Molecular Functions and Pathways of Plastidial Starch Phosphorylase (PHO1) in Starch Metabolism: Current and Future Perspectives

Genome-wide identification of starch phosphorylase gene family in Rosa chinensis and expression in response to abiotic stress

Chinese rose (Rosa chinensis) is an important ornamental plant, with economic, cultural, and symbolic significance. During the application of outdoor greening, adverse environments such as high temperature and drought are often encountered, which affect its application scope and ornamental quality. The starch phosphorylase (Pho) gene family participate in the synthesis and decomposition of starch, not only related to plant energy metabolism, but also plays an important role in plant stress resistance. The role of Pho in combating salinity and high temperature stress in R. chinensis remains unknown. In this work, 4 Phos from R. chinensis were detected with Pfam number of Pho (PF00343.23) and predicted by homolog-based prediction (HBP). The Phos are characterized by sequence lengths of 821 to 997 bp, and the proteins are predicted to subcellularly located in the plastid and cytoplasm. The regulatory regions of the Phos contain abundant stress and phytohormone-responsive cis-acting elements. Based on transcriptome analysis, the Phos were found to respond to abiotic stress factors such as drought, salinity, high temperature, and plant phytohormone of jasmonic acid and salicylic acid. The response of Phos to abiotic stress factors such as salinity and high temperature was confirmed by qRT-PCR analysis. To evaluate the genetic characteristics of Phos, a total of 69 Phos from 17 species were analyzed and then classified into 3 groups in phylogenetic tree. The collinearity analysis of Phos in R. chinensis and other species was conducted for the first time. This work provides a view of evolution for the Pho gene family and indicates that Phos play an important role in abiotic stress response of R. chinensis.

Serine 31 Phosphorylation-Driven Regulation of AGPase Activity: Potential Implications for Enhanced Starch Yields in Crops

ADP-Glc pyrophosphorylase (AGPase), which catalyzes the transformation of ATP and glucose-1-phosphate (Glc-1-P) into adenosine diphosphate glucose (ADP-Glc), acts as a rate-limiting enzyme in crop starch biosynthesis. Prior research has hinted at the regulation of AGPase by phosphorylation in maize. However, the identification and functional implications of these sites remain to be elucidated. In this study, we identified the phosphorylation site (serine at the 31st position of the linear amino acid sequence) of the AGPase large subunit (Sh2) using iTRAQTM. Subsequently, to ascertain the impact of Sh2 phosphorylation on AGPase, we carried out site-directed mutations creating Sh2-S31A (serine residue replaced with alanine) to mimic dephosphorylation and Sh2-S31D (serine residue replaced with aspartic acid) or Sh2-S31E (serine residue replaced with glutamic acid) to mimic phosphorylation. Preliminary investigations were performed to determine Sh2 subcellular localization, its interaction with Bt2, and the resultant AGPase enzymatic activity. Our findings indicate that phosphorylation exerts no impact on the stability or localization of Sh2. Furthermore, none of these mutations at the S31 site of Sh2 seem to affect its interaction with Bt2 (smaller subunit). Intriguingly, all S31 mutations in Sh2 appear to enhance AGPase activity when co-transfected with Bt2, with Sh2-S31E demonstrating a substantial five-fold increase in AGPase activity compared to Sh2. These novel insights lay a foundational groundwork for targeted improvements in AGPase activity, thus potentially accelerating the production of ADP-Glc (the primary substrate for starch synthesis), promising implications for improved starch biosynthesis, and holding the potential to significantly impact agricultural practices.

Site-Directed Mutations at Phosphorylation Sites in Zea mays PHO1 Reveal Modulation of Enzymatic Activity by Phosphorylation at S566 in the L80 Region

Starch phosphorylase (PHO) is a pivotal enzyme within the GT35-glycogen-phosphorylase (GT; glycosyltransferases) superfamily. Despite the ongoing debate surrounding the precise role of PHO1, evidence points to its substantial influence on starch biosynthesis, supported by its gene expression profile and subcellular localization. Key to PHO1 function is the enzymatic regulation via phosphorylation; a myriad of such modification sites has been unveiled in model crops. However, the functional implications of these sites remain to be elucidated. In this study, we utilized site-directed mutagenesis on the phosphorylation sites of Zea mays PHO1, replacing serine residues with alanine, glutamic acid, and aspartic acid, to discern the effects of phosphorylation. Our findings indicate that phosphorylation exerts no impact on the stability or localization of PHO1. Nonetheless, our enzymatic assays unveiled a crucial role for phosphorylation at the S566 residue within the L80 region of the PHO1 structure, suggesting a potential modulation or enhancement of PHO1 activity. These data advance our understanding of starch biosynthesis regulation and present potential targets for crop yield optimization.

Gradual Analytics of Starch-Interacting Proteins Revealed the Involvement of Starch-Phosphorylating Enzymes during Synthesis of Storage Starch in Potato (Solanum tuberosum L.) Tubers

The complete mechanism behind starch regulation has not been fully characterized. However, significant progress can be achieved through proteomic approaches. In this work, we aimed to characterize the starch-interacting proteins in potato (Solanum tuberosum L. cv. Desiree) tubers under variable circumstances. Starch-interacting proteins were extracted from developing tubers of wild type and transgenic lines containing antisense inhibition of glucan phosphorylases. Further, proteins were separated by SDS-PAGE and characterized through mass spectrometry. Additionally, starch-interacting proteins were analyzed in potato tubers stored at different temperatures. Most of the proteins strongly interacting with the potato starch granules corresponded to proteins involved in starch metabolism. GWD and PWD, two dikinases associated with starch degradation, were consistently found bound to the starch granules. This indicates that their activity is not only restricted to degradation but is also essential during storage starch synthesis. We confirmed the presence of protease inhibitors interacting with the potato starch surface as previously revealed by other authors. Starch interacting protein profiles of transgenic tubers appeared differently from wild type when tubers were stored under different temperatures, indicating a differential expression in response to changing environmental conditions.

Pho1a (plastid starch phosphorylase) is duplicated and essential for normal starch granule phenotype in tubers of Solanum tuberosum L

Reserve starch from seeds and tubers is a crucial plant product for human survival. Much research has been devoted to quantitative and qualitative aspects of starch synthesis and its relation to abiotic factors of importance in agriculture. Certain aspects of genetic factors and enzymes influencing carbon assimilation into starch granules remain elusive after many decades of research. Starch phosphorylase (Pho) can operate, depending on metabolic conditions, in a synthetic and degradative pathway. The plastidial form of the enzyme is one of the most highly expressed genes in potato tubers, and the encoded product is imported into starch-synthesizing amyloplasts. We identified that the genomic locus of a Pho1a-type starch phosphorylase is duplicated in potato. Our study further shows that the enzyme is of importance for a normal starch granule phenotype in tubers. Null mutants created by genome editing display rounded starch granules in an increased number that contained a reduced ratio of apparent amylose in the starch.

Transcriptomic and metabolomic profiling reveal the mechanism underlying the inhibition of wound healing by ascorbic acid in fresh-cut potato

Ascorbic acid (AsA) inhibits wound healing in fresh-cut potatoes (FCP); however, the comprehensive regulatory mechanisms of the chemical during wound healing remain unclear. Here, physiobiochemical, transcriptomic, and metabolomic analyses were performed. In total, 685 differentially expressed genes (DEGs) and 1921 differentially accumulated metabolites (DAMs) were identified between control and AsA-treated samples. The level of the majority of DEGs expression and DAMs abundance in AsA-treated samples were similar to data of newly cut samples. The collective data indicated that the AsA treatment inhibited wound healing in FCPs by regulating glutathione metabolism, enhancing starch metabolism, and inhibiting phenylalanine metabolism, sucrose degradation, and fatty acid synthesis. Major genes and metabolites affected by AsA treatment included StGST, StPAL, StPHO1 and StLOX5, and starch, sucrose, and linoleic acid. AsA treatment increased starch content and amylase and lipoxygenase activity and decreased free fatty acid level. Our research provides fundamental insights into wound healing mechanisms in FCP.

The Plastidial Glucan Phosphorylase Affects the Maltooligosaccharide Metabolism in Parenchyma Cells of Potato (Solanum tuberosum L.) Tuber Discs

Maltodextrin metabolism is thought to be involved in both starch initiation and degradation. In this study, potato tuber discs from transgenic lines containing antisense constructs against the plastidial and cytosolic isoforms of α-glucan phosphorylase and phosphoglucomutase were used to evaluate their influences on the conversion of externally supplied glucose-1-phosphate into soluble maltodextrins, as compared to wild-type potato tubers (Solanum tuberosum L. cv. Desiree). Relative maltodextrin amounts analyzed by capillary electrophoresis with laser-induced fluorescence revealed that tuber discs could immediately uptake glucose-1-phosphate and use it to produce maltooligosaccharides with a degree of polymerization of up to 30, as opposed to tubers repressing the plastidial glucan phosphorylase. The results presented here support previous indications that a specific transporter for glucose-1-phosphate may exist in both the plant cells and the plastidial membranes, thereby allowing a glucose-6-phosphate-independent transport. Furthermore, it confirms that the plastidial glucan phosphorylase is responsible for producing longer maltooligosaccharides in the plastids by catalyzing a glucosyl polymerization reaction when glucose-1-phosphate is available. All these findings contribute to a better understanding of the role of the plastidial phosphorylase as a key enzyme directly involved in the synthesis and degradation of glucans and their implication on starch metabolism.

Dependence of the content of starch and reducing sugars on the level of expression of the genes of β-amylases StBAM1 and StBAM9 and the amylase inhibitor StAI during long-term low-temperature storage of potato tubers

Solanum tuberosum L. is the most important non-grain starch crop with a potential yield of 38-48 t/ha and a starch content of 13.2-18.7 %. Potato tubers are stored at a low temperature (2-4 °C) in a state of physiological dormancy. A disadvantage of this type of storage is the degradation of starch and the accumulation of reducing sugars (cold-induced sweetening), including due to an increase in the activity of β-amylases that hydrolyze starch to maltose. In this study, a comparative analysis of the β-amylase (StBAM1, StBAM9) and amylase inhibitor (StAI ) gene expression, as well as starch and reducing sugar content in tubers during long-term low-temperature storage (September, February, April) was performed using potato cultivars Nadezhda, Barin, Krasavchik, Severnoe siyanie and Utro. The β-amylase genes, StBAM9 and one of the two StBAM1 homologs (with the highest degree of homology with AtBAM1), were selected based on phylogenetic analysis data. Evaluation of the expression of these genes and the amylase inhibitor gene showed a tendency to decrease in transcription for all analyzed cultivars. The starch content also significantly decreased during tuber storage. The amount of reducing sugars increased in the September-April period, while in February-April, their content did not change (Krasavchik), decreased (Barin, Severnoe siyanie) or continued to grow (Utro, Nadezhda). It can be assumed that the gene activity of StBAM1 and StBAM9 correlates with the amount of starch (positively) and monosaccharides (negatively). The level of StAI expression, in turn, may be directly dependent on the level of StBAM1 expression. At the same time, there is no relationship between the degree of cultivar predisposition to cold-induced sweetening and the expression profile of the StBAM1, StBAM9, and StAI genes.

Screening of Induced Mutants Led to the Identification of Starch Biosynthetic Genes Associated with Improved Resistant Starch in Wheat

Several health benefits are obtained from resistant starch, also known as healthy starch. Enhancing resistant starch with genetic modification has huge commercial importance. The variation of resistant starch content is narrow in wheat, in relation to which limited improvement has been attained. Hence, there is a need to produce a wheat population that has a wide range of variations in resistant starch content. In the present study, stable mutants were screened that showed significant variation in the resistant starch content. A megazyme kit was used for measuring the resistant starch content, digestible starch, and total starch. The analysis of variance showed a significant difference in the mutant population for resistant starch. Furthermore, four diverse mutant lines for resistant starch content were used to study the quantitative expression patterns of 21 starch metabolic pathway genes; and to evaluate the candidate genes for resistant starch biosynthesis. The expression pattern of 21 starch metabolic pathway genes in two diverse mutant lines showed a higher expression of key genes regulating resistant starch biosynthesis (GBSSI and their isoforms) in the high resistant starch mutant lines, in comparison to the parent variety (J411). The expression of SBEs genes was higher in the low resistant starch mutants. The other three candidate genes showed overexpression (BMY, Pho1, Pho2) and four had reduced (SSIII, SBEI, SBEIII, ISA3) expression in high resistant starch mutants. The overexpression of AMY and ISA1 in the high resistant starch mutant line JE0146 may be due to missense mutations in these genes. Similarly, there was a stop_gained mutation for PHO2; it also showed overexpression. In addition, the gene expression analysis of 21 starch metabolizing genes in four different mutants (low and high resistant starch mutants) shows that in addition to the important genes, several other genes (phosphorylase, isoamylases) may be involved and contribute to the biosynthesis of resistant starch. There is a need to do further study about these new genes, which are responsible for the fluctuation of resistant starch in the mutants.

LIKE EARLY STARVATION 1 alters the glucan structures at the starch granule surface and thereby influences the action of both starch-synthesizing and starch-degrading enzymes

For starch metabolism to take place correctly, various enzymes and proteins acting on the starch granule surface are crucial. Recently, two non-catalytic starch-binding proteins, pivotal for normal starch turnover in Arabidopsis leaves, namely, EARLY STARVATION 1 (ESV1) and its homolog LIKE EARLY STARVATION 1 (LESV), have been identified. Both share nearly 38% sequence homology. As ESV1 has been found to influence glucan phosphorylation via two starch-related dikinases, α-glucan, water dikinase (GWD) and phosphoglucan, water dikinase (PWD), through modulating the surface glucan structures of the starch granules and thus affecting starch degradation, we assess the impact of its homolog LESV on starch metabolism. Thus, the 65-kDa recombinant protein LESV and the 50-kDa ESV1 were analyzed regarding their influence on the action of GWD and PWD on the surface of the starch granules. We included starches from various sources and additionally assessed the effect of these non-enzymatic proteins on other starch-related enzymes, such as starch synthases (SSI and SSIII), starch phosphorylases (PHS1), isoamylase and β-amylase. The data obtained indicate that starch phosphorylation, hydrolyses and synthesis were affected by LESV and ESV1. Furthermore, incubation with LESV and ESV1 together exerted an additive effect on starch phosphorylation. In addition, a stable alteration of the glucan structures at the starch granule surface following treatment with LESV and ESV1 was observed. Here, we discuss all the observed changes that point to modifications in the glucan structures at the surface of the native starch granules and present a model to explain the existing processes.

Comparative Study of Starch Phosphorylase Genes and Encoded Proteins in Various Monocots and Dicots with Emphasis on Maize

Starch phosphorylase (PHO) is a multimeric enzyme with two distinct isoforms: plastidial starch phosphorylase (PHO1) and cytosolic starch phosphorylase (PHO2). PHO1 specifically resides in the plastid, while PHO2 is found in the cytosol. Both play a critical role in the synthesis and degradation of starch. This study aimed to report the detailed structure, function, and evolution of genes encoding PHO1 and PHO2 and their protein ligand-binding sites in eight monocots and four dicots. "True" orthologs of PHO1 and PHO2 of Oryza sativa were identified, and the structure of the enzyme at the protein level was studied. The genes controlling PHO2 were found to be more conserved than those controlling PHO1; the variations were mainly due to the variable sequence and length of introns. Cis-regulatory elements in the promoter region of both genes were identified, and the expression pattern was analyzed. The real-time quantitative polymerase chain reaction indicated that PHO2 was expressed in all tissues with a uniform pattern of transcripts, and the expression pattern of PHO1 indicates that it probably contributes to the starch biosynthesis during seed development in Zea mays. Under abscisic acid (ABA) treatment, PHO1 was found to be downregulated in Arabidopsis and Hordeum vulgare. However, we found that ABA could up-regulate the expression of both PHO1 and PHO2 within 12 h in Zea mays. In all monocots and dicots, the 3D structures were highly similar, and the ligand-binding sites were common yet fluctuating in the position of aa residues.