Integrative analysis reveals evolutionary patterns and potential functions of SWEET transporters in Euphorbiaceae

SWEET transporters of Medicago lupulina in the arbuscular-mycorrhizal system in the presence of medium level of available phosphorus

Arbuscular mycorrhiza (AM) fungi receive photosynthetic products and sugars from plants in exchange for contributing to the uptake of minerals, especially phosphorus, from the soil. The identification of genes controlling AM symbiotic efficiency may have practical application in the creation of highly productive plant-microbe systems. The aim of our work was to evaluate the expression levels of SWEET sugar transporter genes, the only family in which sugar transporters specific to AM symbiosis can be detected. We have selected a unique "host plant-AM fungus" model system with high response to mycorrhization under medium phosphorus level. This includes a plant line which is highly responsive to inoculation by AM fungi, an ecologically obligate mycotrophic line MlS-1 from black medick (Medicago lupulina) and the AM fungus Rhizophagus irregularis strain RCAM00320, which has a high efficiency in a number of plant species. Using the selected model system, differences in the expression levels of 11 genes encoding SWEET transporters in the roots of the host plant were evaluated during the development of or in the absence of symbiosis of M. lupulina with R. irregularis at various stages of the host plant development in the presence of medium level of phosphorus available for plant nutrition in the substrate. At most stages of host plant development, mycorrhizal plants had higher expression levels of MlSWEET1b, MlSWEET3c, MlSWEET12 and MlSWEET13 compared to AM-less controls. Also, increased expression relative to control during mycorrhization was observed for MlSWEET11 at 2nd and 3rd leaf development stages, for MlSWEET15c at stemming (stooling) stage, for MlSWEET1a at 2nd leaf development, stemming and lateral branching stages. The MlSWEET1b gene can be confidently considered a good marker with specific expression for effective development of AM symbiosis between M. lupulina and R. irregularis in the presence of medium level of phosphorus available to plants in the substrate.

The rubber tree kinome: Genome-wide characterization and insights into coexpression patterns associated with abiotic stress responses

The protein kinase (PK) superfamily constitutes one of the largest and most conserved protein families in eukaryotic genomes, comprising core components of signaling pathways in cell regulation. Despite its remarkable relevance, only a few kinase families have been studied in Hevea brasiliensis. A comprehensive characterization and global expression analysis of the PK superfamily, however, is currently lacking. In this study, with the aim of providing novel inferences about the mechanisms associated with the stress response developed by PKs and retained throughout evolution, we identified and characterized the entire set of PKs, also known as the kinome, present in the Hevea genome. Different RNA-sequencing datasets were employed to identify tissue-specific expression patterns and potential correspondences between different rubber tree genotypes. In addition, coexpression networks under several abiotic stress conditions, such as cold, drought and latex overexploitation, were employed to elucidate associations between families and tissues/stresses. A total of 1,809 PK genes were identified using the current reference genome assembly at the scaffold level, and 1,379 PK genes were identified using the latest chromosome-level assembly and combined into a single set of 2,842 PKs. These proteins were further classified into 20 different groups and 122 families, exhibiting high compositional similarities among family members and with two phylogenetically close species Manihot esculenta and Ricinus communis. Through the joint investigation of tandemly duplicated kinases, transposable elements, gene expression patterns, and coexpression events, we provided insights into the understanding of the cell regulation mechanisms in response to several conditions, which can often lead to a significant reduction in rubber yield.

The core triacylglycerol toolbox in woody oil plants reveals targets for oil production bioengineering

Woody oil plants are the most productive oil-bearing species that produce seeds with high levels of valuable triacylglycerols (TAGs). TAGs and their derivatives are the raw materials for many macromolecular bio-based products, such as nylon precursors, and biomass-based diesel. Here, we identified 280 genes encoding seven distinct classes of enzymes (i.e., G3PAT, LPAAT, PAP, DGAT, PDCT, PDAT, and CPT) involved in TAGs-biosynthesis. Several multigene families are expanded by large-scale duplication events, such as G3PATs, and PAPs. RNA-seq was used to survey the expression profiles of these TAG pathway-related genes in different tissues or development, indicating functional redundancy for some duplicated genes originated from the large-scale duplication events, and neo-functionalization or sub-functionalization for some of them. Sixty-two genes showed strong, preferential expression during the period of rapid seed lipid synthesis, suggesting that their might represented the core TAG-toolbox. We also revealed for the first time that there is no PDCT pathway in Vernicia fordii and Xanthoceras sorbifolium. The identification of key genes involved in lipid biosynthesis will be the foundation to plan strategies to develop woody oil plant varieties with enhanced processing properties and high oil content.

Divergence of flowering-related genes to control flowering in five Euphorbiaceae genomes

Reproductive growth and vegetative growth are a pair of main contradictions in the process of plant growth. Flowering, as part of reproductive growth, is a key switch in the life cycle of higher plants, which affects the yield and economic benefits of plants to a certain extent. The Euphorbiaceae species, including castor bean (Ricinus communis), physic nut (Jatropha curcas), tung tree (Vernicia fordii), cassava (Manihot esculenta), and rubber tree (Hevea brasiliensis), have important economic values because they are raw materials for the production of biodiesel, rubber, etc. The flowering mechanisms are still excluded in the Euphorbiaceae species. The flowering-related genes of Arabidopsis thaliana (Arabidopsis) were used as a reference to determine the orthologs of these genes in Euphorbiaceae genomes. The result showed that 146, 144, 114, 114, and 149 of 207 A. thaliana genes were respectively matched to R. communis, V. fordii, J. curcas, H. brasiliensis, and M. esculenta. These identified genes were clustered into seven pathways including gibberellins, floral meristem identity (FMI), vernalization, photoperiod, floral pathway integrators (FPIs), and autonomous pathways. Then, some key numbers of flowering-related genes are widely conserved in the Euphorbiaceae genomes including but not limited to FPI genes LFY, SOC1, FT, and FMI genes AG, CAL, and FUL. However, some genes, including FRI, FLC, and GO, were missing in several or all five Euphorbiaceae species. In this study, we proposed the putative mechanisms of flowering-related genes to control flowering and provided new candidate flowering genes for using marker-assisted breeding to improve variety quality.

Evolution and function of ubiquitin-specific proteases (UBPs): Insight into seed development roles in tung tree (Vernicia fordii)

The tung oil produced by the tung tree (Vernicia fordii) provides resources for the manufacture of biodiesel. Ubiquitin-specific proteases (UBPs) are the largest group of deubiquitinases and play key roles in regulating development and stress responses. Here, 21 UBPs were identified in V. fordii, roughly one-half the number found in Manihot esculenta and Hevea brasiliensis. Most UBP duplications are produced from whole-genome duplication (WGD), and significant differences in gene retention existed among Euphorbiaceae. The great majority of UBP-containing blocks in V. fordii, V. montana, Ricinus communis, and Jatropha curcas exhibited extensive conservation with the duplicated regions of M. esculenta and H. brasiliensis. These blocks formed 14 orthologous groups, indicating they shared WGD with UBPs in M. esculenta and H. brasiliensis, but most of these UBPs copies were lost. The UBP orthologs contained significant functional divergence which explained the susceptibility of V. fordii to Fusarium wilt and the resistance of V. montana to Fusarium wilt. The expression patterns and experiments suggested that Vf03G1417 could affect the seed-related traits and positively regulate the seed oil accumulation. This study provided important insights into the evolution of UBPs in Euphorbiaceae and identified important candidate VfUBPs for marker-assisted breeding in V. fordii.

Functional Heterogeneity of the Young and Old Duplicate Genes in Tung Tree (Vernicia fordii)

Genes are subject to birth and death during the long evolutionary period. Here, young and old duplicate genes were identified in Vernicia fordii. We performed integrative analyses, including expression pattern, gene complexity, evolution, and functional divergence between young and old duplicate genes. Compared with young genes, old genes have higher values of Ka and Ks, lower Ka/Ks values, and lower average intrinsic structural disorder (ISD) values. Gene ontology and RNA-seq suggested that most young and old duplicate genes contained asymmetric functions. Only old duplicate genes are likely to participate in response to Fusarium wilt infection and exhibit divergent expression patterns. Our data suggest that young genes differ from older genes not only by evolutionary properties but also by their function and structure. These results highlighted the characteristics and diversification of the young and old genes in V. fordii and provided a systematic analysis of these genes in the V. fordii genome.

AHLs' life in plants: Especially their potential roles in responding to Fusarium wilt and repressing the seed oil accumulation

Members of the AT-hook motif nuclear localized (AHL) family contain diverse but poorly understood biological functions. We identified 371 AHLs in 20 land plants, varying from the early diverging lycophyte Selagineila moellendorfi to a variety of higher plants. The AHLs were divided into two clades (Clade-A and Clade-B) with three different types (Type-I, Type-II, and Type-III AHLs). The divergence between Clade-A and Clade-B likely occurred before the separation of S. moellendorfi from the vascular plant lineages. Members of the AHLs family expanded with the specific whole-genome duplication (WGD)/segmental duplication in some genomes, such as Hevea brasiliensis. The ortholog (Vf00G1914/Amo018442) exhibited opposite expression patterns between two Vernicia species (V. fordii and V. montana), indicating that it was implicated in resistance to Fusarium wilt disease. The expression of Vf09G2138 exhibited a negative correlation with lipid biosynthesis in V. fordii seeds during different stages of development, suggesting that this gene might repress the seed oil accumulation. The core AT-hook motif and PPC domain were responsible for guiding the localization of AHL in the nucleus. This study helps us to understand the evolution of AHLs in multiple plants, further highlight their functions during V. fordii seed development and response to Fusarium wilt disease.

Mobile forms of carbon in trees: metabolism and transport

Plants constitute 80% of the biomass on earth, and almost two-thirds of this biomass is found in wood. Wood formation is a carbon (C)-demanding process and relies on C transport from photosynthetic tissues. Thus, understanding the transport process is of major interest for understanding terrestrial biomass formation. Here, we review the molecules and mechanisms used to transport and allocate C in trees. Sucrose is the major form in which C is transported in plants, and it is found in the phloem sap of all tree species investigated so far. However, in several tree species, sucrose is accompanied by other molecules, notably polyols and the raffinose family of oligosaccharides. We describe the molecules that constitute each of these transport groups, and their distribution across different tree species. Furthermore, we detail the metabolic reactions for their synthesis, the mechanisms by which trees load and unload these compounds in and out of the vascular system, and how they are radially transported in the trunk and finally catabolized during wood formation. We also address a particular C recirculation process between phloem and xylem that occurs in trees during the annual cycle of growth and dormancy. A search of possible evolutionary drivers behind the diversity of C-carrying molecules in trees reveals no consistent differences in C transport mechanisms between angiosperm and gymnosperm trees. Furthermore, the distribution of C forms across species suggests that climate-related environmental factors will not explain the diversity of C transport forms. However, the consideration of C-transport mechanisms in relation to tree-rhizosphere coevolution deserves further attention. To conclude the review, we identify possible future lines of research in this field.

Sugar transporters of the SWEET family and their role in arbuscular mycorrhiza

Plant sugar transporters play an essential role in the organism’s productivity by carrying out carbohydrate transportation from source cells in the leaves to sink cells in the cortex. In addition, they aid in the regulation of a substantial part of the exchange of nutrients with microorganisms in the rhizosphere (bacteria and fungi), an ty essential to the formation of symbiotic relationships. This review pays special attention to carbohydrate nutrition during the development of arbuscular mycorrhiza (AM), a symbiosis of plants with fungi from the Glomeromycotina subdivision. This relationship results in the host plant receiving micronutrients from the mycosymbiont, mainly phosphorus, and the fungus receiving carbon assimilation products in return. While the eff icient nutrient transport pathways in AM symbiosis are yet to be discovered, SWEET sugar transporters are one of the three key families of plant carbohydrate transporters. Specif ic AM symbiosis transporters can be identif ied among the SWEET proteins. The survey provides data on the study history, structure and localization, phylogeny and functions of the SWEET proteins. A high variability of both the SWEET proteins themselves and their functions is noted along with the fact that the same proteins may perform different functions in different plants. A special role is given to the SWEET transporters in AM development. SWEET transporters can also play a key role in abiotic stress tolerance, thus allowing plants to adapt to adverse environmental conditions. The development of knowledge about symbiotic systems will contribute to the creation of microbial preparations for use in agriculture in the Russian Federation.

Deciphering Evolutionary Dynamics of WRKY I Genes in Rosaceae Species

WRKY transcription factors participate in various regulation processes at different developmental stages in higher plants. Here, 98 WRKY I genes were identified in seven Rosaceae species. The WRKY I genes are highly enriched in some subgroups and are selectively expanded in Chinese pear [Pyrus bretschneideri (P. bretschneideri)] and apple [Malus domestica (M. domestica)]. By searching for intra-species gene microsynteny, we found the majority of chromosomal segments for WRKY I-containing segments in both P. bretschneideri and M. domestica genomes, while paired segments were hardly identified in the other five genomes. Furthermore, we analyzed the environmental selection pressure of duplicated WRKY I gene pairs, which indicated that the strong purifying selection for WRKY domains may contribute to the stability of its structure and function. The expression patterns of duplication PbWRKY genes revealed that functional redundancy for some of these genes was derived from common ancestry and neo-functionalization or sub-functionalization for some of them. This study traces the evolution of WRKY I genes in Rosaceae genomes and lays the foundation for functional studies of these genes in the future. Our results also show that the rates of gene loss and gain in different Rosaceae genomes are far from equilibrium.

Deciphering the roles of leucine-rich repeat receptor-like protein kinases (LRR-RLKs) in response to Fusarium wilt in the Vernicia fordii (Tung tree)

Leucine-rich repeat receptor-like protein kinases (LRR-RLKs) are vital for plant growth and development, signal transduction, immunity, and play diverse roles in plant defense responses. However, the LRR-RLK genes have not been systematically studied in Vernicia fordii (tung tree), especially its response to Fusarium wilt. Here, we carried out an integrative analysis of LRR-RLKs among five Euphorbiaceae species: Hevea brasiliensis (rubber tree), Manihot esculenta (cassava), Jatropha curcas (physic nut), Ricinus communis (castor bean), and V. fordii, which contained 223, 311, 186, 138, and 167 LRR-RLKs, respectively. Maximum-likelihood tree was estimated using LRR-RLKs of Arabidopsis thaliana as a template, and they allowed us to divide Euphorbiaceae LRR-RLKs into 22 groups. There are 126 segmental and 30 tandem duplications in these Euphorbiaceae genomes by synteny analysis. The tissue-specific expression patterns revealed that V. fordii LRR-RLKs (VfLRR-RLKs) were differentially expressed in various tissues, and some of them exhibited specific expression in meristems tissues, which suggested their potential functions during organ formation and cell fate specification. Two VfLRR-RLK pairs (Vf01G2125 and Vf03G1740, Vf06G2687 and Vf10G1659), which generated by tandem duplication events, were associated with possible resistance to Fusarium wilt infection. The qRT-PCR confirmed these four VfLRR-RLKs contained opposite expression profiles during pathogen infection in V. fordii and V. montana. Taken together, our data systematically analyzed the LRR-RLK family in Euphorbiaceae genomes for the first time. We highlight the putative roles of VfLRR-RLKs in response to Fusarium wilt infection, and VfLRR-RLKs may be further applied in marker-assisted breeding to control Fusarium wilt in V. fordii.

SWEET Transporters and the Potential Functions of These Sequences in Tea (Camellia sinensis)

Tea (Camellia sinensis) is an important economic beverage crop. Its flowers and leaves could be used as healthcare tea for its medicinal value. SWEET proteins were recently identified in plants as sugar transporters, which participate in diverse physiological processes, including pathogen nutrition, seed filling, nectar secretion, and phloem loading. Although SWEET genes have been characterized and identified in model plants, such as Arabidopsis thaliana and Oryza sativa, there is very little knowledge of these genes in C. sinensis. In this study, 28 CsSWEETs were identified in C. sinensis and further phylogenetically divided into four subfamilies with A. thaliana. These identified CsSWEETs contained seven transmembrane helixes (TMHs) which were generated by an ancestral three-TMH unit with an internal duplication experience. Microsynteny analysis revealed that the large-scale duplication events were the main driving forces for members from CsSWEET family expansion in C. sinensis. The expression profiles of the 28 CsSWEETs revealed that some genes were highly expressed in reproductive tissues. Among them, CsSWEET1a might play crucial roles in the efflux of sucrose, and CsSWEET17b could control fructose content as a hexose transporter in C. sinensis. Remarkably, CsSWEET12 and CsSWEET17c were specifically expressed in flowers, indicating that these two genes might be involved in sugar transport during flower development. The expression patterns of all CsSWEETs were differentially regulated under cold and drought treatments. This work provided a systematic understanding of the members from the CsSWEET gene family, which would be helpful for further functional studies of CsSWEETs in C. sinensis.

Comparative genomic analysis of superoxide dismutase (SOD) genes in three Rosaceae species and expression analysis in Pyrus bretschneideri

Superoxide dismutases (SODs) are antioxidant enzymes that play a critical role in the polymerization of lignin monomers. Although current research has indicated that SODs are involved in plant growth and development, information on SODs in pear (Pyrus bretschneideri) and their function in lignin formation is scarce. In this study, 25 SODs, containing three kinds of plant SODs (Cu/Zn-SODs, Mn-SODs, and Fe-SODs), were identified from three Rosaceae species, and 11 of these genes were found in pear. According to the evolutionary analysis, the genes were divided into four subgroups, the division of which is consistent with the intron-exon and conserved motif analyses. These PbSODs were randomly scattered across 7 chromosomes. We have analysed the conserved domains and gene family evolution and predicted the cis-elements of the promoter. Ka/Ks analysis pointed that SOD genes mainly underwent purifying selection. Subsequently, the expression patterns of 11 PbSODs were examined in different tissues, at different developmental periods, in different pear varieties and under different hormone treatments. Gene expression analysis showed that PbCSD3 exhibited transcript levels consistent with the typical changes in lignin content. The changes in SOD activity and hydrogen peroxide (H₂O₂) content combined with the results of a spatio-temporal expression analysis showed that PbCSD3 was a candidate gene in reactive oxygen species (ROS) metabolism during the lignification of pear stone cells. Thus, our research reveals the evolutionary features of the SOD family in Rosaceae species and provide useful information for analysis of functional genome of the SOD family in pear.

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Systematic analysis of the OFP genes in six Rosaceae genomes and their roles in stress response in Chinese pear (Pyrus bretschneideri)

OVATE family proteins (OFPs) are the plant-specific transcription factors, and have significant functions in regulating plant growth, development and resistance. The OFP genes have been investigated in several plants, but they still lack a systematic analysis of OFP genes in Chinese pear and some other five Rosaceae genomes. Here, 28 PbrOFPs were identified within Chinese pear and compared them with those of other five Rosaceae genomes. Evolutionary tree revealed that all OFP genes from six Rosaceae genomes were divided into eight groups. Seventeen conserved microsynteny regions were detected in Chinese pear genome, suggested that these PbrOFP genes might be considered to have originated from the large-scale duplication events., indicating these PbrOFP genes might contain specialized regulatory mechanisms in these tissues, such as flower, ovary and fruit. Remarkably, two PbrOFP genes (Pbr010426.1 and Pbr010427.1) were up-regulated under Venturia nashicola treatment, and five PbrOFP genes were up-regulated under PEG treatment, suggesting that these genes might play crucial roles in defence to environmental stresses. Our data presented a systematic analysis and might aid in the selection of appropriate PbrOFPs for further functional studies in Chinese pear, especially in response to the mechanism of biotic and abiotic stresses.

Micro-Evolution Analysis Reveals Diverged Patterns of Polyol Transporters in Seven Gramineae Crops

Polyol transporters (PLTs), also called polyol/monosaccharide transporters, is of significance in determining plant development and sugar transportation. However, the diverged evolutionary patterns of the PLT gene family in Gramineae crops are still unclear. Here a micro-evolution analysis was performed among the seven Gramineae representative crops using whole-genome sequences, i.e., Brachypodium distachyon (Bd), Hordeum vulgare (Hv), Oryza rufipogon (Or), Oryza sativa (Os), Sorghum bicolor (Sb), Setaria italica (Si), and Zea mays (Zm), leading to the identification of 12, 11, 12, 15, 20, 24, and 20 PLT genes, respectively. In this study, all PLT genes were divided into nine orthogroups (OGs). However, the number of PLT genes and the distribution of PLT OGs were not the same in these seven Gramineae species, and different OGs were also subject to different purification selection pressures. These results indicated that the PLT OGs of the PLT gene family have been expanded or lost unevenly in all tested species. Then, our results of gene duplication events confirmed that gene duplication events promoted the expansion of the PLT gene family in some Gramineous plants, namely, Bd, Or, Os, Si, Sb, and Zm, but the degree of gene family expansion, the type of PLT gene duplication, and the differentiation time of duplicate gene pairs varied greatly among these species. In addition, the sequence alignment and the internal repeat analysis of all PLTs protein sequences implied that the PLT protein sequences may originate from an internal repeat duplication of an ancestral six transmembrane helical units. Besides that, the protein motifs result highlighted that the PLT protein sequences were highly conserved, whereas the functional differentiation of the PLT genes was characterized by different gene structures, upstream elements, as well as co-expression analysis. The gene expression analysis of rice and maize showed that the PLT genes have a wide range of expression patterns, suggesting diverse biological functions. Taken together, our finding provided a perspective on the evolution differences and the functional characterizations of PLT genes in Gramineae representative crops.

Network Analysis of Transcriptome and LC-MS Reveals a Possible Biosynthesis Pathway of Anthocyanins in Dendrobium officinale

Anthocyanins, a group of flavonoids, are widely present in plants and determine the colors of the peels of stems, fruits, and flowers. In this study, we used UHPLC-ESI-MS to identify anthocyanins in the herbal plant Dendrobium officinale, which has been used for centuries in China. The results indicated that the total anthocyanin content in samples from Guangxi was the highest. Seven anthocyanins were identified, and the fragmentation pathways were proposed from D. officinale. Most of the identified anthocyanins were composed of cyanidin and sinapoyl groups. We also carried out that the sinapoyl group had active sites on breast cancer receptors by using Schrödinger. The relative levels of the 7 anthocyanins in the samples from the three locations were determined. Transcriptomic analysis was used to analyze the sinapoyl anthocyanin synthesis-related genes in plants, such as genes encoding UGTs and serine carboxypeptidase. We speculated that sinapoyl anthocyanin biosynthesis was associated with the activities of certain enzymes, including chalcone flavonone isomerase-like, hydroxycinnamoyltransferase 1, UGT-83A1, UGT-88B1 isoform X1, serine carboxypeptidase-like 18 isoform X3, and serine carboxypeptidase-like 18.

MYB Transcription Factors as Regulators of Secondary Metabolism in Plants

MYB transcription factors (TFs), as one of the largest gene families in plants, play important roles in multiple biological processes, such as plant growth and development, cell morphology and pattern building, physiological activity metabolism, primary and secondary metabolic reactions, and responses to environmental stresses. The function of MYB TFs in crops has been widely studied, but few studies have been done on medicinal plants. In this review, we summarized the MYB TFs that play important roles in secondary metabolism and emphasized the possible mechanisms underlying how MYB TFs are regulated at the protein, posttranscriptional, and transcriptional levels, as well as how they regulate the downstream target gene networks related to secondary metabolism in plants, especially in medicinal plants.

Comparative Analysis and Expression Patterns of the PLP_deC Genes in Dendrobium officinale

Studies have shown that the type II pyridoxal phosphate-dependent decarboxylase (PLP_deC) genes produce secondary metabolites and flavor volatiles in plants, and TDC (tryptophan decarboxylase), a member of the PLP_deC family, plays an important role in the biosynthesis of terpenoid indole alkaloids (TIAs). In this study, we identified eight PLP_deC genes in Dendrobium officinale (D. officinale) and six in Phalaenopsis equestris (P. equestris), and their structures, physicochemical properties, response elements, evolutionary relationships, and expression patterns were preliminarily predicted and analyzed. The results showed that PLP_deC genes play important roles in D. officinale and respond to different exogenous hormone treatments; additionally, the results support the selection of appropriate candidates for further functional characterization of PLP_deC genes in D. officinale.