Protein and gene expression characteristics of a rice phosphoenolpyruvate carboxylase Osppc3; its unique role for seed cell maturation

Over-expression of a plant-type phosphoenolpyruvate carboxylase derails Arabidopsis stamen formation

We examined whether plant-type phosphoenolpyruvate carboxylase (PEPC) is involved in flower organ formation or not by over-expression in Arabidopsis. A wheat PEPC isogene Tappc3A, belonging to the ppc3 group, was targeted due to its preferential expression pattern in pistils and stamens. Transgenic Arabidopsis over-expressing Tappc3A exhibited irregular stamen formation, i.e., a lesser number of stamens per flower and shorter filaments in T2 and T3 generations. Irregular stamens were frequently observed in homozygous T4 lines, but no morphological change was observed in other floral organs. High-degree gene co-expression of Tappc3 isogenes with wheat SEEDSTICKs but not with other homeotic transcription factor genes for flower formation implicates that Tappc3 is under control by the class D genes of the ABCDE model to flower development. In addition, the conservation of CArG box sequences on the Tappc3 promoters supported the developmentally programmed gene expression of ppc3 in wheat flowering organs. Thus, this study provides the first experimental evidence for the critical regulation of plant-type PEPC for flower formation.

The Original Form of C4-Photosynthetic Phosphoenolpyruvate Carboxylase Is Retained in Pooids but Lost in Rice

Poaceae is the most prominent monocot family that contains the primary cereal crops wheat, rice, and maize. These cereal species exhibit physiological diversity, such as different photosynthetic systems and environmental stress tolerance. Phosphoenolpyruvate carboxylase (PEPC) in Poaceae is encoded by a small multigene family and plays a central role in C4-photosynthesis and dicarboxylic acid metabolism. Here, to better understand the molecular basis of the cereal species diversity, we analyzed the PEPC gene family in wheat together with other grass species. We could designate seven plant-type and one bacterial-type grass PEPC groups, ppc1a, ppc1b, ppc2a, ppc2b, ppc3, ppc4, ppcC4, and ppc-b, respectively, among which ppc1b is an uncharacterized type of PEPC. Evolutionary inference revealed that these PEPCs were derived from five types of ancient PEPCs (ppc1, ppc2, ppc3, ppc4, and ppc-b) in three chromosomal blocks of the ancestral Poaceae genome. C4-photosynthetic PEPC (ppcC4 ) had evolved from ppc1b, which seemed to be arisen by a chromosomal duplication event. We observed that ppc1b was lost in many Oryza species but preserved in Pooideae after natural selection. In silico analysis of cereal RNA-Seq data highlighted the preferential expression of ppc1b in upper ground organs, selective up-regulation of ppc1b under osmotic stress conditions, and nitrogen response of ppc1b. Characterization of wheat ppc1b showed high levels of gene expression in young leaves, transcriptional responses under nitrogen and abiotic stress, and the presence of a Dof1 binding site, similar to ppcC4 in maize. Our results indicate the evolving status of Poaceae PEPCs and suggest the functional association of ppc1-derivatives with adaptation to environmental changes.

Prokaryotic Expression of Phosphoenolpyruvate Carboxylase Fragments from Peanut and Analysis of Osmotic Stress Tolerance of Recombinant Strains

Phosphoenolpyruvate carboxylase (PEPC) is a ubiquitous cytosolic enzyme that catalyzes the irreversible β-carboxylation of phosphoenolpyruvate (PEP) in presence of HCO₃⁻ to produce oxaloacetate (OAA) during carbon fixation and photosynthesis. It is well accepted that PEPC genes are expressed in plants upon stress. PEPC also supports the biosynthesis of biocompatible osmolytes in many plant species under osmotic stress. There are five isoforms of PEPC found in peanut (Arachis hypogaea L.), namely, AhPEPC1, AhPEPC2, AhPEPC3, AhPEPC4, and AhPEPC5. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed that the gene expression patterns of these AhPEPC genes were different in mature seeds, stems, roots, flowers, and leaves. The expression of all the plant type PEPC (PTPCs) (AhPEPC1, AhPEPC2, AhPEPC3, and AhPEPC4) was relatively high in roots, while the bacterial type PEPC (BTPC) (AhPEPC5) showed a remarkable expression level in flowers. Principal component analysis (PCA) result showed that AhPEPC3 and AhPEPC4 are correlated with each other, indicating comparatively associations with roots, and AhPEPC5 have a very close relationship with flowers. In order to investigate the function of these AhPEPCs, the fragments of these five AhPEPC cDNA were cloned and expressed in Escherichia coli (E. coli). The recombinant proteins contained a conserved domain with a histidine site, which is important for enzyme catalysis. Results showed that protein fragments of AhPEPC1, AhPEPC2, and AhPEPC5 had remarkable expression levels in E. coli. These three recombinant strains were more sensitive at pH 9.0, and recombinant strains carrying AhPEPC2 and AhPEPC5 fragments exhibited more growth than the control strain with the presence of PEG6000. Our findings showed that the expression of the AhPEPC fragments may enhance the resistance of transformed E. coli to osmotic stress.

Rapidly evolving phosphoenolpyruvate carboxylase Gmppc1 and Gmppc7 are highly expressed in the external seed coat of immature soybean seeds

Phosphoenolpyruvate carboxylase (PEPC) is a carbon fixation enzyme which probably plays crucial roles in seed development. A greater number of PEPC isoforms are encoded in the soybean genome, while most of the PEPC isoforms are functionally unknown. In this study, we investigated on soybean PEPC expressed in the external layer of seed coat (ELSC) during seed formation. PEPC activity in ELSC ranged from 0.24 to 1.0 U/g F.W., which could be comparable to those in whole seeds at U per dry matter. Public RNA-Seq data in separated soybean seed tissues revealed that six plant-type PEPC isogenes were substantially expressed in ELSC, and Gmppc1 and Gmppc7 were highly expressed in hourglass cells of ELSC. Gene Ontology enrichment of co-expressed genes with Gmppc1 and Gmppc7 implicated a role of these isogenes in assisting energy production and cellulose biosynthesis. Comparison of PEPC sequences from 16 leguminous species hypothesized adaptive evolution of the Gmppc1 and Gmppc7 lineage after divergence from the other plant-type PEPC lineages. Molecular diversification of these plant-type PEPC was possibly accomplished by adaptation to the functions of the soybean seed tissues. This study indicates that energy demand in immature seeds may be a driving force for the molecular evolution of PEPC.

The plant-type phosphoenolpyruvate carboxylase Gmppc2 is developmentally induced in immature soy seeds at the late maturation stage: a potential protein biomarker for seed chemical composition

Phosphoenolpyruvate carboxylase (PEPC) is a carbon-fixing enzyme with critical roles in seed development. Previously we observed a positive correlation between PEPC activity and protein content in mature seeds among soybean cultivars and varietal differences of PEPC activity in immature seeds, which is concordant with seed protein accumulation. Here, we report a PEPC isoform (Gmppc2) which is preferentially expressed in immature soybean seeds at the late maturation stage. Gmppc2 was co-expressed with enzyme genes involved in starch degradation: α-amylase, hexokinase, and α-glucan phosphorylase. Gmppc2 was developmentally induced in the external seed coats, internal seed coats, hypocotyls, and cotyledons at the late maturation stage. The expression of Gmppc2 protein was negatively regulated by the application of a nitrogen fertilizer, which suppressed nodule formation. These results imply that Gmppc2 is involved in the metabolism of nitrogen originated from nodules into seeds, and Gmppc2 might be applicable as a biomarker of seed protein content.Abbreviations: PEP: phosphoenolpyruvate; PEPC: phosphoenolpyruvate carboxylase; RNA-Seq: RNA sequencing; PCA: principal component analysis; SE: standard error.

Pattern analysis suggests that phosphoenolpyruvate carboxylase in maturing soybean seeds promotes the accumulation of protein

The protein and oil contents in soybean seeds are major factors in seed quality. Seed proteins and oils are synthesized from sucrose and nitrogenous compounds transported into maturing seeds. In this study, we compared changes in the activity of phosphoenolpyruvate carboxylase (PEPC) and the accumulation profiles of protein and oil in maturing seeds of two soybean cultivars, which exhibit different protein and oil contents in seeds, to determine the interrelationships of them. A principal component analysis indicated a concordance of seed PEPC activity with the protein content, but did not with the oil content. PEPC activity per seed was highest in the late maturation stage, when the physiological status of the vegetative organs drastically changed. The high-protein cultivar had higher PEPC activity compared to the low-protein cultivar. These results highlight the biological role of PEPC in the synthesis of protein, therefore it was implied that PEPC could be a biomarker in soybean breeding.Abbreviations: ANOVA: analysis of variance; DS: developmental stage; DW: dry weight; FW: fresh weight; NIR: near infrared; PEP(C): phosphoenolpyruvate (carboxylase); PC(A): principal component (analysis); S.E.: standard error; WC: water content.