Gene duplications facilitate C4-CAM compatibility in common purslane

Common purslane (Portulaca oleracea) integrates both C4 and crassulacean acid metabolism (CAM) photosynthesis pathways and is a promising model plant to explore C4-CAM plasticity. Here, we report a high-quality chromosome-level genome of nicotinamide adenine dinucleotide (NAD)-malic enzyme (ME) subtype common purslane that provides evidence for 2 rounds of whole-genome duplication (WGD) with an ancient WGD (P-β) in the common ancestor to Portulacaceae and Cactaceae around 66.30 million years ago (Mya) and another (Po-α) specific to common purslane lineage around 7.74 Mya. A larger number of gene copies encoding key enzymes/transporters involved in C4 and CAM pathways were detected in common purslane than in related species. Phylogeny, conserved functional site, and collinearity analyses revealed that the Po-α WGD produced the phosphoenolpyruvate carboxylase-encoded gene copies used for photosynthesis in common purslane, while the P-β WGD event produced 2 ancestral genes of functionally differentiated (C4- and CAM-specific) beta carbonic anhydrases involved in the C4 + CAM pathways. Additionally, cis-element enrichment analysis in the promoters showed that CAM-specific genes have recruited both evening and midnight circadian elements as well as the Abscisic acid (ABA)-independent regulatory module mediated by ethylene-response factor cis-elements. Overall, this study provides insights into the origin and evolutionary process of C4 and CAM pathways in common purslane, as well as potential targets for engineering crops by integrating C4 or CAM metabolism.

Modulation of Immune-Related Gene Expressions in Zebrafish (Danio rerio) by Dietary Purslane (Portulaca oleracea) Extract

To promote fish's immunity against pathogens in the aquaculture industry, fish dietary fortification with additives or compounds has increasingly attracted attention. In the present study, zebrafish (Danio rerio) was used as an animal model to investigate the effects of purslane, Portulaca oleracea, extract (PE) on the relative expression level of some immune-related genes. A total of 300 zebrafish were randomly divided into four treatment groups and fed for 8 weeks with the basal diets supplemented with 0.5, 1, 1.5, and 2% of PE. The control group was fed with a basal diet without PE. At the end of 8 weeks, the mRNA expression levels of interleukin 1-beta (IL-1β), interleukin 10 (IL-10), transforming growth factor-beta (TGF-β), tumor necrosis factor-alpha (TNF-α), superoxide dismutase (SOD), and lysozyme (LYZ) in the fish were evaluated. The results showed that the mRNA expression level of IL-1β was significantly upregulated in the fish fed with 1 and 2% PE compared to the control group (p < 0.05). Moreover, the evaluation of the mRNA expression level of TGF-β was significantly increased in a dose-dependent manner in the 1.5 and 2% fed groups compared to the control group (p < 0.05). However, the IL-10 was significantly downregulated in all treated groups compared to the control group (p < 0.05). The expression of the TNF-α gene was not affected amongst all groups by the inclusion of PE in the zebrafish diet (p > 0.05). Based on the results, the diet supplemented with 1.5 and 2% PE significantly upregulated the mRNA expression levels of LYZ and SOD, respectively, compared to the control group (p < 0.05). In conclusion, dietary inclusion of PE may result in beneficial effects on some immune responses via upregulation of some immune genes in zebrafish.

A matter of time: regulatory events behind the synchronization of C4 and crassulacean acid metabolism in Portulaca oleracea

Portulaca species can switch between C4 and crassulacean acid metabolism (CAM) depending on environmental conditions. However, the regulatory mechanisms behind this rare photosynthetic adaptation remain elusive. Using Portulaca oleracea as a model system, here we investigated the involvement of the circadian clock, plant hormones, and transcription factors in coordinating C4 and CAM gene expression. Free-running experiments in constant conditions suggested that C4 and CAM gene expression are intrinsically connected to the circadian clock. Detailed time-course, drought, and rewatering experiments revealed distinct time frames for CAM induction and reversion (days versus hours, respectively), which were accompanied by changes in abscisic acid (ABA) and cytokinin metabolism and signaling. Exogenous ABA and cytokinins were shown to promote and repress CAM expression in P. oleracea, respectively. Moreover, the drought-induced decline in C4 transcript levels was completely recovered upon cytokinin treatment. The ABA-regulated transcription factor genes HB7, NFYA7, NFYC9, TT8, and ARR12 were identified as likely candidate regulators of CAM induction following this approach, whereas NFYC4 and ARR9 were connected to C4 expression patterns. Therefore, we provide insights into the signaling events controlling C4-CAM transitions in response to water availability and over the day/night cycle, highlighting candidate genes for future functional studies in the context of facultative C4-CAM photosynthesis.

Developing Portulaca oleracea as a model system for functional genomics analysis of C4/CAM photosynthesis

Previously regarded as an intriguing photosynthetic curiosity, the occurrence of C4 and Crassulacean acid metabolism (CAM) photosynthesis within a single organism has recently emerged as a source of information for future biotechnological use. Among C4/CAM facultative species, Portulaca oleracea L. has been used as a model for biochemical and gene expression analysis of C4/CAM under field and laboratory conditions. In the present work, we focussed on developing molecular tools to facilitate functional genomics studies in this species, from the optimisation of RNA isolation protocols to a method for stable genetic transformation. Eleven variations of RNA extraction procedures were tested and compared for RNA quantity and quality. Also, 7 sample sets comprising total RNA from hormonal and abiotic stress treatments, distinct plant organs, leaf developmental stages, and subspecies were used to select, among 12 reference genes, the most stable reference genes for RT-qPCR analysis of each experimental condition. Furthermore, different explant sources, Agrobacterium tumefaciens strains, and regeneration and antibiotic selection media were tested in various combinations to optimise a protocol for stable genetic transformation of P. oleracea. Altogether, we provide essential tools for functional gene analysis in the context of C4/CAM photosynthesis, including an efficient RNA isolation method, preferred reference genes for RT-qPCR normalisation for a range of experimental conditions, and a protocol to produce P. oleracea stable transformants using A. tumefaciens.

An insight from tolerance to salinity stress in halophyte Portulaca oleracea L.: Physio-morphological, biochemical and molecular responses

Salinity represents one of the environmental conditions with adverse effects on the productivity of most crops throughout the world. The response of plants to salt stress is of great interest for research to understand the mechanism involved in tolerance to salinity and highlight insights into the improvement of salt tolerance-crops of importance. In this study, the effect of salt stress was observed in wild and cultivated populations of P. oleracea originated from Tunisia and Italy. The results showed that at various concentrations of NaCl (0 mM, 50 mM, 100 mM and 150 mM), salinity has led to changes in growth parameters marked mainly by an increase in fresh and dry biomass. Beside, one of the salinity-induced side effects corresponds to the competition of Na⁺ and K⁺ ions for potassium root transporters. Our results suggested that purslane deployed an important element of tolerance such as the transporters ability to discriminate cations. In addition, the variation of PC5S gene expression tested by semi-quantitative RT-PCR revealed that proline synthesis is important in plants adaptation in saline conditions. A correlation between the gene expression varying by population and saline concentration and the level of proline assayed on the leaves of P. oleracea was highlighted.

Leaf lipidome and transcriptome profiling of Portulaca oleracea: characterization of lysophosphatidylcholine acyltransferase

Portulaca leaves serve as an alternative bioresource for edible PUFAs. Transcriptome data provide information to explore Portulaca as a model system for galactolipids, leaf lipid metabolism, and PUFA-rich designer lipids. Poly-unsaturated fatty acids (PUFAs) are gaining importance due to their innumerable health benefits, and hence, understanding their biosynthesis in plants has attained prominence in recent years. The most common source of PUFAs is of marine origin. Although reports have identified Portulaca oleracea (purslane) as a leaf source of omega-3 fatty acids in the form of alpha-linolenic acid (ALA), the mechanism of ALA accumulation and its distribution into various lipids has not been elucidated. Here, we present the lipid profiles of leaves and seeds of several accessions of P. oleracea. Among the nineteen distinct accessions, the RR04 accession has the highest amount of ALA and is primarily associated with galactolipids. In addition, we report the transcriptome of RR04, and we have mapped the potential genes involved in lipid metabolism. Phosphatidylcholine (PC) is the major site of acyl editing, which is catalyzed by lysophosphatidylcholine acyltransferase (LPCAT), an integral membrane protein that plays a major role in supplying oleate to the PC pool for further unsaturation. Our investigations using mass spectrometric analysis of leaf microsomal fractions identified LPCAT as part of a membrane protein complex. Both native and recombinant LPCAT showed strong acyltransferase activity with various acyl-CoA substrates. Altogether, the results suggest that ALA-rich glycerolipid biosynthetic machinery is highly active in nutritionally important Portulaca leaves. Furthermore, lipidome, transcriptome, and mass spectrometric analyses of RR04 provide novel information for exploring Portulaca as a potential resource and a model system for studying leaf lipid metabolism.

Chemical Composition and Yield of Six Genotypes of Common Purslane (Portulaca oleracea L.): An Alternative Source of Omega-3 Fatty Acids

Common purslane (Portulaca oleracea L.) is an annual weed rich in omega-3 fatty acids which is consumed for its edible leaves and stems. In the present study six different genotypes of common purslane (A-F) were evaluated for their nutritional value and chemical composition. Nutritional value and chemical composition depended on genotype. Oxalic acid content was the lowest for genotype D, whereas genotypes E and F are more promising for commercial cultivation, since they have low oxalic acid content. Genotype E had a very good antioxidant profile and a balanced composition of omega-3 and omega-6 fatty acids. Regarding yield, genotype A had the highest yield comparing to the other genotypes, whereas commercial varieties (E and F) did not differ from genotypes B and C. This study provides new information regarding common purslane bioactive compounds as affected by genotype and could be further implemented in food industry for products of high quality and increased added value.

Identification of genes involved in the drought adaptation and recovery in Portulaca oleracea by differential display

Portulaca oleracea is one of the richest plant sources of ω-3 and ω-6 fatty acids and other compounds potentially valuable for nutrition. It is broadly established in arid, semiarid and well-watered fields, thus making it a promising candidate for research on abiotic stress resistance mechanisms. It is capable of withstanding severe drought and then of recovering upon rehydration. Here, the adaptation to drought and the posterior recovery was evaluated at transcriptomic level by differential display validated by qRT-PCR. Of the 2279 transcript-derived fragments amplified, 202 presented differential expression. Ninety of them were successfully isolated and sequenced. Selected genes were tested against different abiotic stresses in P. oleracea and the behavior of their orthologous genes in Arabidopsis thaliana was also explored to seek for conserved response mechanisms. In drought adapted and in recovered plants changes in expression of many protein metabolism-, lipid metabolism- and stress-related genes were observed. Many genes with unknown function were detected, which also respond to other abiotic stresses. Some of them are also involved in the seed desiccation/imbibition process and thus would be of great interest for further research. The potential use of candidate genes to engineer drought tolerance improvement and recovery is discussed.

Shared origins of a key enzyme during the evolution of C4 and CAM metabolism

CAM and C4 photosynthesis are two key plant adaptations that have evolved independently multiple times, and are especially prevalent in particular groups of plants, including the Caryophyllales. We investigate the origin of photosynthetic PEPC, a key enzyme of both the CAM and C4 pathways. We combine phylogenetic analyses of genes encoding PEPC with analyses of RNA sequence data of Portulaca, the only plants known to perform both CAM and C4 photosynthesis. Three distinct gene lineages encoding PEPC exist in eudicots (namely ppc-1E1, ppc-1E2 and ppc-2), one of which (ppc-1E1) was recurrently recruited for use in both CAM and C4 photosynthesis within the Caryophyllales. This gene is present in multiple copies in the cacti and relatives, including Portulaca. The PEPC involved in the CAM and C4 cycles of Portulaca are encoded by closely related yet distinct genes. The CAM-specific gene is similar to genes from related CAM taxa, suggesting that CAM has evolved before C4 in these species. The similar origin of PEPC and other genes involved in the CAM and C4 cycles highlights the shared early steps of evolutionary trajectories towards CAM and C4, which probably diverged irreversibly only during the optimization of CAM and C4 phenotypes.

Two plastid DNA lineages--Rapa/Oleracea and Nigra--within the tribe Brassiceae can be best explained by reciprocal crosses at hexaploidy: evidence from divergence times of the plastid genomes and R-block genes of the A and B genomes of Brassica juncea

Brassica species (tribe Brassiceae) belonging to U's triangle--B. rapa (AA), B. nigra (BB), B. oleracea (CC), B. juncea (AABB), B. napus (AACC) and B. carinata (BBCC)--originated via two polyploidization rounds: a U event producing the three allopolyploids, and a more ancient b genome-triplication event giving rise to the A-, B-, and C-genome diploid species. Molecular mapping studies, in situ hybridization, and genome sequencing of B. rapa support the genome triplication origin of tribe Brassiceae, and suggest that these three diploid species diversified from a common hexaploid ancestor. Analysis of plastid DNA has revealed two distinct lineages--Rapa/Oleracea and Nigra--that conflict with hexaploidization as a single event defining the tribe Brassiceae. We analysed an R-block region of A. thaliana present in six copies in B. juncea (AABB), three copies each on A- and B-genomes to study gene fractionation pattern and synonymous base substitution rates (Ks values). Divergence time of paralogues within the A and B genomes and homoeologues between the A and B genomes was estimated. Homoeologous R blocks of the A and B genomes exhibited high gene collinearity and a conserved gene fractionation pattern. The three progenitors of diploid Brassicas were estimated to have diverged approximately 12 mya. Divergence of B. rapa and B. nigra, calculated from plastid gene sequences, was estimated to have occurred approximately 12 mya, coinciding with the divergence of the three genomes participating in the b event. Divergence of B. juncea A and B genome homoeologues was estimated to have taken place around 7 mya. Based on divergence time estimates and the presence of distinct plastid lineages in tribe Brassiceae, it is concluded that at least two independent triplication events involving reciprocal crosses at the time of the b event have given rise to Rapa/Oleracea and Nigra lineages.

Omega-3 fatty acid desaturase genes isolated from purslane (Portulaca oleracea L.): expression in different tissues and response to cold and wound stress

Two full-length cDNA clones PoleFAD7 and PoleFAD8, encoding plastidial omega-3 fatty acid desaturases were isolated from purslane (Portulaca oleracea). The encoded enzymes convert linoleic to alpha-linolenic acid (C18:3n-3). Three histidine clusters characteristic of fatty acid desaturases, a putative chloroplast transit peptide in the N-terminal, and three putative transmembrane domains were identified in the sequence. Both genes were expressed in all analyzed tissues showing different levels of expression. PoleFAD7 was up-regulated by wounding but not by low temperature. PoleFAD8 was up-regulated by cold stress but not by wounding. Total fatty acid and linolenic acid content were higher both, in wounded and intact leaves of plants exposed to low temperature.

Characterization and functional identification of a novel plant 4,5-extradiol dioxygenase involved in betalain pigment biosynthesis in Portulaca grandiflora

Betalains are pigments that replace anthocyanins in the majority of families of the plant order Caryophyllales. Betalamic acid is the common chromophore of betalains. The key enzyme of the betalain biosynthetic pathway is an extradiol dioxygenase that opens the cyclic ring of dihydroxy-phenylalanine (DOPA) between carbons 4 and 5, thus producing an unstable seco-DOPA that rearranges nonenzymatically to betalamic acid. A gene for a 4,5-DOPA-dioxygenase has already been isolated from the fungus Amanita muscaria, but no homolog was ever found in plants. To identify the plant gene, we constructed subtractive libraries between different colored phenotypes of isogenic lines of Portulaca grandiflora (Portulacaceae) and between different stages of flower bud formation. Using in silico analysis of differentially expressed cDNAs, we identified a candidate showing strong homology at the level of translated protein with the LigB domain present in several bacterial extradiol 4,5-dioxygenases. The gene was expressed only in colored flower petals. The function of this gene in the betalain biosynthetic pathway was confirmed by biolistic genetic complementation in white petals of P. grandiflora genotypes lacking the gene for color formation. This gene named DODA is the first characterized member of a novel family of plant dioxygenases phylogenetically distinct from Amanita sp. DOPA-dioxygenase. Homologs of DODA are present not only in betalain-producing plants but also, albeit with some changes near the catalytic site, in other angiosperms and in the bryophyte Physcomitrella patens. These homologs are part of a novel conserved plant gene family probably involved in aromatic compound metabolism.