Genomics of the origin and evolution of Citrus.

The genus Citrus, comprising some of the most widely cultivated fruit crops worldwide, includes an uncertain number of species. Here we describe ten natural citrus species, using genomic, phylogenetic and biogeographic analyses of 60 accessions representing diverse citrus germ plasms, and propose that citrus diversified during the late Miocene epoch through a rapid southeast Asian radiation that correlates with a marked weakening of the monsoons. A second radiation enabled by migration across the Wallace line gave rise to the Australian limes in the early Pliocene epoch. Further identification and analyses of hybrids and admixed genomes provides insights into the genealogy of major commercial cultivars of citrus. Among mandarins and sweet orange, we find an extensive network of relatedness that illuminates the domestication of these groups. Widespread pummelo admixture among these mandarins and its correlation with fruit size and acidity suggests a plausible role of pummelo introgression in the selection of palatable mandarins. This work provides a new evolutionary framework for the genus Citrus.

A chromosome-scale reference genome of trifoliate orange (Poncirus trifoliata) provides insights into disease resistance, cold tolerance and genome evolution in Citrus.

Trifoliate orange (Poncirus trifoliata), a deciduous close relative of evergreen Citrus, has important traits for citrus production, including tolerance/resistance to citrus greening disease (Huanglongbing, HLB) and other major diseases, and cold tolerance. It has been one of the most important rootstocks, and one of the most valuable sources of resistance and tolerance genes for citrus. Here we present a high-quality, chromosome-scale genome assembly of P. trifoliata. The 264.9-Mb assembly contains nine chromosomal pseudomolecules with 25 538 protein-coding genes, covering 97.2% of the estimated gene space. Comparative analyses of P. trifoliata and nine Citrus genomes revealed 605 species-specific genes and six rapidly evolving gene families in the P. trifoliata genome. Poncirus trifoliata has evolved specific adaptation in the C-repeat/DREB binding factor (CBF)-dependent and CBF-independent cold signaling pathways to tolerate cold. We identified candidate genes within quantitative trait loci for HLB tolerance, and at the loci for resistance to citrus tristeza virus and citrus nematode. Genetic diversity analysis of Poncirus accessions and Poncirus/Citrus hybrids shows a narrow genetic base in the US germplasm collection, and points to the importance of collecting and preserving more natural genetic variation. Two phenotypically divergent Poncirus accessions are found to be clonally related, supporting a previous conjecture that dwarf Flying Dragon originated as a mutant of a non-dwarfing type. The high-quality genome reveals features and evolutionary insights of Poncirus, and it will serve as a valuable resource for genetic, genomic and molecular research and manipulation in citrus.

Whole-transcriptome analysis of differentially expressed genes in the ray florets and disc florets of Chrysanthemum morifolium.

BACKGROUND: Chrysanthemum morifolium is one of the most important global cut flower and pot plants, and has been cultivated worldwide. However, limited genomic resources are available and the molecular mechanisms involved in the two morphologically distinct floret developmental cycles in chrysanthemum remain unclear. RESULTS: The transcriptomes of chrysanthemum ray florets, disc florets and leaves were sequenced using Illumina paired-end sequencing technology. In total, 16.9 G reads were assembled into 93,138 unigenes with an average length of 738 bp, of which 44,364 unigenes showed similarity to known proteins in the Swissprot or NCBI non-redundant protein databases. Additionally, 26,320, 22,304 and 13,949 unigenes were assigned to 54 gene ontology (GO) categories, 25 EuKaryotic Orthologous Groups (KOG) categories, and 280 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, respectively. A total of 1863 differentially expressed genes (DEGs) (1210 up-regulated and 653 down-regulated) were identified between ray florets and disc florets, including genes encoding transcription factors and protein kinases. GO and KEGG pathway enrichment analyses were performed on the DEGs to identify differences in the biological processes and pathways between ray florets and disc florets. The important regulatory genes controlling flower development and flower organ determination, as well as important functional genes in the anthocyanin biosynthetic pathway, were identified, of which two leucoanthocyanidin dioxygenase-encoding genes showed specific expression in ray florets. Lastly, reverse transcription quantitative PCR was conducted to validate the DEGs identified in our study. CONCLUSIONS: Comparative transcriptome analysis revealed significant differences in patterns of gene expression and signaling pathways between ray florets and disc florets in Chrysanthemum morifolium. This study provided the first step to understanding the molecular mechanism of the differential development of ray florets and disc florets in chrysanthemum, and also provided valuable genomic resources for candidate genes applicable for the breeding of novel varieties in chrysanthemum.

Comprehensive meta-analysis, co-expression, and miRNA nested network analysis identifies gene candidates in citrus against Huanglongbing disease.

BACKGROUND: Huanglongbing (HLB), the most devastating disease of citrus, is associated with infection by Candidatus Liberibacter asiaticus (CaLas) and is vectored by the Asian citrus psyllid (ACP). Recently, the molecular basis of citrus-HLB interactions has been examined using transcriptome analyses, and these analyses have identified many probe sets and pathways modulated by CaLas infection among different citrus cultivars. However, lack of consistency among reported findings indicates that an integrative approach is needed. This study was designed to identify the candidate probe sets in citrus-HLB interactions using meta-analysis and gene co-expression network modelling. RESULTS: Twenty-two publically available transcriptome studies on citrus-HLB interactions, comprising 18 susceptible (S) datasets and four resistant (R) datasets, were investigated using Limma and RankProd methods of meta-analysis. A combined list of 7,412 differentially expressed probe sets was generated using a Teradata in-house Structured Query Language (SQL) script. We identified the 65 most common probe sets modulated in HLB disease among different tissues from the S and R datasets. Gene ontology analysis of these probe sets suggested that carbohydrate metabolism, nutrient transport, and biotic stress were the core pathways that were modulated in citrus by CaLas infection and HLB development. We also identified R-specific probe sets, which encoded leucine-rich repeat proteins, chitinase, constitutive disease resistance (CDR), miraculins, and lectins. Weighted gene co-expression network analysis (WGCNA) was conducted on 3,499 probe sets, and 21 modules with major hub probe sets were identified. Further, a miRNA nested network was created to examine gene regulation of the 3,499 target probe sets. Results suggest that csi-miR167 and csi-miR396 could affect ion transporters and defence response pathways, respectively. CONCLUSION: Most of the potential candidate hub probe sets were co-expressed with gibberellin pathway (GA)-related probe sets, implying the role of GA signalling in HLB resistance. Our findings contribute to the integration of existing citrus-HLB transcriptome data that will help to elucidate the holistic picture of the citrus-HLB interaction. The citrus probe sets identified in this analysis signify a robust set of HLB-responsive candidates that are useful for further validation.

Genome-wide identification, characterisation and expression analysis of the MADS-box gene family in Prunus mume.

MADS-box genes encode transcription factors that play crucial roles in plant development, especially in flower and fruit development. To gain insight into this gene family in Prunus mume, an important ornamental and fruit plant in East Asia, and to elucidate their roles in flower organ determination and fruit development, we performed a genome-wide identification, characterisation and expression analysis of MADS-box genes in this Rosaceae tree. In this study, 80 MADS-box genes were identified in P. mume and categorised into MIKC, Mα, Mß, Mγ and Mδ groups based on gene structures and phylogenetic relationships. The MIKC group could be further classified into 12 subfamilies. The FLC subfamily was absent in P. mume and the six tandemly arranged DAM genes might experience a species-specific evolution process in P. mume. The MADS-box gene family might experience an evolution process from MIKC genes to Mδ genes to Mα, Mß and Mγ genes. The expression analysis suggests that P. mume MADS-box genes have diverse functions in P. mume development and the functions of duplicated genes diverged after the duplication events. In addition to its involvement in the development of female gametophytes, type I genes also play roles in male gametophytes development. In conclusion, this study adds to our understanding of the roles that the MADS-box genes played in flower and fruit development and lays a foundation for selecting candidate genes for functional studies in P. mume and other species. Furthermore, this study also provides a basis to study the evolution of the MADS-box family.

A comparative analysis of characteristic floral scent compounds in Prunus mume and related species.

In order to investigate the difference in their characteristic floral scents between Prunus mume Siebold & Zucc. and the related Prunus species, their headspace volatiles and endogenous extraction were analyzed by gas chromatography-mass spectrometry. The efficiency of substrate utilization of the flowers was studied by incubating them with different alcohol substrates. Our results indicated that benzyl acetate is a dominant compound influencing the characteristic floral scent of P. mume. An alcohol substrate concentration of 4 mmol L(-1) and a reaction time of 2 h were constituted the reaction condition for catalysis of exogenous alcohol substrates by the flowers. Under these conditions, Prunus sibirica exhibited the highest utilization efficiency for benzyl alcohol substrate while the utilization efficiency of Prunus persica was the lowest. Comparative analysis of several alcohol substrates indicated that the flowers of the tested species had selective specificity for benzyl alcohol substrates.

The functional role of L-fucose on dendritic cell function and polarization.

Despite significant advances in the development and refinement of immunotherapies administered to combat cancer over the past decades, a number of barriers continue to limit their efficacy. One significant clinical barrier is the inability to mount initial immune responses towards the tumor. As dendritic cells are central initiators of immune responses in the body, the elucidation of mechanisms that can be therapeutically leveraged to enhance their functions to drive anti-tumor immune responses is urgently needed. Here, we report that the dietary sugar L-fucose can be used to enhance the immunostimulatory activity of dendritic cells (DCs). L-fucose polarizes immature myeloid cells towards specific DC subsets, specifically cDC1 and moDC subsets. In vitro, L-fucose treatment enhances antigen uptake and processing of DCs. Furthermore, our data suggests that L-fucose-treated DCs increase stimulation of T cell populations. Consistent with our functional assays, single-cell RNA sequencing of intratumoral DCs from melanoma- and breast tumor-bearing mice confirmed transcriptional regulation and antigen processing as pathways that are significantly altered by dietary L-fucose. Together, this study provides the first evidence of the ability of L-fucose to bolster DC functionality and provides rational to further investigate how L-fucose can be used to leverage DC function in order to enhance current immunotherapy.

Genome-wide identification and analysis of late embryogenesis abundant (LEA) genes in Prunus mume.

Late embryogenesis abundant (LEA) proteins play important roles in plant desiccation tolerance. In this study, 30 LEA genes were identified from Chinese plum (Prunus mume) through genome-wide analysis. The PmLEA genes are distributed on all Chinese plum chromosomes except chromosome 3. Twelve (40 %) and five PmLEA genes are arranged in tandem and segmental duplications, respectively. The PmLEA genes could be divided into eight groups (LEA_1, LEA_2, LEA_3, LEA_4, LEA_5, PvLEA18, dehydrin and seed maturation protein). Ten gene conversion events were observed and most of them (70 %) were identified in dehydrin group. Most PmLEA genes were highly expressed in flower (22/30) and up-regulated by ABA treatment (19/30).

RBFOX2 deregulation promotes pancreatic cancer progression and metastasis through alternative splicing.

RNA splicing is an important biological process associated with cancer initiation and progression. However, the contribution of alternative splicing to pancreatic cancer (PDAC) development is not well understood. Here, we identify an enrichment of RNA binding proteins (RBPs) involved in splicing regulation linked to PDAC progression from a forward genetic screen using Sleeping Beauty insertional mutagenesis in a mouse model of pancreatic cancer. We demonstrate downregulation of RBFOX2, an RBP of the FOX family, promotes pancreatic cancer progression and liver metastasis. Specifically, we show RBFOX2 regulates exon splicing events in transcripts encoding proteins involved in cytoskeletal remodeling programs. These exons are differentially spliced in PDAC patients, with enhanced exon skipping in the classical subtype for several RBFOX2 targets. RBFOX2 mediated splicing of ABI1, encoding the Abelson-interactor 1 adapter protein, controls the abundance and localization of ABI1 protein isoforms in pancreatic cancer cells and promotes the relocalization of ABI1 from the cytoplasm to the periphery of migrating cells. Using splice-switching antisense oligonucleotides (AONs) we demonstrate the ABI1 ∆Ex9 isoform enhances cell migration. Together, our data identify a role for RBFOX2 in promoting PDAC progression through alternative splicing regulation.

Gain of Chromosome 1q Perturbs a Competitive Endogenous RNA Network to Promote Melanoma Metastasis.

Somatic copy-number alterations (CNA) promote cancer, but the underlying driver genes may not be comprehensively identified if only the functions of the encoded proteins are considered. mRNAs can act as competitive endogenous RNAs (ceRNA), which sponge miRNAs to posttranscriptionally regulate gene expression in a protein coding-independent manner. We investigated the contribution of ceRNAs to the oncogenic effects of CNAs. Chromosome 1q gains promoted melanoma progression and metastasis at least in part through overexpression of three mRNAs with ceRNA activity: CEP170, NUCKS1, and ZC3H11A. These ceRNAs enhanced melanoma metastasis by sequestering tumor suppressor miRNAs. Orthogonal genetic assays with miRNA inhibitors and target site blockers, along with rescue experiments, demonstrated that miRNA sequestration is critical for the oncogenic effects of CEP170, NUCKS1, and ZC3H11A mRNAs. Furthermore, chromosome 1q ceRNA-mediated miRNA sequestration alleviated the repression of several prometastatic target genes. This regulatory RNA network was evident in other cancer types, suggesting chromosome 1q ceRNA deregulation as a common driver of cancer progression. Taken together, this work demonstrates that ceRNAs mediate the oncogenicity of somatic CNAs. SIGNIFICANCE: The function of CEP170, NUCKS1, and ZC3H11A mRNAs as competitive endogenous RNAs that sequester tumor suppressor microRNAs underlies the oncogenic activity of chromosome 1q gains.

Genome-Wide Association Study of Healthful Flavonoids among Diverse Mandarin Accessions.

Mandarins have many unique flavonoids with documented health benefits and that help to prevent chronic human diseases. Flavonoids are difficult to measure and cannot be phenotyped without the use of specialized equipment; consequently, citrus breeders have not used flavonoid contents as selection criteria to develop cultivars with increased benefits for human health or increased tolerance to diseases. In this study, peel, pulp, and seed samples collected from many mandarin accessions and their hybrids were analyzed for the presence of selected flavonoids with documented human health benefits. A genome-wide association study (GWAS) was used to identify SNPs associated with biosynthesis of flavonoids in these mandarin accessions, and there were 420 significant SNPs were found to be associated with 28 compounds in peel, pulp, or seed samples. Four candidate genes involved in flavonoid biosynthesis were identified by enrichment analysis. SNPs that were found to be associated with compounds in pulp samples have the potential to be used as markers to select mandarins with improved phytonutrient content to benefit human health. Mandarin cultivars bred with increased flavonoid content may provide value to growers and consumers.

Comprehensive analysis of the GALACTINOL SYNTHASE (GolS) gene family in citrus and the function of CsGolS6 in stress tolerance.

Galactinol synthase (GolS) catalyzes the first and rate-limiting step in the synthesis of raffinose family of oligosaccharides (RFOs), which serve as storage and transport sugars, signal transducers, compatible solutes and antioxidants in higher plants. The present work aimed to assess the potential functions of citrus GolS in mechanisms of stress response and tolerance. By homology searches, eight GolS genes were found in the genomes of Citrus sinensis and C. clementina. Phylogenetic analysis showed that there is a GolS ortholog in C. clementina for each C. sinensis GolS, which have evolved differently from those of Arabidopsis thaliana. Transcriptional analysis indicated that most C. sinensis GolS (CsGolS) genes show a low-level tissue-specific and stress-inducible expression in response to drought and salt stress treatments, as well as to 'Candidatus Liberibacter asiaticus' infection. CsGolS6 overexpression resulted in improved tobacco tolerance to drought and salt stresses, contributing to an increased mesophyll cell expansion, photosynthesis and plant growth. Primary metabolite profiling revealed no significant changes in endogenous galactinol, but different extents of reduction of raffinose in the transgenic plants. On the other hand, a significant increase in the levels of metabolites with antioxidant properties, such as ascorbate, dehydroascorbate, alfa-tocopherol and spermidine, was observed in the transgenic plants. These results bring evidence that CsGolS6 is a potential candidate for improving stress tolerance in citrus and other plants.

Racial and ethnic differences in clonal hematopoiesis, tumor markers, and outcomes of patients with multiple myeloma.

Multiple myeloma (MM) incidence, mortality, and survival vary by race and ethnicity, but the causes of differences remain unclear. We investigated demographic, clinical, and molecular features of diverse MM patients to elucidate mechanisms driving clinical disparities. This study included 495 MM patients (self-reported Hispanic, n = 45; non-Hispanic Black, n = 52; non-Hispanic White, n = 398). Hispanic and non-Hispanic Black individuals had an earlier age of onset than non-Hispanic White individuals (53 and 57 vs 63 years, respectively, P < .001). There were no differences in treatment by race and ethnicity groups, but non-Hispanic Black patients had a longer time to hematopoietic cell transplant than non-Hispanic White patients (376 days vs 248 days; P = .01). Overall survival (OS) was improved for non-Hispanic Black compared with non-Hispanic White patients (HR, 0.50; 95% CI, 0.31-0.81; P = .005), although this association was attenuated after adjusting for clinical features (HR, 0.62; 95% CI, 0.37-1.03; P = .06). Tumor mutations in IRF4 were most common in Hispanic patients, and mutations in SP140, AUTS2, and SETD2 were most common in non-Hispanic Black patients. Differences in tumor expression of BCL7A, SPEF2, and ANKRD26 by race and ethnicity were observed. Clonal hematopoiesis was detected in 12% of patients and associated with inferior OS in non-Hispanic Black patients compared with patients without clonal hematopoiesis (HR, 4.36; 95% CI, 1.36-14.00). This study provides insight into differences in molecular features that may drive clinical disparities in MM patients receiving comparable treatment, with the novel inclusion of Hispanic individuals.

Proteogenomic, Epigenetic, and Clinical Implications of Recurrent Aberrant Splice Variants in Clear Cell Renal Cell Carcinoma.

BACKGROUND: Alternative mRNA splicing can be dysregulated in cancer, resulting in the generation of aberrant splice variants (SVs). Given the paucity of actionable genomic mutations in clear cell renal cell carcinoma (ccRCC), aberrant SVs may be an avenue to novel mechanisms of pathogenesis. OBJECTIVE: To identify and characterize aberrant SVs enriched in ccRCC. DESIGN, SETTING, AND PARTICIPANTS: Using RNA-seq data from the Cancer Cell Line Encyclopedia, we identified neojunctions uniquely expressed in ccRCC. Candidate SVs were then checked for expression across normal tissue in the Genotype-Tissue Expression Project and primary tumor tissue from The Cancer Genome Atlas (TCGA), Clinical Proteomic Tumor Analysis Consortium (CPTAC), and our institutional Total Cancer Care database. OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS: Clinicopathologic, genomic, and survival data were available for all cohorts. Epigenetic data were available for the TCGA and CPTAC cohorts. Proteomic data were available for the CPTAC cohort. The association of aberrant SV expression with these variables was examined using the Kruskal-Wallis test, pairwise t test, Spearman correlation test, and Cox regression analysis. RESULTS AND LIMITATIONS: Our pipeline identified 16 ccRCC-enriched SVs. EGFR, HPCAL1-SV and RNASET2-SV expression was negatively correlated with gene-specific CpG methylation. We derived a survival risk score based primarily on the expression of five SVs (RNASET2, FGD1, PDZD2, COBLL1, and PTPN14), which was consistent and applicable across multiple cohorts on multivariate analysis. The splicing factor RBM4, which modulates splicing of HIF-1α, exhibited significantly lower expression at the protein level in the high-risk group, as defined by our SV-based score. CONCLUSIONS: We describe 16 aberrant SVs enriched in ccRCC, many of which are associated with disease biology and/or clinical outcomes. This study provides a novel strategy for identifying and characterizing disease-specific aberrant SVs. PATIENT SUMMARY: We describe a method to identify disease targets and biomarkers using transcriptomic analysis beyond somatic mutations or gene expression. Kidney tumors express unique splice variants that may provide additional prognostic information following surgery.

Correlating Immune Cell Infiltration Patterns with Recurrent Somatic Mutations in Advanced Clear Cell Renal Cell Carcinoma.

BACKGROUND: Clear cell renal cell carcinoma (ccRCC) tumors have low frequencies of genetic alterations compared with other malignancies, but very high levels of immune cell infiltration and favorable response rates to immunotherapy. Currently, the interplay between specific ccRCC somatic mutations and immune infiltration pattern is unclear. OBJECTIVE: To analyze the associations between common ccRCC somatic mutations and immune cell infiltration patterns within the tumor immune microenvironment (TIME). DESIGN, SETTING, AND PARTICIPANTS: The study included tumor samples (24 primary and 24 metastatic) from 48 patients with stage IV ccRCC. Targeted sequencing was performed for well-characterized recurrent somatic mutations in ccRCC, with the analysis focusing on the six most common ones: VHL, BAP1, PBRM1, SETD2, TP53, and KDM5C. For each sample, multiplex immunofluorescence (IF) was performed in lymphoid and myeloid panels, for seven regions of interest in three zones (tumor core, stroma, and tumor-stroma interface). IF-derived cellular densities were compared across patients, stratified by their somatic mutation status, using a linear mixed-model analysis. External validation was pursued using RNA-seq enrichment scoring from three large external data sources. RESULTS AND LIMITATIONS: Tumors with SETD2 mutations demonstrated significantly decreased levels of FOXP3+ T cells in the tumor core, stroma, and tumor-stroma interface. PBRM1 mutations were associated with decreased FOXP3+ T cells in the tumor core. Primary KDM5C mutations were associated with significantly increased CD206+ macrophage tumor infiltration in the tumor core. A computational method estimating immune cell types in the TIME using bulk RNA-seq data, xCell scoring, failed to validate associations from the IF analysis in large external data sets. A major limitation of the study is the relatively small patient population studied. CONCLUSIONS: This study provides evidence that common somatic mutations in ccRCC, such as SETD2, PBRM1, and KDM5C, are associated with distinct immune infiltration patterns within the TIME. PATIENT SUMMARY: In this study, we analyzed tumor samples from patients with metastatic kidney cancer to determine whether common genetic mutations that arise from the cancer cells are associated with the density of immune cells found within those tumors. We found several distinct immune cell patterns that were associated with specific genetic mutations. These findings provide insight into the interaction between cancer genetics and the immune system in kidney cancer.

Characterizing batch effects and binding site-specific variability in ChIP-seq data.

Multiple sources of variability can bias ChIP-seq data toward inferring transcription factor (TF) binding profiles. As ChIP-seq datasets increase in public repositories, it is now possible and necessary to account for complex sources of variability in ChIP-seq data analysis. We find that two types of variability, the batch effects by sequencing laboratories and differences between biological replicates, not associated with changes in condition or state, vary across genomic sites. This implies that observed differences between samples from different conditions or states, such as cell-type, must be assessed statistically, with an understanding of the distribution of obscuring noise. We present a statistical approach that characterizes both differences of interests and these source of variability through the parameters of a mixed effects model. We demonstrate the utility of our approach on a CTCF binding dataset composed of 211 samples representing 90 different cell-types measured across three different laboratories. The results revealed that sites exhibiting large variability were associated with sequence characteristics such as GC-content and low complexity. Finally, we identified TFs associated with high-variance CTCF sites using TF motifs documented in public databases, pointing the possibility of these being false positives if the sources of variability are not properly accounted for.

Phloem Regeneration Is a Mechanism for Huanglongbing-Tolerance of "Bearss" Lemon and "LB8-9" Sugar Belle® Mandarin.

Huanglongbing (HLB) is an extremely destructive and lethal disease of citrus worldwide, presumably caused by phloem-limited bacteria, Candidatus Liberibacter asiaticus (CLas). The widespread invasiveness of the HLB pathogen and lack of natural HLB-resistant citrus cultivars have underscored the need for identifying tolerant citrus genotypes to support the current citrus industry's survival and potentially to lead to future natural HLB resistance. In this study, transverse sections of leaf lamina and midribs were examined with light and epifluorescence microscopy to determine anatomical characteristics that underlie HLB-tolerant mechanisms operating among "Bearss" lemon, "LB8-9" Sugar Belle® mandarin, and its sibling trees compared with HLB-sensitive "Valencia" sweet orange. The common anatomical aberrations observed in all CLas-infected varieties are as follows: phloem necrosis, hypertrophic phloem parenchyma cells, phloem plugging with abundant callose depositions, phloem collapse with cell wall distortion and thickening, excessive starch accumulation, and sometimes even cambium degeneration. Anatomical distribution of starch accumulation even extended to tracheid elements. Although there were physical, morphological, and pathological similarities in the examined foliage, internal structural preservation in "Bearss" lemon and "LB8-9" Sugar Belle® mandarin was superior compared with HLB-sensitive "Valencia" sweet orange and siblings of "LB8-9" Sugar Belle® mandarin. Intriguingly, there was substantial phloem regeneration in the tolerant types that may compensate for the dysfunctional phloem, in comparison with the sensitive selections. The lower levels of phloem disruption, together with greater phloem regeneration, are two key elements that contribute to HLB tolerance in diverse citrus cultivars.

Construction of High-Density Genetic Maps and Detection of QTLs Associated With Huanglongbing Tolerance in Citrus.

Huanglongbing (HLB), or citrus greening, is the most devastating disease in citrus worldwide. Commercial citrus varieties including sweet orange (Citrus sinensis) are highly susceptible to HLB, and trifoliate orange (Poncirus trifoliata, a close Citrus relative) is widely considered resistant or highly tolerant to HLB. In this study, an intergeneric F1 population of sweet orange and trifoliate orange was genotyped by Genotyping-by-Sequencing, and high-density SNP-based genetic maps were constructed separately for trifoliate orange and sweet orange. The two genetic maps exhibited high synteny and high coverage of the citrus genome. Progenies of the F1 population and their parents were planted in a replicated field trial, exposed to intense HLB pressure for 3 years, and then evaluated for susceptibility to HLB over 2 years. The F1 population exhibited a wide range in severity of HLB foliar symptom and canopy damage. Genome-wide QTL analysis based on the phenotypic data of foliar symptom and canopy damage in 2 years identified three clusters of repeatable QTLs in trifoliate orange linkage groups LG-t6, LG-t8 and LG-t9. Co-localization of QTLs for two traits was observed within all three regions. Additionally, one cluster of QTLs in sweet orange (linkage group LG-s7) was also detected. The majority of the identified QTLs each explained 18-30% of the phenotypic variation, indicating their major role in determining HLB responses. These results show, for the first time, a quantitative genetic nature yet the presence of major loci for the HLB tolerance in trifoliate orange. The results suggest that sweet orange also contains useful genetic factor(s) for improving HLB tolerance in commercial citrus varieties. Findings from this study should be very valuable and timely to researchers worldwide as they are hastily searching for genetic solutions to the devastating HLB crisis through breeding, genetic engineering, or genome editing.

Overexpression of Prunus mume Dehydrin Genes in Tobacco Enhances Tolerance to Cold and Drought.

Dehydrins, known as group 2 or D-11 family late-embryogenesis-abundant (LEA) proteins, play important roles in plant growth and stress tolerance. Six dehydrin genes were previously identified from the genome of Prunus mume. In this study, five of them (PmLEA8, PmLEA10, PmLEA19, PmLEA20, and PmLEA29) were cloned from cold-resistant P. mume 'Beijingyudie'. Real-time RT-PCR analysis indicated that all these genes could be up-regulated by one or several treatments (ABA, SA, low temperature, high temperature, PEG, and NaCl treatments). The results of spot assay demonstrated that the expression of all these dehydrins, except PmLEA8, conferred improved osmotic and freezing-resistance to the recombinant Escherichia coli. So four dehydrin genes, PmLEA10, PmLEA19, PmLEA20 and PmLEA29 were chosen for individual over-expression in tobacco plants. The transgenic tobacco plants showed lower relative content of malondialdehyde, relative electrolyte leakage and higher relative content of water than control plants when exposed to cold and drought stress. These results demonstrated that PmLEAs were involved in plant responses to cold and drought.

Reprogramming of a defense signaling pathway in rough lemon and sweet orange is a critical element of the early response to 'Candidatus Liberibacter asiaticus'.

Huanglongbing (HLB) in citrus infected by Candidatus Liberibacter asiaticus (CLas) has caused tremendous losses to the citrus industry. No resistant genotypes have been identified in citrus species or close relatives. Among citrus varieties, rough lemon (Citrus jambhiri) has been considered tolerant due to its ability to produce a healthy flush of new growth after infection. The difference between tolerance and susceptibility is often defined by the speed and intensity of a plant's response to a pathogen, especially early defense responses. RNA-seq data were collected from three biological replicates of CLas- and mock-inoculated rough lemon and sweet orange at week 0 and 7 following infection. Functional analysis of the differentially expressed genes (DEGs) indicated that genes involved in the mitogen activated protein kinase (MAPK) signaling pathway were highly upregulated in rough lemon. MAPK induces the transcription of WRKY and other transcription factors which potentially turn on multiple defense-related genes. A Subnetwork Enrichment Analysis further revealed different patterns of regulation of several functional categories, suggesting DEGs with different functions were subjected to reprogramming. In general, the amplitude of the expression of defense-related genes is much greater in rough lemon than in sweet orange. A quantitative disease resistance response may contribute to the durable tolerance level to HLB observed in rough lemon.