Genome-wide identification and functional analysis of S-RNase involved in the self-incompatibility of citrus

Transposable elements cause the loss of self-incompatibility in citrus

Self-incompatibility (SI) is a widespread prezygotic mechanism for flowering plants to avoid inbreeding depression and promote genetic diversity. Citrus has an S-RNase-based SI system, which was frequently lost during evolution. We previously identified a single nucleotide mutation in Sm-RNase, which is responsible for the loss of SI in mandarin and its hybrids. However, little is known about other mechanisms responsible for conversion of SI to self-compatibility (SC) and we identify a completely different mechanism widely utilized by citrus. Here, we found a 786-bp miniature inverted-repeat transposable element (MITE) insertion in the promoter region of the FhiS2-RNase in Fortunella hindsii Swingle (a model plant for citrus gene function), which does not contain the Sm-RNase allele but are still SC. We demonstrate that this MITE plays a pivotal role in the loss of SI in citrus, providing evidence that this MITE insertion prevents expression of the S-RNase; moreover, transgenic experiments show that deletion of this 786-bp MITE insertion recovers the expression of FhiS2-RNase and restores SI. This study identifies the first evidence for a role for MITEs at the S-locus affecting the SI phenotype. A family-wide survey of the S-locus revealed that MITE insertions occur frequently adjacent to S-RNase alleles in different citrus genera, but only certain MITEs appear to be responsible for the loss of SI. Our study provides evidence that insertion of MITEs into a promoter region can alter a breeding strategy and suggests that this phenomenon may be broadly responsible for SC in species with the S-RNase system.

The miR159a-DUO1 module regulates pollen development by modulating auxin biosynthesis and starch metabolism in citrus

Achieving seedlessness in citrus varieties is one of the important objectives of citrus breeding. Male sterility associated with abnormal pollen development is an important factor in seedlessness. However, our understanding of the regulatory mechanism underlying the seedlessness phenotype in citrus is still limited. Here, we determined that the miR159a-DUO1 module played an important role in regulating pollen development in citrus, which further indirectly modulated seed development and fruit size. Both the overexpression of csi-miR159a and the knocking out of DUO1 in Hong Kong kumquat (Fortunella hindsii) resulted in small and seedless fruit phenotypes. Moreover, pollen was severely aborted in both transgenic lines, with arrested pollen mitotic I and abnormal pollen starch metabolism. Through additional cross-pollination experiments, DUO1 was proven to be the key target gene for miR159a to regulate male sterility in citrus. Based on DNA affinity purification sequencing (DAP-seq), RNA-seq, and verified interaction assays, YUC2/YUC6, SS4 and STP8 were identified as downstream target genes of DUO1, those were all positively regulated by DUO1. In transgenic F. hindsii lines, the miR159a-DUO1 module down-regulated the expression of YUC2/YUC6, which decreased indoleacetic acid (IAA) levels and modulated auxin signaling to repress pollen mitotic I. The miR159a-DUO1 module reduced the expression of the starch synthesis gene SS4 and sugar transport gene STP8 to disrupt starch metabolism in pollen. Overall, this work reveals a new mechanism by which the miR159a-DUO1 module regulates pollen development and elucidates the molecular regulatory network underlying male sterility in citrus.

Two Secretory T2 RNases Act as Cytotoxic Factors Contributing to the Virulence of an Insect Fungal Pathogen

RNase T2 members are secreted by several pathogens or parasites during infection, playing various roles in pathogen-host interaction. However, functions of those members in biocontrol microbes targeting their hosts are still unknown. Here, we report that an insect fungal pathogen, Beauveria bassiana, produces two secretory RNase T2 members that act as cytotoxic factors, which were examined by insect bioassays using the targeted gene(s) disruption and overexpression strains. Overexpression strains displayed dramatically increased virulence, which was concurrent with few fungal cells and hemocytes in hemocoel, suggesting a cytotoxicity of the overexpressed gene products. In vitro assays using yeast-expressed proteins verified the cytotoxicity of the two members against insect cells, to which the cytotoxic effect was dependent on their RNases enzyme activities and glycosylation modification. Moreover, the excessive humoral immune responses triggered by the two ribonucleases were examined. These results suggested prospects of these two T2 ribonucleases for improvement of biocontrol agents.

Transcriptome Analysis of the Late-Acting Self-Incompatibility Associated with RNase T2 Family in Camellia oleifera

The Camellia oil tree (Camellia oleifera Abel.) is an important nonwood forest species in China, and the majority of its cultivars are late-acting self-incompatibility (LSI) types. Although several studies have examined the mechanism of LSI, the process is quite complicated and unclear. In this study, pollen tube growth and fruit setting of two Camellia oil tree cultivars Huashuo (HS) and Huajin (HJ) were investigated after non and self-pollination, and transcriptomic analysis of the ovaries was performed 48 h after self-pollination to identify the potential genes implicated in the LSI of Camellia oil trees. The results showed that the fruit set of HS was significantly higher than that of HJ after self-pollination. Transcriptomic analysis revealed that plant hormone signal transduction, the phosphatidylinositol signaling system, ATP-binding cassette (ABC) transporters, reactive oxygen species (ROS) metabolism, and Ca2+ signaling were mainly contributed in the LSI of reaction of Camellia oil tree. Moreover, nine RNase T2 genes were identified from the transcriptome analysis, which also showed that CoRNase7 participated in the self-incompatibility reaction in HS. Based on phylogenetic analysis, CoRNase6 was closely related to S-RNase from coffee, and CoRNase7 and CoRNase8 were closely related to S-RNase from Camellia sinensis. The 9 RNase T2 genes successfully produced proteins in prokaryotes. Subcellular localization indicated that CoRNase1 and CoRNase5 were cytoplasmic proteins, while CoRNase7 was a plasma membrane protein. These results screened the main metabolic pathways closely related to LSI in Camellia oil tree, and SI signal transduction might be regulated by a large molecular regulatory network. The discovery of T2 RNases provided evidence that Camellia oil tree might be under RNase-based gametophytic self-incompatibility.

Genome-wide screening of the RNase T2 gene family and functional analyses in jujube (Ziziphus jujuba Mill.)

BACKGROUND

Ribonuclease (RNase T2) plays crucial roles in plant evolution and breeding. However, there have been few studies on the RNase T2 gene family in Ziziphus jujuba Mill., one of important dried fruit tree species. Recently, the released sequences of the reference genome of jujube provide a good chance to perform genome-wide identification and characterization of ZjRNase gene family in the jujube.

RESULTS

In this study, we identified four members of RNase T2 in jujube distributed on three chromosomes and unassembled chromosomes. They all contained two conserved sites (CASI and CASII). Analysis of the phylogenetic relationships revealed that the RNase T2 genes in jujube could be divided into two groups: ZjRNase1 and ZjRNase2 belonged to class I, while ZjRNase3 and ZjRNase4 belonged to class II. Only ZjRNase1 and ZjRNase2 expression were shown by the jujube fruit transcriptome analysis. So ZjRNase1 and ZjRNase2 were selected functional verification by overexpression transformation of Arabidopsis. The overexpression of these two genes led to an approximately 50% reduction in seed number, which deserve further attention. Moreover, the leaves of the ZjRNase1 overexpression transgenic lines were curled and twisted. Overexpression of ZjRNase2 resulted in shortened and crisp siliques and the production of trichomes, and no seeds were produced.

CONCLUSION

In summary, these findings will provide new insights into the molecular mechanisms of low number of hybrid seeds in jujube and a reference for the future molecular breeding of jujube.

The functional analysis of sugar transporter proteins in sugar accumulation and pollen tube growth in pummelo (Citrus grandis)

Sugar transporter proteins (STPs) play vital roles in sugar transport and allocation of carbon sources in plants. However, the evolutionary dynamics of this important gene family and their functions are still largely unknown in citrus, which is the largest fruit crop in the world. In this study, fourteen non-redundant CgSTP family members were identified in pummelo (Citrus grandis). A comprehensive analysis based on the biochemical characteristics, the chromosomal location, the exon-intron structures and the evolutionary relationships demonstrated the conservation and the divergence of CgSTPs. Moreover, CgSTP4, 11, 13, 14 were proofed to be localized in plasma membrane and have glucose transport activity in yeast. The hexose content were significantly increased with the transient overexpression of CgSTP11 and CgSTP14. In addition, antisense repression of CgSTP4 induced the shorter pollen tube length in vitro, implying the potential role of CgSTP4 in pummelo pollen tube growth. Taken together, this work explored a framework for understanding the physiological role of CgSTPs and laid a foundation for future functional studies of these members in citrus species.

The origin and evolution of RNase T2 family and gametophytic self-incompatibility system in plants

Ribonuclease (RNase) T2 genes are found widely in both eukaryotes and prokaryotes, and genes from this family have been revealed to have various functions in plants. In particular, S-RNase is known to be the female determinant in the S-RNase-based gametophytic self-incompatibility (GSI) system. However, the origin and evolution of the RNase T2 gene family and GSI system are not well understood. In this study, 785 RNase T2 genes were identified in 81 sequenced plant genomes representing broad-scale diversity and divided into three subgroups (Class I, II, and III) based on phylogenetic and synteny network analysis. Class I was found to be of ancient origin and to emerge in green algae, Class II was shown to originate with the appearance of angiosperms, while Class III was discovered to be eudicot-specific. Each of the three major classes could be further classified into several subclasses of which some subclasses were found to be lineage-specific. Furthermore, duplication, deletion, or inactivation of the S/S-like-locus was revealed to be linked to repeated loss and gain of self-incompatibility in different species from distantly related plant families with GSI. Finally, the origin and evolutionary history of S-locus in Rosaceae species was unraveled with independent loss and gain of S-RNase occurred in different subfamilies of Rosaceae. Our findings provide insights into the origin and evolution of the RNase T2 family and the GSI system in plants.

Rapid Identification of a Stripe Rust Resistance Gene YrXK in Chinese Wheat Line Xike01015 Using Specific Locus Amplified Fragment (SLAF) Sequencing

Wheat stripe rust, an airborne fungal disease caused by Puccinia striiformis Westend. f. sp. tritici, is one of the most devastating diseases of wheat. Chinese wheat cultivar Xike01015 displays high levels of all-stage resistance (ASR) to the current predominant P. striiformis f. sp. tritici race CYR33. In this study, a single dominant gene, designated YrXk, was identified in Xike01015 conferring resistance to CYR33 with genetic analysis of F₂ and BC₁ populations from a cross of Mingxian169 (susceptible) and Xike01015. The specific length amplified fragment sequencing (SLAF-seq) strategy was used to construct a linkage map in the F₂ population. Quantitative trait loci (QTL) analysis mapped YrXk to a 12.4-Mb segment on chromosome1 BS, explaining >86.96% of the phenotypic variance. Gene annotation in the QTL region identified three differential expressed candidate genes, TraesCS1B02G168600.1, TraesCS1B02G170200.1, and TraesCS1B02G172400.1. The qRT-PCR results showed that TraesCS1B02G172400.1 and TraesCS1B02G168600.1 are upregulated and that TraesCS1B02G170200.1 is slightly downregulated after inoculation with CYR33 in the seedling stage, which indicates that these genes may function in wheat resistance to stripe rust. The results of this study can be used in wheat breeding for improving resistance to stripe rust.

Downregulated expression of S2-RNase attenuates self-incompatibility in "Guiyou No. 1" pummelo

Self-incompatibility (SI) substantially restricts the yield and quality of citrus. Therefore, breeding and analyzing self-compatible germplasm is of great theoretical and practical significance for citrus. Here, we focus on the mechanism of a self-compatibility mutation in 'Guiyou No. 1' pummelo (Citrus maxima), which is a spontaneous mutant of 'Shatian' pummelo (Citrus maxima, self-incompatibility). The rate of fruit set and the growth of pollen tubes in the pistil confirmed that a spontaneous mutation in the pistil is responsible for the self-compatibility of 'Guiyou No. 1'. Segregation ratios of the S genotype in F₁ progeny, expression analysis, and western blotting validated that the reduced levels of S₂-RNase mRNA contribute to the loss of SI in 'Guiyou No. 1'. Furthermore, we report a phased assembly of the 'Guiyou No. 1' pummelo genome and obtained two complete and well-annotated S haplotypes. Coupled with an analysis of SV variations, methylation levels, and gene expression, we identified a candidate gene (CgHB40), that may influence the regulation of the S₂-RNase promoter. Our data provide evidence that a mutation that affects the pistil led to the loss of SI in 'Guiyou No. 1' by influencing a poorly understood mechanism that affects transcriptional regulation. This work significantly advances our understanding of the genetic basis of the SI system in citrus and provides information on the regulation of S-RNase genes.

SKP1-like protein, CrSKP1-e, interacts with pollen-specific F-box proteins and assembles into SCF-type E3 complex in 'Wuzishatangju' (Citrus reticulata Blanco) pollen

S-ribonuclease (S-RNase)-based self-incompatibility (SI) mechanisms have been extensively studied in Solanaceae, Rosaceae and Plantaginaceae. S-RNase-based SI is controlled by two closely related genes, S-RNase and S-locus F-box (SLF), located at a polymorphic S-locus. In the SI system, the SCF-type (SKP1-CUL1-F-box-RBX1) complex functions as an E3 ubiquitin ligase complex for ubiquitination of non-self S-RNase. Pummelo (Citrus grandis) and several mandarin cultivars are suggested to utilize an S-RNase-based SI system. However, the molecular mechanism of the non-S-factors involved in the SI reaction is not straightforward in Citrus. To investigate the SCF-type E3 complex responsible for the SI reaction in mandarin, SLF, SKP1-like and CUL1 candidates potentially involved in the SI reaction of ‘Wuzishatangju’ (Citrus reticulata Blanco) were identified based on the genome-wide identification and expression analyses. Sixteen pollen-specific F-box genes (CrFBX1-CrFBX16), one pollen-specific SKP1-like gene (CrSKP1-e) and two CUL1 genes (CrCUL1A and CrCUL1B) were identified and cloned from ‘Wuzishatangju’. Yeast two-hybrid (Y2H) and in vitro binding assays showed that five CrFBX proteins could bind to CrSKP1-e, which is an ortholog of SSK1 (SLF-interacting-SKP1-like), a non-S-factor responsible for the SI reaction. Luciferase complementation imaging (LCI) and in vitro binding assays also showed that CrSKP1-e interacts with the N-terminal region of both CrCUL1A and CrCUL1B. These results indicate that CrSKP1-e may serve as a functional member of the SCF-type E3 ubiquitin ligase complex in ‘Wuzishatangju’.

Temperature-Dependent Compatible and Incompatible Pollen-Style Interactions in Citrus clementina Hort. ex Tan. Show Different Transglutaminase Features and Polyamine Pattern

In clementine, failure of fertilization can result in parthenocarpic fruit development, which has several advantages, such as seedless fruit, longer shelf-life, and greater consumer appeal. Recently, S-RNases have been identified in Citrus grandis, thus revealing that the self-incompatibility (SI) reaction relies on the S-RNase gametophytic mechanism. The fundamental role of environmental factors, mostly temperature, in determining the numbers of pollen tubes reaching the ovary is also well established in Citrus. In the present work, temperature-dependent pollen-pistil interactions in C. clementina were analyzed, focusing on several morphological aspects, as well as on polyamine (PA) content and the activity and distribution of transglutaminase (TGase), both reported to be involved in the SI response in pear and in pummelo. Results clearly indicate that temperature contributed to a different activation of the SI response, which occurs at optimal temperature of 25°C but was by-passed at 15°C. TGase activity was stimulated during the SI response, and it localized differently in the compatible and incompatible interaction: in compatible pollinated styles, TGase localized inside the style canal, while it was detected all around it in incompatible crosses. TGase localization and activity were congruent with the levels of soluble and insoluble conjugated PAs and with morphological evidences, which highlighted cell wall modification occurring as a result of SI.

Evolution of self-compatibility by a mutant Sm-RNase in citrus

Self-incompatibility (SI) is an important mechanism that prevents self-fertilization and inbreeding in flowering plants. The most widespread SI system utilizes S ribonucleases (S-RNases) and S-locus F-boxes (SLFs) as S determinants. In citrus, SI is ancestral, and Citrus maxima (pummelo) is self-incompatible, while Citrus reticulata (mandarin) and its hybrids are self-compatible (SC). Here, we identify nine highly polymorphic pistil-specific, developmentally expressed S-RNases from pummelo that segregate with S haplotypes in a gametophytic manner and cluster with authentic S-RNases. We provide evidence that these S-RNases function as the female S determinants in citrus. Moreover, we show that each S-RNase is linked to approximately nine SLFs. In an analysis of 117 citrus SLF and SFL-like (SLFL) genes, we reveal that they cluster into 12 types and that the S-RNases and intra-haplotypic SLF and SLFL genes co-evolved. Our data support the notion that citrus have a S locus comprising a S-RNase and several SLFs that fit the non-self-recognition model. We identify a predominant single nucleotide mutation, S_m-RNase, in SC citrus, which provides a 'natural' loss of function. We show that SI-SC transitions due to the S_m-RNase initially arose in mandarin, spreading to its hybrids and became fixed. Identification of an evolutionarily distant new genus utilizing the S-RNase-based SI system, >100 million years separated from the nearest S-RNase family, is a milestone for evolutionary comparative studies.

Transcriptome analysis of callus from melon

OBJECTIVE

To identify the key genes promoting the differentiation of melon non-embryogenic callus into embryogenic callus.

METHODS

The transcriptome sequencing analysis was used to analyze the mRNA sequence in embryogenic callus (Z) and non-embryogenic callus (F); transcript mapping, gene expression analysis, cluster analysis, classification analysis and enrichment analysis were then used to detect the differentially expressed genes and enriched pathways.

RESULTS

The correlation coefficient between sample Z and sample F was 0.929 after transcript mapping. The overall gene expression levels in sample Z were higher as compared with sample F. Furthermore, cluster analysis showed that the expression of genes involved in photosynthesis was increased in sample Z when comparing to F. Besides, the classification of differential Gene Ontology (GO) showed that many metabolic processes were affected with the metabolism enhanced in embryogenic callus. Interestingly, Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis further demonstrated the high metabolic activity and active secondary metabolite formation in the embryogenic callus.

CONCLUSION

The genes associated with photosynthesis, metabolic pathways and biosynthesis of secondary metabolites may promote the differentiation of callus into embryogenic callus.

Reproduction in woody perennial Citrus: an update on nucellar embryony and self-incompatibility

Review on citrus reproduction. Citrus is one of the most important and widely grown fruit crops. It possesses several special reproductive characteristics, such as nucellar embryony and self-incompatibility. The special phenomenon of nucellar embryony in citrus, also known as the polyembryony, is a kind of sporophytic apomixis. During the past decade, the emergence of novel technologies and the construction of multiple citrus reference genomes have facilitated rapid advances to our understanding of nucellar embryony. Indeed, several research teams have preliminarily determined the genetic basis of citrus apomixis. On the other hand, the phenomenon of self-incompatibility that promotes genetic diversity by rejecting self-pollen and accepting non-self-pollen is difficult to study in citrus because the long juvenile period of citrus presents challenges to identifying candidate genes that control this phenomenon. In this review, we focus on advances to our understanding of reproduction in citrus from the last decade and discuss priorities for the coming decade.