Global transcriptome and gene co-expression network analyses on the development of distyly in Primula oreodoxa

PfPIN5 promotes style elongation by regulating cell length in Primula forbesii Franch

BACKGROUND AND AIMS

Style dimorphism is one of the polymorphic characteristics of flowers in heterostylous plants, which have two types of flowers: the pin morph, with long styles and shorter anthers, and the thrum morph, with short styles and longer anthers. The formation of dimorphic styles has received attention in the plant world. Previous studies showed that CYP734A50 in Primula determined style length and limited style elongation and that the brassinosteroid metabolic pathway was involved in regulation of style length. However, it is unknown whether there are other factors affecting the style length of Primula.

METHODS

Differentially expressed genes highly expressed in pin morph styles were screened based on Primula forbesii transcriptome data. Virus-induced gene silencing was used to silence these genes, and the style length and anatomical changes were observed 20 days after injection.

KEY RESULTS

PfPIN5 was highly expressed in pin morph styles. When PfPIN5 was silenced, the style length was shortened in pin and long-homostyle plants by shortening the length of style cells. Moreover, silencing CYP734A50 in thrum morph plants increased the expression level of PfPIN5 significantly, and the style length increased. The results indicated that PfPIN5, an auxin efflux transporter gene, contributed to regulation of style elongation in P. forbesii.

CONCLUSIONS

The results implied that the auxin pathway might also be involved in the formation of styles of P. forbesii, providing a new pathway for elucidating the molecular mechanism of style elongation in P. forbesii.

Novel Tri-Segmented Rhabdoviruses: A Data Mining Expedition Unveils the Cryptic Diversity of Cytorhabdoviruses

Cytorhabdoviruses (genus Cytorhabdovirus, family Rhabdoviridae) are plant-infecting viruses with enveloped, bacilliform virions. Established members of the genus Cytorhabdovirus have unsegmented single-stranded negative-sense RNA genomes (ca. 10-16 kb) which encode four to ten proteins. Here, by exploring large publicly available metatranscriptomics datasets, we report the identification and genomic characterization of 93 novel viruses with genetic and evolutionary cues of cytorhabdoviruses. Strikingly, five unprecedented viruses with tri-segmented genomes were also identified. This finding represents the first tri-segmented viruses in the family Rhabdoviridae, and they should be classified in a novel genus within this family for which we suggest the name "Trirhavirus". Interestingly, the nucleocapsid and polymerase were the only typical rhabdoviral proteins encoded by those tri-segmented viruses, whereas in three of them, a protein similar to the emaravirus (family Fimoviridae) silencing suppressor was found, while the other predicted proteins had no matches in any sequence databases. Genetic distance and evolutionary insights suggest that all these novel viruses may represent members of novel species. Phylogenetic analyses, of both novel and previously classified plant rhabdoviruses, provide compelling support for the division of the genus Cytorhabdovirus into three distinct genera. This proposed reclassification not only enhances our understanding of the evolutionary dynamics within this group of plant rhabdoviruses but also illuminates the remarkable genomic diversity they encompass. This study not only represents a significant expansion of the genomics of cytorhabdoviruses that will enable future research on the evolutionary peculiarity of this genus but also shows the plasticity in the rhabdovirus genome organization with the discovery of tri-segmented members with a unique evolutionary trajectory.

Haplotype-resolved genome assembly provides insights into the evolution of S-locus supergene in distylous Nymphoides indica

Distyly has evolved independently in numerous animal-pollinated angiosperm lineages. Understanding of its molecular basis has been restricted to a few species, primarily Primula. Here, we investigate the genetic architecture of the single diallelic locus (S-locus) supergene, a linkage group of functionally associated genes, and explore how it may have evolved in distylous Nymphoides indica, a lineage of flowering plants not previously investigated. We assembled haplotype-resolved genomes, used read-coverage-based genome-wide association study (rb-GWAS) to locate the S-locus supergene, co-expression network analysis to explore gene networks underpinning the development of distyly, and comparative genomic analyses to investigate the origins of the S-locus supergene. We identified three linked candidate S-locus genes - NinBAS1, NinKHZ2, and NinS1 - that were only evident in the short-styled morph and were hemizygous. Co-expression network analysis suggested that brassinosteroids contribute to dimorphic sex organs in the short-styled morph. Comparative genomic analyses indicated that the S-locus supergene likely evolved via stepwise duplications and has been affected by transposable element activities. Our study provides novel insight into the structure, regulation, and evolution of the supergene governing distyly in N. indica. It also provides high-quality genomic resources for future research on the molecular mechanisms underlying the striking evolutionary convergence in form and function across heterostylous taxa.

Two floral forms in the same species-distyly

MAIN CONCLUSION

This paper reviews the progress of research on the morphology, physiology and molecular biology of distyly in plants. It will help to elucidate the mysteries of distyly in plants. Distyly is a unique representative type of heterostyly in plants, primarily characterized by the presence of long style and short style within the flowers of the same species. This interesting trait has always fascinated researchers. With the rapid development of molecular biology, the molecular mechanism for the production of dimorphic styles in plants is also gaining ground. Researchers have been studying plant dimorphic styles from various perspectives. The researchers are gradually unravelling the mechanisms by which plants produce distyly traits. This paper reviews advances in the study of plant dimorphic style characteristics, mainly in terms of the morphology, physiology and molecular biology of plants with dimorphic styles. The aim is to provide a theoretical basis for the study of the mechanism of distyly formation in plants.

Comprehensive transcriptomic profiling reveals complex molecular mechanisms in the regulation of style-length dimorphism in Guettarda speciosa (Rubiaceae), a species with "anomalous" distyly

BACKGROUND

The evolution of heterostyly, a genetically controlled floral polymorphism, has been a hotspot of research since the 19th century. In recent years, studies on the molecular mechanism of distyly (the most common form of heterostyly) revealed an evolutionary convergence in genes for brassinosteroids (BR) degradation in different angiosperm groups. This floral polymorphism often exhibits considerable variability that some taxa have significant stylar dimorphism, but anther height differs less. This phenomenon has been termed "anomalous" distyly, which is usually regarded as a transitional stage in evolution. Compared to "typical" distyly, the genetic regulation of "anomalous" distyly is almost unknown, leaving a big gap in our understanding of this special floral adaptation strategy.

METHODS

Here we performed the first molecular-level study focusing on this floral polymorphism in Guettarda speciosa (Rubiaceae), a tropical tree with "anomalous" distyly. Comprehensive transcriptomic profiling was conducted to examine which genes and metabolic pathways were involved in the genetic control of style dimorphism and if they exhibit similar convergence with "typical" distylous species.

RESULTS

"Brassinosteroid homeostasis" and "plant hormone signal transduction" was the most significantly enriched GO term and KEGG pathway in the comparisons between L- and S-morph styles, respectively. Interestingly, homologs of all the reported S-locus genes either showed very similar expressions between L- and S-morph styles or no hits were found in G. speciosa. BKI1, a negative regulator of brassinosteroid signaling directly repressing BRI1 signal transduction, was identified as a potential gene regulating style length, which significantly up-regulated in the styles of S-morph.

DISCUSSION

These findings supported the hypothesis that style length in G. speciosa was regulated through a BR-related signaling network in which BKI1 may be one key gene. Our data suggested, in species with "anomalous" distyly, style length was regulated by gene differential expressions, instead of the "hemizygous" S-locus genes in "typical" distylous flowers such as Primula and Gelsemium, representing an "intermediate" stage in the evolution of distyly. Genome-level analysis and functional studies in more species with "typical" and "anomalous" distyly would further decipher this "most complex marriage arrangement" in angiosperms and improve our knowledge of floral evolution.

Gene coexpression analysis reveals key pathways and hub genes related to late-acting self-incompatibility in Camellia oleifera

INTRODUCTION

Self-incompatibility (SI) is an important strategy for plants to maintain abundant variation to enhance their adaptability to the environment. Camellia oleifera is one of the most important woody oil plants and is widely cultivated in China. Late acting self-incompatibility (LSI) in C. oleifera results in a relatively poor fruit yield in the natural state, and understanding of the LSI mechanism remains limited.

METHODS

To better understand the molecular expression and gene coexpression network in the LSI reaction in C. oleifera, we conducted self- and cross-pollination experiments at two different flower bud developmental stages (3-4 d before flowering and 1 d before flowering), and cytological observation, fruit setting rate (FSR) investigation and RNA-Seq analysis were performed to investigate the mechanism of the male -female interaction and identify hub genes responsible for the LSI in C. oleifera.

RESULTS

Based on the 21 ovary transcriptomes, a total of 7669 DEGs were identified after filtering out low-expression genes. Weighted gene coexpression network analysis (WGCNA) divided the DEGs into 15 modules. Genes in the blue module (1163 genes) were positively correlated with FSR, and genes in the pink module (339 genes) were negatively correlated with FSR. KEGG analysis indicated that flavonoid biosynthesis, plant MAPK signaling pathways, ubiquitin-mediated proteolysis, and plant-pathogen interaction were the crucial pathways for the LSI reaction. Fifty four transcription factors (TFs) were obtained in the two key modules, and WRKY and MYB were potentially involved in the LSI reaction in C. oleifera. Network establishment indicated that genes encoding G-type lectin S-receptor-like serine (lecRLK), isoflavone 3'-hydroxylase-like (CYP81Q32), cytochrome P450 87A3-like (CYP87A3), and probable calcium-binding protein (CML41) were the hub genes that positively responded to the LSI reaction. The other DEGs inside the two modules, including protein RALF-like 10 (RALF), F-box and pectin acetylesterase (MTERF5), might also play vital roles in the LSI reaction in C. oleifera.

DISCUSSION

Overall, our study provides a meaningful resource for gene network studies of the LSI reaction process and subsequent analyses of pollen-pistil interactions and TF roles in the LSI reaction, and it also provides new insights for exploring the mechanisms of the LSI response.

Different molecular changes underlie the same phenotypic transition: Origins and consequences of independent shifts to homostyly within species

The repeated transition from outcrossing to selfing is a key topic in evolutionary biology. However, the molecular basis of such shifts has been rarely examined due to lack of knowledge of the genes controlling these transitions. A classic example of mating system transition is the repeated shift from heterostyly to homostyly. Occurring in 28 angiosperm families, heterostyly is characterized by the reciprocal position of male and female sexual organs in two (or three) distinct, usually self-incompatible floral morphs. Conversely, homostyly is characterized by a single, self-compatible floral morph with reduced separation of male and female organs, facilitating selfing. Here, we investigate the origins of homostyly in Primula vulgaris and its microevolutionary consequences by integrating surveys of the frequency of homostyles in natural populations, DNA sequence analyses of the gene controlling the position of female sexual organs (CYPᵀ), and microsatellite genotyping of both progeny arrays and natural populations characterized by varying frequencies of homostyles. As expected, we found that homostyles displace short-styled individuals, but long-style morphs are maintained at low frequencies within populations. We also demonstrated that homostyles repeatedly evolved from short-styled individuals in association with different types of loss-of-function mutations in CYPᵀ. Additionally, homostyly triggers a shift to selfing, promoting increased inbreeding within and genetic differentiation among populations. Our results elucidate the causes and consequences of repeated transitions to homostyly within species, and the putative mechanisms precluding its fixation in P. vulgaris. This study represents a benchmark for future analyses of losses of heterostyly in other angiosperms.

The S-Gene YUC6 Pleiotropically Determines Male Mating Type and Pollen Size in Heterostylous Turnera (Passifloraceae): A Novel Neofunctionalization of the YUCCA Gene Family

In heterostylous, self-incompatible Turnera species, a member of the YUCCA gene family, YUC6, resides at the S-locus and has been hypothesized to determine the male mating type. YUCCA gene family members synthesize the auxin, indole-3-acetic acid, via a two-step process involving the TAA gene family. Consequently, it has been speculated that differences in auxin concentration in developing anthers are the biochemical basis underlying the male mating type. Here, we provide empirical evidence that supports this hypothesis. Using a transgenic knockdown approach, we show that YUC6 acts pleiotropically to control both the male physiological mating type and pollen size, but not the filament length dimorphism associated with heterostyly in Turnera. Using qPCR to assess YUC6 expression in different transgenic lines, we demonstrate that the level of YUC6 knockdown correlates with the degree of change observed in the male mating type. Further assessment of YUC6 expression through anther development, in the knockdown lines, suggests that the male mating type is irreversibly determined during a specific developmental window prior to microsporogenesis, which is consistent with the genetically sporophytic nature of this self-incompatibility system. These results represent the first gene controlling male mating type to be characterized in any species with heterostyly.

Unlocking the Hidden Genetic Diversity of Varicosaviruses, the Neglected Plant Rhabdoviruses

The genus Varicosavirus is one of six genera of plant-infecting rhabdoviruses. Varicosaviruses have non-enveloped, flexuous, rod-shaped virions and a negative-sense, single-stranded RNA genome. A distinguishing feature of varicosaviruses, which is shared with dichorhaviruses, is a bi-segmented genome. Before 2017, a sole varicosavirus was known and characterized, and then two more varicosaviruses were identified through high-throughput sequencing in 2017 and 2018. More recently, the number of known varicosaviruses has substantially increased in concert with the extensive use of high-throughput sequencing platforms and data mining approaches. The novel varicosaviruses have revealed not only sequence diversity, but also plasticity in terms of genome architecture, including a virus with a tentatively unsegmented genome. Here, we report the discovery of 45 novel varicosavirus genomes which were identified in publicly available metatranscriptomic data. The identification, assembly, and curation of the raw Sequence Read Archive reads has resulted in 39 viral genome sequences with full-length coding regions and 6 with nearly complete coding regions. The highlights of the obtained sequences include eight varicosaviruses with unsegmented genomes, which are linked to a phylogenetic clade associated with gymnosperms. These findings have resulted in the most complete phylogeny of varicosaviruses to date and shed new light on the phylogenetic relationships and evolutionary landscape of this group of plant rhabdoviruses. Thus, the extensive use of sequence data mining for virus discovery has allowed us to unlock of the hidden genetic diversity of varicosaviruses, the largely neglected plant rhabdoviruses.

Integrated transcriptome and metabolome analyses revealed regulatory mechanisms of flavonoid biosynthesis in Radix Ardisia

Background

Radix Ardisia (Jab Bik Lik Jib) is a common Miao medicine and is widely distributed in the Guizhou region of southern China. The botanical origin of Radix Ardisia includes the dry root and rhizome of Ardisia Crenata Sims (ACS) or Ardisia Crispa (Thunb.) A.DC. (AC), which are closely related species morphologically. However, the secondary metabolites in their roots are different from one another, especially the flavonoids, and these differences have not been thoroughly explored at the molecular level. This project preliminarily identified regulatory molecular mechanisms in the biosynthetic pathways of the flavonoids between ACS and AC using a multi-omics association analysis.

Methods

In this study, we determined the total levels of saponin, flavonoid, and phenolic in Radix Ardisia from different origins. Integrated transcriptome and metabolome analyses were used to identify the differentially expressed genes (DEGs) and differentially expressed metabolites (DEM). We also performed conjoint analyses on DEGs and DEMs to ascertain the degree pathways, and explore the regulation of flavonoid biosynthesis.

Results

The total flavonoid and phenolic levels in ACS were significantly higher than in AC (P < 0.05). There were 17,685 DEGs between ACS vs. AC, 8,854 were upregulated and 8,831 were downregulated. Based on this, we continued to study the gene changes in the flavonoid biosynthesis pathway, and 100 DEGs involving flavonoid biosynthesis were differentially expressed in ACS and AC. We validated the accuracy of the RNA-seq data using qRT-PCR. Metabolomic analyses showed that 11 metabolites were involved in flavonoid biosynthesis including: Naringenin, Luteolin, Catechin, and Quercetin. A conjoint analysis of the genome-wide connection network revealed the differences in the types and levels of flavonoid compounds between ACS and AC. The correlation analysis showed that Naringenin, Luteolin, Catechin, and Quercetin were more likely to be key compounds in the flavonoid biosynthesis pathway also including 4CL, AOMT, CHS, CHI, DFR, F3'5'H, FLS, and LAR.

Conclusions

This study provides useful information for revealing the regulation of flavonoid biosynthesis and the regulatory relationship between metabolites and genes in the flavonoid biosynthesis pathway in Radix Ardisia from different origins.

Transcriptome and Network Analyses of Heterostyly in Turnera subulata Provide Mechanistic Insights: Are S-Loci a Red-Light for Pistil Elongation?

Heterostyly employs distinct hermaphroditic floral morphs to enforce outbreeding. Morphs differ structurally in stigma/anther positioning, promoting cross-pollination, and physiologically blocking self-fertilization. Heterostyly is controlled by a self-incompatibility (S)-locus of a small number of linked S-genes specific to short-styled morph genomes. Turnera possesses three S-genes, namely TsBAHD (controlling pistil characters), TsYUC6, and TsSPH1 (controlling stamen characters). Here, we compare pistil and stamen transcriptomes of floral morphs of T. subulata to investigate hypothesized S-gene function(s) and whether hormonal differences might contribute to physiological incompatibility. We then use network analyses to identify genetic networks underpinning heterostyly. We found a depletion of brassinosteroid-regulated genes in short styled (S)-morph pistils, consistent with hypothesized brassinosteroid-inactivating activity of TsBAHD. In S-morph anthers, auxin-regulated genes were enriched, consistent with hypothesized auxin biosynthesis activity of TsYUC6. Evidence was found for auxin elevation and brassinosteroid reduction in both pistils and stamens of S- relative to long styled (L)-morph flowers, consistent with reciprocal hormonal differences contributing to physiological incompatibility. Additional hormone pathways were also affected, however, suggesting S-gene activities intersect with a signaling hub. Interestingly, distinct S-genes controlling pistil length, from three species with independently evolved heterostyly, potentially intersect with phytochrome interacting factor (PIF) network hubs which mediate red/far-red light signaling. We propose that modification of the activities of PIF hubs by the S-locus could be a common theme in the evolution of heterostyly.