvestable organs of many crop plants, and various endeavors have been taken to improve the yields of starchy organs through the manipulation of starch synthesis. Substantial efforts have centered on the starch regulatory enzyme ADPglucose pyrophosphorylase (AGPase) due to its pivotal role in starch biosynthesis. These efforts include over-expression of this enzyme in cereal plants such as maize, rice and wheat as well as potato and cassava, as they supply the bulk of the staple food worldwide. In this perspective, we describe efforts to increase starch yields in cereal grains by first providing an introduction about the importance of source-sink relationship and the motives behind the efforts to alter starch biosynthesis and turnover in leaves. We then discuss the catalytic and regulatory properties of AGPase and the molecular approaches used to enhance starch synthesis by manipulation of this process during grain filling using seed-specific promoters. Several studies have demonstrated increases in starch content per seed using endosperm-specific promoters, but other studies have demonstrated an increase in seed number with only marginal impact on seed weight. Potential mechanisms that may be responsible for this paradoxical increase in seed number will also be discussed. Finally, we describe current efforts and future prospects to improve starch yield in cereals. These efforts include further enhancement of starch yield in rice by augmenting the process of ADPglucose transport into amyloplast as well as other enzymes involved in photoassimilate partitioning in seeds.

A rice mutant lacking a large subunit of ADP-glucose pyrophosphorylase has drastically reduced starch content in the culm but normal plant morphology and yield

A mutant of rice (Oryza sativa L.) was identified with a Tos17 insertion in Os05g50380, a gene encoding a plastidial large subunit (LSU) of ADP-glucose pyrophosphorylase (AGPase) that was previously called OsAPL3 or OsAGPL1. The insertion prevents the production of a normal transcript. Characterisation of the mutant showed that this LSU is required for 97% of the starch synthesised in the flowering stem (culm), approximately half of the AGPase activity in developing embryos and that it contributes to AGPase activity in the endosperm. Despite the near absence of starch in the culms and reduced starch content in the embryos, the mutant rice plants grow and develop normally, and show no reduction in productivity. The starch content of leaves is increased in the mutant, revealing plasticity in the distribution of photosynthates among different temporary carbohydrate storage pools within the plant.

Phenomics of rice early vigour and drought response: Are sugar related and morphogenetic traits relevant?

BACKGROUND

Early vigour (biomass accumulation) is a useful but complex trait in rainfed rice (Oryza sativa L). Little is known on trade-offs with drought tolerance. This study explored the relevance of (sugar) metabolic and morphogenetic traits to describe the genetic diversity of rice early vigour and its phenotypic plasticity under drought conditions. A greenhouse experiment was conducted to characterize on a panel of 43 rice genotypes plant morphogenesis and sugar concentration in expanded (source) and expanding (sink) leaves.

RESULTS

Across genotypes in control treatment, leaf starch concentration was negatively correlated with organogenetic development rate (DR, defined as leaf appearance rate on main stem). Genotypes with small leaves had high DR and tiller number but low leaf starch concentration. Under drought, vigorous genotypes showed stronger growth reduction. Starch concentration decreased in source leaves, by contrast with soluble sugars and with that observed in sink leaves. Accordingly, genotypes were grouped in three clusters differing in constitutive vigour, starch storage and growth maintenance under drought showing a trade off between constitutive vigour and drought tolerance.

CONCLUSIONS

It was therefore suggested that non structural carbohydrates, particularly starch, were relevant markers of early vigour. Their relevance as markers of growth maintenance under drought needs to be further explored. Results are discussed regarding novel process based traits to be introduced in the GRiSP (Global Rice Science Partnership) phenotyping network.

Exploiting leaf starch synthesis as a transient sink to elevate photosynthesis, plant productivity and yields

Improvements in plant productivity (biomass) and yield have centered on increasing the efficiency of leaf CO(2) fixation and utilization of products by non-photosynthetic sink organs. We had previously demonstrated a correlation between photosynthetic capacity, plant growth, and the extent of leaf starch synthesis utilizing starch-deficient mutants. This finding suggested that leaf starch is used as a transient photosynthetic sink to recycle inorganic phosphate and, in turn, maximize photosynthesis. To test this hypothesis, Arabidopsis thaliana and rice (Oryza sativa L.) lines were generated with enhanced capacity to make leaf starch with minimal impact on carbon partitioning to sucrose. The Arabidopsis engineered plants exhibited enhanced photosynthetic capacity; this translated into increased growth and biomass. These enhanced phenotypes were displayed by similarly engineered rice lines. Manipulation of leaf starch is a viable alternative strategy to increase photosynthesis and, in turn, the growth and yields of crop and bioenergy plants.

The evolution of the starch biosynthetic pathway in cereals and other grasses

In most species, the precursor for starch synthesis, ADPglucose, is made exclusively in the plastids by the enzyme ADPglucose pyrophosphorylase (AGPase). However, in the endosperm of grasses, including the economically important cereals, ADPglucose is also made in the cytosol via a cytosolic form of AGPase. Cytosolic ADPglucose is imported into plastids for starch synthesis via an ADPglucose/ADP antiporter (ADPglucose transporter) in the plastid envelope. The genes encoding the two subunits of cytosolic AGPase and the ADPglucose transporter are unique to grasses. In this review, the evolutionary origins of this unique endosperm pathway of ADPglucose synthesis and its functional significance are discussed. It is proposed that the genes encoding the pathway originated from a whole-genome-duplication event in an early ancestor of the grasses.

Loss of cytosolic fructose-1,6-bisphosphatase limits photosynthetic sucrose synthesis and causes severe growth retardations in rice (Oryza sativa)

During photosynthesis, triose-phosphates (trioseP) exported from the chloroplast to the cytosol are converted to sucrose via cytosolic fructose-1,6-bisphosphatase (cFBPase). Expression analysis in rice suggests that OscFBP1 plays a major role in the cytosolic conversion of trioseP to sucrose in leaves during the day. The isolated OscFBP1 mutants exhibited markedly decreased photosynthetic rates and severe growth retardation with reduced chlorophyll content, which results in plant death. Analysis of primary carbon metabolites revealed both significantly reduced levels of sucrose, glucose, fructose and starch in leaves of these mutants, and a high accumulation of sucrose to starch in leaves of rice plants. In the oscfbp1 mutants, products of glycolysis and the TCA cycle were significantly increased. A partitioning experiment of (14)C-labelled photoassimilates revealed altered carbon distributions including a slight increase in the insoluble fraction representing transitory starch, a significant decrease in the neutral fraction corresponding to soluble sugars and a high accumulation of phosphorylated intermediates and carboxylic acid fractions in the oscfbp1 mutants. These results indicate that the impaired synthesis of sucrose in rice cannot be sufficiently compensated for by the transitory starch-mediated pathways that have been found to facilitate plant growth in the equivalent Arabidopsis mutants.

Determining the role of Tie-dyed1 in starch metabolism: epistasis analysis with a maize ADP-glucose pyrophosphorylase mutant lacking leaf starch

In regions of their leaves, tdy1-R mutants hyperaccumulate starch. We propose 2 alternative hypotheses to account for the data, that Tdy1 functions in starch catabolism or that Tdy1 promotes sucrose export from leaves. To determine whether Tdy1 might function in starch breakdown, we exposed plants to extended darkness. We found that the tdy1-R mutant leaves retain large amounts of starch on prolonged dark treatment, consistent with a defect in starch catabolism. To further test this hypothesis, we identified a mutant allele of the leaf expressed small subunit of ADP-glucose pyrophosphorylase (agps-m1), an enzyme required for starch synthesis. We determined that the agps-m1 mutant allele is a molecular null and that plants homozygous for the mutation lack transitory leaf starch. Epistasis analysis of tdy1-R; agps-m1 double mutants demonstrates that Tdy1 function is independent of starch metabolism. These data suggest that Tdy1 may function in sucrose export from leaves.