Jujube witches' broom phytoplasma effectors SJP1 and SJP2 induce lateral bud outgrowth by repressing the ZjBRC1-controlled auxin efflux channel

Sequencing and comparative analyses of 'Candidatus Phytoplasma solani' genomes reveal diversity of effectors and potential mobile units

Phytoplasmas (genus 'Candidatus Phytoplasma') encompass a group of uncultivated bacteria affecting numerous plant species and causing significant damage in agriculture worldwide. They have a dual parasitic cycle, including colonization of both plant phloem and insect cells. Their genomes are small, diverse, repetitive, prone to rearrangements and harbour transposon-like elements known as potential mobile units (PMUs). In the Euro-Mediterranean region, 'Ca. P. solani' is an important species due to its broad range of plant hosts and insect vectors. To provide insights into the genomic diversity of this species, particularly the repertoire of putative effectors and PMUs, this study conducted genome sequencing and analyses of two 'Ca. P. solani' strains originating from different plants and transmitted by different insects. Based on de novo assembly, we obtained 19 contigs totalling 656 141 bp for strain STOL and 28 contigs totalling 707 036 bp for strain ST19. The prevalence of repetitive sequences and PMUs contributed to the fragmentation of these draft assemblies. The annotation identified 28 and 26 genes that encode putative secreted proteins in these two strains, respectively, including several homologues of previously characterized phytoplasma effectors. Our comparative analyses further identified species- and strain-specific genes. Frequently, genes that encode putative secreted proteins and effectors were found within PMU-like regions in both genomes. Moreover, strain STOL showed characteristics of a more reduced genome, having fewer PMU-like repetitive elements and genome rearrangements, while strain ST19 exhibited a higher level of sequence divergence in its PMU genes. The high levels of genomic diversity among 'Ca. P. solani' strains suggested rapid evolution of this species, which may contribute to its wide host range and adaptability potential. This study provides novel data on the diversification of 'Ca. P. solani' genomes. These results provide a foundation for future functional studies of putative effectors and their interactions with host targets, which could facilitate deciphering the pathogenicity strategies of this successful and versatile pathogen.

The Efficacy of Orange Terpene and Bacillus mycoides Strain BM103 on the Control of Periwinkle Leaf Yellowing Phytoplasma

Phytoplasmas are obligate phytopathogenic bacteria belonging to the class Mollicutes. The pathogens, transmitted by insect vectors, are associated with hundreds of plant diseases worldwide. Because of the regulations banning the use of antibiotics and the limited efficacy of traditional disease management manners, an eco-friendly alternative is needed. Given that terpene and probiotics have antibiotic activity and the ability to induce systemic resistance, in this study, the effectiveness of orange terpene and a Bacillus mycoides strain, BM103, was evaluated in periwinkle plants infected with periwinkle leaf yellowing (PLY) phytoplasma derived from a shoot-tip tissue culture system. Weekly drenching of 1,000 ppm diluted orange terpene emulsion or preactivated strain BM103 liquid culture dilution exhibited the ability to inhibit PLY phytoplasma accumulation. The expression of the genes associated with plant defense response and flower development was upregulated after treatment. Moreover, pretreatment of orange terpene or strain BM103 delayed PLY infection via cleft-grafting inoculation. Although orange terpene did not suppress the symptoms, strain BM103 did result in a milder symptom expression that might partially attribute to its plant growth-promoting characteristics. Additionally, the preactivation of strain BM103 may contribute to its efficacy. Taken together, this research indicates that orange terpene and B. mycoides BM103, with the ability to rapidly induce plant defense responses, could potentially be developed into biologic control materials as preventive agents or biofertilizers.

Unraveling Morphological, Physiological, and Transcriptomic Alterations Underlying the Formation of Little Leaves in Phytoplasma-Infected Sweet Cherry Trees

Phytoplasmas are minute phytopathogenic bacteria that induce excessive vegetative growth, known as witches' broom (WB), in many infected plant species during the later stages of infection. The WB structure is characterized by densely clustered little (small) leaves, which are frequently accompanied by chlorosis (yellowing). The mechanisms behind the formation of little leaves within WB structures (LL-WB) are poorly understood. To address this gap, the LL-WB formation was extensively studied using sweet cherry virescence (SCV) phytoplasma-infected sweet cherry plants. Based on morphological examinations, signs of premature leaf senescence were observed in LL-WB samples, including reduced leaf size, chlorosis, and alterations in shape. Subsequent physiological analyses indicated decreased sucrose and glucose levels and changes in hormone concentrations in LL-WB samples. Additionally, the transcriptomic analysis revealed impaired ribosome biogenesis and DNA replication. As an essential process in protein production, the compromised ribosome biogenesis and the inhibited DNA replication led to cell cycle arrest, thus affecting leaf morphogenesis and further plant development. Moreover, the expression of marker genes involved in premature leaf senescence was significantly altered. These results indicate a complicated interplay between the development of leaves, premature leaf senescence, and pathogen-induced stress responses in SCV phytoplasma-infected sweet cherry trees. The results of this study provide insight into understanding the underlying molecular mechanisms driving the formation of little leaves and interactions between plants and pathogens. The findings might help control phytoplasma diseases in sweet cherry cultivation.

Decoding Plant-Pathogen Interactions: A Comprehensive Exploration of Effector-Plant Transcription Factor Dynamics

In the coevolutionary process between plant pathogens and hosts, pathogen effectors, primarily proteinaceous, engage in interactions with host proteins, such as plant transcription factors (TFs), during the infection process. This review delves into the intricate interplay between TFs and effectors, a key aspect in the prolonged and complex battle between plants and pathogens. Effectors strategically manipulate TFs using diverse tactics. These include modulating activity of TFs, influencing their incorporation into multimeric complexes, directly changing TF expression levels, promoting their degradation via the ubiquitin-proteasome system, and inducing their subcellular relocalization. The review systematically presents documented interactions, elucidating key mechanisms and their profound impact on host-pathogen dynamics. It emphasises the central role of TFs in plant defence and investigates the convergent evolution of effectors targeting TFs. By providing this overview, we offer valuable insights into this dynamic interaction landscape and suggest potential directions for future research.

The effector PHYL1JWB from Candidatus Phytoplasma ziziphi induces abnormal floral development by destabilising flower development proteins

Phytoplasmas can induce complex and substantial phenotypic changes in their hosts in ways that favour their colonisation, but the mechanisms underlying these changes remain largely unknown. Jujube witches' broom (JWB) disease is a typical phytoplasma disease causing great economic loss in Chinese jujube (Ziziphus jujuba Mill.). Here, we reported an effector, PHYL1JWB from Candidatus Phytoplasma ziziphi, which implicated in inducing abnormal floral organogenesis. Utilising a combination of in vivo and in vitro methods, we investigated the influence of PHYL1JWB on the proteins associated with floral development. Our findings reveal that PHYL1JWB facilitates the proteasome-mediated degradation of essential flower morphogenetic regulators, including AP1, SEP1, SEP2, SEP3, SEP4, CAL, and AGL6, through a distinctive pathway that is dependent on the activity of the 26S proteasome, thus obviating the requirement for lysine ubiquitination of the substrates. Further, the Y2H analysis showed that the leucine at position 75th in second α helix of PHYL1JWB is fundamental for the interactions of PHYL1JWB with AP1 and SEP1-4 in jujube and Arabidopsis. Our research carry profound implications for elucidating the contribution of PHYL1JWB to the aberrant floral development in diseased jujube, and help to establish a robust theoretical underpinning for the prophylaxis and therapy of JWB disease.

The effector SJP3 interferes with pistil development by sustaining SHORT VEGETATIVE PHASE 3 expression in jujube

Jujube witches' broom (JWB) is a phytoplasma disease that causes severe damage to jujube (Ziziphus jujuba) crops worldwide. Diseased jujube plants show enhanced vegetative growth after floral reversion, including leafy flower structures (phyllody) and the fourth whorl converting into a vegetative shoot. In previous research, secreted JWB protein 3 (SJP3) was identified as an inducer of phyllody. However, the molecular mechanisms of SJP3-mediated pistil reversion remain unknown. Here, the effector SJP3 was found to interact with the MADS-box protein SHORT VEGETATIVE PHASE 3 (ZjSVP3). ZjSVP3 was expressed in young leaves and during the initial flower bud differentiation of healthy jujube-bearing shoots but was constitutively expressed in JWB phytoplasma-infected flowers until the later stage of floral development. The SJP3 effector showed the same expression pattern in the diseased buds and promoted ZjSVP3 accumulation in SJP3 transgenic jujube calli. The N-terminal domains of ZjSVP3 contributed to its escape from protein degradation in the presence of SJP3. Heterologous expression of ZjSVP3 in Nicotiana benthamiana produced typical pistil abnormalities, including trichome-enriched style and stemlike structures within the leaflike ovary, which were consistent with those in the mildly malformed lines overexpressing SJP3. Furthermore, ectopic expression of ZjSVP3 directly bound to the zinc finger protein 8 (ZjZFP8) and MADS-box gene SHATTERPROOF 1 (ZjSHP1) promoters to regulate their expression, resulting in abnormal pistil development. Overall, effector SJP3-mediated derepression of ZjSVP3 sustained its expression to interfere with pistil development, providing insight into the mechanisms of pistil reversion caused by JWB phytoplasma in specific perennial woody plant species.

Mechanism of zju-miR156c-mediated network in regulating witches' broom symptom of Chinese jujube

Jujube witches' broom, caused by phytoplasma, is a destructive disease of Chinese jujube. Studies have shown that zju-miR156s play an important role in phytoplasma infection in jujube, but the regulatory mechanism between zju-miR156c and witches' broom remains unexplored. In the current study, miRNA-seq and gene expression analysis showed that zju-miR156c was more highly induced in infected jujube plants than the other miRNAs and its target gene was ZjSPL3. In addition, the expression levels of thymidylate kinase gene (TMKJWB) and secreted jujube protein (SJP1JWB) in diseased materials were higher than those in healthy controls. The expression level of zju-miR156c was significantly upregulated, while ZjSPL3 was sharply downregulated and the content of cytokinin (CTK) significantly increased. Overexpression of zju-miR156c in Arabidopsis significantly reduced the expression of AtSPL10 (homologous gene of ZjSPL3) but increased the content of CTK, and the transgenic plants exhibited witches' broom symptoms. In addition, yeast two-hybrid and co-immunoprecipitation assays confirmed that SJP1JWB interacted with ZjERF18. Yeast one-hybrid analysis showed that ZjERF18 could interact with the promoter of zju-MIR156c. In conclusion, our results demonstrated a novel pathogenic module of ZjERF18-zju-miR156c-ZjSPL3-CTK has an important function in the formation of witches' broom caused by SJP1JWB.

Analyzes of pan-genome and resequencing atlas unveil the genetic basis of jujube domestication

Jujube (Ziziphus jujuba Mill.), belonging to the Rhamnaceae family, is gaining increasing prominence as a perennial fruit crop with significant economic and medicinal values. Here, we conduct de novo assembly of four reference-grade genomes, encompassing one wild and three cultivated jujube accessions. We present insights into the population structure, genetic diversity, and genomic variations within a diverse collection of 1059 jujube accessions. Analyzes of the jujube pan-genome, based on our four assemblies and four previously released genomes, reveal extensive genomic variations within domestication-associated regions, potentially leading to the discovery of a candidate gene that regulates flowering and fruit ripening. By leveraging the pan-genome and a large-scale resequencing population, we identify two candidate genes involved in domestication traits, including the seed-setting rate, the bearing-shoot length and the leaf size in jujube. These genomic resources will accelerate evolutionary and functional genomics studies of jujube.

ZmABF4-ZmVIL2/ZmFIP37 module enhances drought tolerance in maize seedlings

Drought, as a primary environmental factor, imposes significant constraints on developmental processes and productivity of plants. PHDs were identified as stress-responsive genes in a wide range of eukaryotes. However, the regulatory mechanisms governing PHD genes in maize under abiotic stress conditions are still largely unknown and require further investigation. Here, we identified a mutant, zmvil2, in the EMS mutant library with a C to T mutation in the exon of the Zm00001d053875 (VIN3-like protein 2, ZmVIL2), resulting in premature termination of protein coding. ZmVIL2 belongs to PHD protein family. Compared to WT, zmvil2 mutant exhibited increased sensitivity to drought stress. Consistently, overexpression of ZmVIL2 enhances drought resistance in maize. Y2H, BiFC, and Co-IP experiments revealed that ZmVIL2 directly interacts with ZmFIP37 (FKBP12-interacting protein of 37). zmfip37 knockout mutants also exhibit decreased drought tolerance. Interestingly, we demonstrated that ZmABF4 directly binds to the ZmVIL2 promoter to enhance its activity in yeast one hybrid (Y1H), electrophoretic mobility shift assay (EMSA) and dual luciferase reporter assays. Therefore, we uncovered a novel model ZmABF4-ZmVIL2/ZmFIP37 that promotes drought tolerance in maize. Overall, these findings have enriched the knowledge of the functions of PHD genes in maize and provides genetic resources for breeding stress-tolerant maize varieties.

The 'Candidatus phytoplasma ziziphi' effectors SJP1 and SJP2 destabilise the bifunctional regulator ZjTCP7 to modulate floral transition and shoot branching

Phytoplasmic SAP11 effectors alter host plant architecture and flowering time. However, the exact mechanisms have yet to be elucidated. Two SAP11-like effectors, SJP1 and SJP2, from 'Candidatus Phytoplasma ziziphi' induce shoot branching proliferation. Here, the transcription factor ZjTCP7 was identified as a central target of these two effectors to regulate floral transition and shoot branching. Ectopic expression of ZjTCP7 resulted in enhanced bolting and earlier flowering than did the control. Interaction and expression assays demonstrated that ZjTCP7 interacted with the ZjFT-ZjFD module, thereby enhancing the ability of these genes to directly bind to the ZjAP1 promoter. The effectors SJP1 and SJP2 unravelled the florigen activation complex by specifically destabilising ZjTCP7 and ZjFD to delay floral initiation. Moreover, the shoot branching of the ZjTCP7-SRDX transgenic Arabidopsis lines were comparable to those of the SJP1/2 lines, suggesting the involvement of ZjTCP7 in the regulation of shoot branching. ZjTCP7 interacted with the branching repressor ZjBRC1 to enhance suppression of the auxin efflux carrier ZjPIN3 expression. ZjTCP7 also directly bound to and upregulated the auxin biosynthesis gene ZjYUCCA2, thereby promoting auxin accumulation. Our findings confirm that ZjTCP7 serves as a bifunctional regulator destabilised by the effectors SJP1 and SJP2 to modulate plant development.

MeGT2.6 increases cellulose synthesis and active gibberellin content to promote cell enlargement in cassava

Cassava, a pivotal tropical crop, exhibits rapid growth and possesses a substantial biomass. Its stem is rich in cellulose and serves as a crucial carbohydrate storage organ. The height and strength of stems restrict the mechanised operation and propagation of cassava. In this study, the triple helix transcription factor MeGT2.6 was identified through yeast one-hybrid assay using MeCesA1pro as bait, which is critical for cellulose synthesis. Over-expression and loss-of-function lines were generated, and results revealed that MeGT2.6 could promote a significant increase in the plant height, stem diameter, cell size and thickness of SCW of cassava plant. Specifically, MeGT2.6 upregulated the transcription activity of MeGA20ox1 and downregulated the expression level of MeGA2ox1, thereby enhancing the content of active GA3, resulting in a large cell size, high plant height and long stem diameter in cassava. Moreover, MeGT2.6 upregulated the transcription activity of MeCesA1, which promoted the synthesis of cellulose and hemicellulose and produced a thick secondary cell wall. Finally, MeGT2.6 could help supply additional substrates for the synthesis of cellulose and hemicellulose by upregulating the invertase genes (MeNINV1/6). Thus, MeGT2.6 was found to be a multiple regulator; it was involved in GA metabolism and sucrose decomposition and the synthesis of cellulose and hemicellulose.

The 'Candidatus Phytoplasma ziziphi' effectors SJP1/2 negatively control leaf size by stabilizing the transcription factor ZjTCP2 in jujube

Phytoplasmas manipulate host plant development to benefit insect vector colonization and their own invasion. However, the virulence factors and mechanisms underlying small-leaf formation caused by jujube witches' broom (JWB) phytoplasmas remain largely unknown. Here, effectors SJP1 and SJP2 from JWB phytoplasmas were identified to induce small-leaf formation in jujube (Ziziphus jujuba). In vivo interaction and expression assays showed that SJP1 and SJP2 interacted with and stabilized the transcription factor ZjTCP2. Overexpression of SJP1 and SJP2 in jujube induced ZjTCP2 accumulation. In addition, the abundance of miRNA319f_1 was significantly reduced in leaves of SJP1 and SJP2 transgenic jujube plants and showed the opposite pattern to the expression of its target, ZjTCP2, which was consistent with the pattern in diseased leaves. Overexpression of ZjTCP2 in Arabidopsis promoted ectopic leaves arising from the adaxial side of cotyledons and reduced leaf size. Constitutive expression of the miRNA319f_1 precursor in the 35S::ZjTCP2 background reduced the abundance of ZjTCP2 mRNA and reversed the cotyledon and leaf defects in Arabidopsis. Therefore, these observations suggest that effectors SJP1 and SJP2 induced small-leaf formation, at least partly, by interacting with and activating ZjTCP2 expression both at the transcriptional and the protein level, providing new insights into small-leaf formation caused by phytoplasmas in woody plants.

The crotonylated and succinylated proteins of jujube involved in phytoplasma-stress responses

BACKGROUND

Protein posttranslational modifications (PTMs) are fast and early responses to environmental changes, including pathogen infection. Jujube witches' broom (JWB) is a phytoplasma disease causing great economic loss in jujube production. After phytoplasma infection, the transcriptional, translational, and metabolic levels in jujube were activated, enabling it to survive during phytoplasma invasion. However, no study has yet reported on PTMs in jujube. Lysine crotonylation (Kcr) and lysine succinylation (Ksu) have been popular studies in recent years and their function in plant phytoplasma-stress responses remains unclear.

RESULTS

Here, 1656 crotonylated and 282 succinylated jujube proteins were first identified under phytoplasma-stress, of which 198 were simultaneously crotonylated and succinylated. Comparative analysis revealed that 656 proteins, 137 crotonylated and 43 succinylated proteins in jujube were regulated by phytoplasma infection, suggesting that Kcr was more universal than Ksu. Kcr differentially expressed proteins (DEPs) were related to ribosomes, photosynthetic and carbon metabolism, while Ksu DEPs were mainly involved in carbon metabolism, the TCA cycle and secondary metabolite biosynthesis. The crosstalk network among proteome, crotonylome and succinylome showed that DEPs related to ribosomal, peroxidases and glutathione redox were enriched. Among them, ZjPOD51 and ZjPHGPX2 significantly increased at the protein and Kcr level under phytoplasma-stress. Notably, 7 Kcr sites were identified in ZjPHGPX2, a unique antioxidant enzyme. After inhibitor nicotinamide (NAM) treatment, GPX enzyme activity in jujube seedlings was reduced. Further, site-directed mutagenesis of key Kcr modification sites K130 and/or K135 in ZjPHGPX2 significantly reduced its activity.

CONCLUSIONS

This study firstly provided large-scale datasets of Kcr and Ksu in phytoplasma-infected jujube and revealed that Kcr modification in ZjPHGPX2 positively regulates its activity.

Phytoplasma: A plant pathogen that cannot be ignored in agricultural production-Research progress and outlook

Phytoplasmas are phloem-restricted plant-pathogenic bacteria transmitted by insects. They cause diseases in a wide range of host plants, resulting in significant economic and ecological losses worldwide. Research on phytoplasmas has a long history, with significant progress being made in the past 30 years. Notably, with the rapid development of phytoplasma research, scientists have identified the primary agents involved in phytoplasma transmission, established classification and detection systems for phytoplasmas, and 243 genomes have been sequenced and assembled completely or to draft quality. Multiple possible phytoplasma effectors have been investigated, elucidating the molecular mechanisms by which phytoplasmas manipulate their hosts. This review summarizes recent advances in phytoplasma research, including identification techniques, host range studies, whole- or draft-genome sequencing, effector pathogenesis and disease control methods. Additionally, future research directions in the field of phytoplasma research are discussed.

Jujube Witches' Broom Phytoplasma Effector Zaofeng3, a Homologous Effector of SAP54, Induces Abnormal Floral Organ Development and Shoot Proliferation

Plant-pathogenic phytoplasmas secrete specific virulence proteins into a host plant to modulate plant function for their own benefit. Identification of phytoplasmal effectors is a key step toward clarifying the pathogenic mechanisms of phytoplasma. In this study, Zaofeng3, also known as secreted jujube witches' broom phytoplasma protein 3 (SJP3), was a homologous effector of SAP54 and induced a variety of abnormal phenotypes, such as phyllody, malformed floral organs, witches' broom, and dwarfism in Arabidopsis thaliana. Zaofeng3 can also induce small leaves, dwarfism, and witches' broom in Ziziphus jujuba. Further experiments showed that the three complete α-helix domains predicted in Zaofeng3 were essential for induction of disease symptoms in jujube. Yeast two-hybrid library screening showed that Zaofeng3 mainly interacts with proteins involved in flower morphogenesis and shoot proliferation. Bimolecular fluorescence complementation assays confirmed that Zaofeng3 interacted with these proteins in the whole cell. Overexpression of zaofeng3 in jujube shoot significantly altered the expression patterns of ZjMADS19, ZjMADS47, ZjMADS48, ZjMADS77, and ZjTCP7, suggesting that overexpressing zaofeng3 might induce floral organ malformation and witches' broom by altering the expression of the transcriptional factors involved in jujube morphogenesis.

'Candidatus Phytoplasma ziziphi' Changes the Metabolite Composition of Jujube Tree Leaves and Affects the Feeding Behavior of Its Insect Vector Hishimonus hamatus Kuoh

Hishimonus hamatus Kuoh is a leafhopper species native to China that feeds on Chinese jujube leaves. This leafhopper species has been verified to transmit jujube witches' broom (JWB) disease, caused by phytoplasma, a fatal plant pathogen, which belongs to the phytoplasma subgroup 16SrV-B. The transmission of JWB phytoplasma largely relies on the feeding behavior of piercing-sucking leafhoppers. However, the specific mechanisms behind how and why the infection of JWB influences the feeding behavior of these leafhoppers are not fully understood. To address this, a study was conducted to compare the feeding patterns of H. hamatus when feeding JWB-infested jujube leaves to healthy leaves using the electrical penetration graph (EPG) technique. Then, a widely targeted metabolome analysis was performed to identify differences in the metabolite composition of JWB-infected jujube leaves and that of healthy jujube leaves. The results of EPG analyses revealed that when feeding on JWB-infected jujube leaves, H. hamatus exhibited an increased frequency of phloem ingestion and spent longer in the phloem feeding phase compared to when feeding on healthy leaves. In addition, the results of metabolomic analyses showed that JWB-infected leaves accumulated higher levels of small-molecular carbohydrates, free amino acids, and free fatty acids, as well as lower levels of lignans, coumarins and triterpenoids compared to healthy leaves. The above results indicated that the H. hamatus preferentially fed on the phloem of infected leaves, which seems to be linked to the transmission of the JWB phytoplasma. The results of metabolomic analyses partially imply that the chemical compounds might play a role in making the infected leaves more attractive to H. hamatus for feeding.

Jujube witches' broom phytoplasmas inhibit ZjBRC1-mediated abscisic acid metabolism to induce shoot proliferation

Jujube witches' broom (JWB) phytoplasmas parasitize the sieve tubes of diseased phloem and cause an excessive proliferation of axillary shoots from dormant lateral buds to favour their transmission. In previous research, two JWB effectors, SJP1 and SJP2, were identified to induce lateral bud outgrowth by disrupting ZjBRC1-mediated auxin flux. However, the pathogenesis of JWB disease remains largely unknown. Here, tissue-specific transcriptional reprogramming was examined to gain insight into the genetic mechanisms acting inside jujube lateral buds under JWB phytoplasma infection. JWB phytoplasmas modulated a series of plant signalling networks involved in lateral bud development and defence, including auxin, abscisic acid (ABA), ethylene, jasmonic acid, and salicylic acid. JWB-induced bud outgrowth was accompanied by downregulation of ABA synthesis within lateral buds. ABA application rescued the bushy appearances of transgenic Arabidopsis overexpressing SJP1 and SJP2 in Col-0 and ZjBRC1 in the brc1-2 mutant. Furthermore, the expression of ZjBRC1 and ABA-related genes ZjHB40 and ZjNCED3 was negatively correlated with lateral main bud outgrowth in decapitated healthy jujube. Molecular evidence showed that ZjBRC1 interacted with ZjBRC2 via its N-terminus to activate ZjHB40 and ZjNCED3 expression and ABA accumulation in transgenic jujube calli. In addition, ZjBRC1 widely regulated differentially expressed genes related to ABA homeostasis and ABA signalling, especially by binding to and suppressing ABA receptors. Therefore, these results suggest that JWB phytoplasmas hijack the ZjBRC1-mediated ABA pathways to stimulate lateral bud outgrowth and expansion, providing a strategy to engineer plants resistant to JWB phytoplasma disease and regulate woody plant architecture to promote crop yield and quality.

Candidate pathogenicity factor/effector proteins of 'Candidatus Phytoplasma solani' modulate plant carbohydrate metabolism, accelerate the ascorbate-glutathione cycle, and induce autophagosomes

The pathogenicity of intracellular plant pathogenic bacteria is associated with the action of pathogenicity factors/effectors, but their physiological roles for most phytoplasma species, including 'Candidiatus Phytoplasma solani' are unknown. Six putative pathogenicity factors/effectors from six different strains of 'Ca. P. solani' were selected by bioinformatic analysis. The way in which they manipulate the host cellular machinery was elucidated by analyzing Nicotiana benthamiana leaves after Agrobacterium-mediated transient transformation with the pathogenicity factor/effector constructs using confocal microscopy, pull-down, and co-immunoprecipitation, and enzyme assays. Candidate pathogenicity factors/effectors were shown to modulate plant carbohydrate metabolism and the ascorbate-glutathione cycle and to induce autophagosomes. PoStoSP06, PoStoSP13, and PoStoSP28 were localized in the nucleus and cytosol. The most active effector in the processes studied was PoStoSP06. PoStoSP18 was associated with an increase in phosphoglucomutase activity, whereas PoStoSP28, previously annotated as an antigenic membrane protein StAMP, specifically interacted with phosphoglucomutase. PoStoSP04 induced only the ascorbate-glutathione cycle along with other pathogenicity factors/effectors. Candidate pathogenicity factors/effectors were involved in reprogramming host carbohydrate metabolism in favor of phytoplasma own growth and infection. They were specifically associated with three distinct metabolic pathways leading to fructose-6-phosphate as an input substrate for glycolysis. The possible significance of autophagosome induction by PoStoSP28 is discussed.

Nuclear effectors of plant pathogens: Distinct strategies to be one step ahead

Nuclear effector proteins released by bacteria, oomycete, nematode, and fungi burden the global environment and crop yield. Microbial effectors are key weapons in the evolutionary arms race between plants and pathogens, vital in determining the success of pathogenic colonization. Secreted effectors undermine a multitude of host cellular processes depending on their target destination. Effectors are classified by their localization as either extracellular (apoplastic) or intracellular. Intracellular effectors can be further subclassified by their compartment such as the nucleus, cytoplasm or chloroplast. Nuclear effectors bring into question the role of the plant nucleus' intrinsic defence strategies and their vulnerability to effector-based manipulation. Nuclear effectors interfere with multiple nuclear processes including the epigenetic regulation of the host chromatin, the impairment of the trans-kingdom antifungal RNAi machinery, and diverse classes of immunity-associated host proteins. These effector-targeted pathways are widely conserved among different hosts and regulate a broad array of plant cellular processes. Thus, these nuclear sites constitute meaningful targets for effectors to subvert the plant defence system and acquire resources for pathogenic propagation. This review provides an extensive and comparative compilation of diverse plant microbe nuclear effector libraries, thereby highlighting the distinct and conserved mechanisms these effectors employ to modulate plant cellular processes for the pathogen's profit.

Identification of High Tolerance to Jujube Witches' Broom in Indian Jujube (Ziziphus mauritiana Lam.) and Mining Differentially Expressed Genes Related to the Tolerance through Transcriptome Analysis

The jujube witches' broom (JWB) disease is a severe threat to jujube trees, with only a few cultivars being genuinely tolerant or resistant to phytoplasma. The defense mechanism of jujube trees against phytoplasma is still unclear. In this study, we aimed to investigate the tolerance mechanism of Indian jujube 'Cuimi' to JWB and identify the key genes that contribute to JWB high tolerance. Based on the symptoms and phytoplasma concentrations after infection, we confirmed the high tolerance of 'Cuimi' to JWB. Comparative transcriptome analysis was subsequently performed between 'Cuimi' and 'Huping', a susceptible cultivar of Chinese jujube. Unique gene ontology (GO) terms were identified in 'Cuimi', such as protein ubiquitination, cell wall biogenesis, cell surface receptor signaling pathway, oxylipin biosynthetic process, and transcription factor activity. These terms may relate to the normal development and growth of 'Cuimi' under phytoplasma infection. We identified 194 differential expressed genes related to JWB high tolerance, involved in various processes, such as reactive oxygen species (ROS), Ca²⁺ sensors, protein kinases, transcription factors (TFs), lignin, and hormones. Calmodulin-like (CML) genes were significantly down-regulated in infected 'Cuimi'. We speculated that the CML gene may act as a negative regulatory factor related to JWB high tolerance. Additionally, the cinnamoyl-CoA reductase-like SNL6 gene was significantly up-regulated in infected 'Cuimi', which may cause lignin deposition, limit the growth of phytoplasma, and mediate immune response of 'Cuimi' to phytoplasma. Overall, this study provides insights into the contribution of key genes to the high tolerance of JWB in Indian jujube 'Cuimi'.

Strigolactone regulates adventitious root formation via the MdSMXL7-MdWRKY6-MdBRC1 signaling cascade in apple

Propagation through stem cuttings is a popular method worldwide for species such as fruit tree rootstocks and forest trees. Adventitious root (AR) formation from stem cuttings is crucial for effective and successful clonal propagation of apple rootstocks. Strigolactones (SLs) are newly identified hormones involved in AR formation. However, the regulatory mechanisms underpinning this process remain elusive. In the present study, weighted gene co-expression network analysis, as well as rooting assays using stable transgenic apple materials, revealed that MdBRC1 served as a key gene in the inhibition of AR formation by SLs. We have demonstrated that MdSMXL7 and MdWRKY6 synergistically regulated MdBRC1 expression, depending on the interactions of MdSMXL7 and MdWRKY6 at the protein level downstream of SLs as well as the direct promoter binding on MdBRC1 by MdWRKY6. Furthermore, biochemical studies and genetic analysis revealed that MdBRC1 inhibited AR formation by triggering the expression of MdGH3.1 in a transcriptional activation pathway. Finally, the present study not only proposes a component, MdWRKY6, that enables MdSMXL7 to regulate MdBRC1 during the process of SL-controlled AR formation in apple, but also provides prospective target genes to enhance AR formation capacity using CRISPR (i.e. clustered regularly interspaced short palindromic repeats) technology, particularly in woody plants.

Pathogen-triggered changes in plant development: Virulence strategies or host defense mechanism?

Plants, as sessile organisms, are constantly exposed to pathogens in nature. Plants rely on physical barriers, constitutive chemical defenses, and sophisticated inducible immunity to fight against pathogens. The output of these defense strategies is highly associated with host development and morphology. Successful pathogens utilize various virulence strategies to colonize, retrieve nutrients, and cause disease. In addition to the overall defense-growth balance, the host-pathogen interactions often lead to changes in the development of specific tissues/organs. In this review, we focus on recent advances in understanding the molecular mechanisms of pathogen-induced changes in plants' development. We discuss that changes in host development could be a target of pathogen virulence strategies or an active defense strategy of plants. Current and ongoing research about how pathogens shape plant development to increase their virulence and causes diseases could give us novel views on plant disease control.

The 'Candidatus Phytoplasma mali' effector protein SAP11CaPm interacts with MdTCP16, a class II CYC/TB1 transcription factor that is highly expressed during phytoplasma infection

'Candidatus Phytoplasma mali', is a bacterial pathogen associated with the so-called apple proliferation disease in Malus × domestica. The pathogen manipulates its host with a set of effector proteins, among them SAP11CaPm, which shares similarity to SAP11AYWB from 'Candidatus Phytoplasma asteris'. SAP11AYWB interacts and destabilizes the class II CIN transcription factors of Arabidopsis thaliana, namely AtTCP4 and AtTCP13 as well as the class II CYC/TB1 transcription factor AtTCP18, also known as BRANCHED1 being an important factor for shoot branching. It has been shown that SAP11CaPm interacts with the Malus × domestica orthologues of AtTCP4 (MdTCP25) and AtTCP13 (MdTCP24), but an interaction with MdTCP16, the orthologue of AtTCP18, has never been proven. The aim of this study was to investigate this potential interaction and close a knowledge gap regarding the function of SAP11CaPm. A Yeast two-hybrid test and Bimolecular Fluorescence Complementation in planta revealed that SAP11CaPm interacts with MdTCP16. MdTCP16 is known to play a role in the control of the seasonal growth of perennial plants and an increase of MdTCP16 gene expression has been detected in apple leaves in autumn. In addition to this, MdTCP16 is highly expressed during phytoplasma infection. Binding of MdTCP16 by SAP11CaPm might lead to the induction of shoot proliferation and early bud break, both of which are characteristic symptoms of apple proliferation disease.

The Complete Genome of the “Flavescence Dorée” Phytoplasma Reveals Characteristics of Low Genome Plasticity

Members of the genus ‘Candidatus Phytoplasma’ are obligate intracellular bacteria restricted to phloem sieve elements and are able to colonize several tissues and the hemolymph in their insect vectors. The current unfeasibility of axenic culture and the low complexity of genomic sequences are obstacles in assembling complete chromosomes. Here, a method combining pathogen DNA enrichment from infected insects and dual deep-sequencing technologies was used to obtain the complete genome of a phytoplasma causing Grapevine Flavescence dorée. The de novo assembly generated a circular chromosome of 654,223 bp containing 506 protein-coding genes. Quality assessment of the draft showed a high degree of completeness. Comparative analysis with other phytoplasmas revealed the absence of potential mobile units and a reduced amount of putative phage-derived segments, suggesting a low genome plasticity. Phylogenetic analyses identified Candidatus Phytoplasma ziziphi as the closest fully sequenced relative. The “Flavescence dorée” phytoplasma strain CH genome also encoded for several putative effector proteins potentially playing a role in pathogen virulence. The availability of this genome provides the basis for the study of the pathogenicity mechanisms and evolution of the Flavescence dorée phytoplasma.

The Jujube TCP Transcription Factor ZjTCP16 Regulates Plant Growth and Cell Size by Affecting the Expression of Genes Involved in Plant Morphogenesis

Jujube production is threatened by jujube witches’ broom (JWB) disease, which is caused by JWB phytoplasma. The jujube TCP transcription factor (TF) ZjTCP16 may be involved in the interaction of jujube plants with JWB phytoplasma. In this study, qRT-PCR proved that the expression pattern of ZjTCP16 was altered by JWB phytoplasma. The gene functions of ZjTCP16 were analyzed by its overexpression in Arabidopsis and jujube, as well as knock-down in. The overexpression of ZjTCP16 in Arabidopsis and jujube resulted in dwarfism and small leaves, while the zjtcp16 CRISPR mutants were higher than the WT. Microscopic observation of paraffin sections of jujube stems showed that ZjTCP16 affected the size of cells. The interactions of ZjTCP16 with ZjAS2 and ZjLOB in both the cytoplasm and nucleus were demonstrated by yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays. Yeast one-hybrid (Y1H) assays and qRT-PCR further confirmed that ZjTCP16 affected the expression of genes involved in leaf morphogenesis and cell proliferation (ZjAS1, ZjKNAT1, ZjKNAT2 and ZjKNAT6) at the mRNA level through the ZjAS2 and ZjLOB pathways. In conclusion, ZjTCP16 regulates plant growth and cell size by altering the expression pattern of morphogenesis-related genes in jujube.

Comparative Genome Analysis of ‘Candidatus Phytoplasma luffae’ Reveals the Influential Roles of Potential Mobile Units in Phytoplasma Evolution

Phytoplasmas are insect-transmitted plant pathogens that cause substantial losses in agriculture. In addition to economic impact, phytoplasmas induce distinct disease symptoms in infected plants, thus attracting attention for research on molecular plant-microbe interactions and plant developmental processes. Due to the difficulty of establishing an axenic culture of these bacteria, culture-independent genome characterization is a crucial tool for phytoplasma research. However, phytoplasma genomes have strong nucleotide composition biases and are repetitive, which make it challenging to produce complete assemblies. In this study, we utilized Illumina and Oxford Nanopore sequencing technologies to obtain the complete genome sequence of ‘Candidatus Phytoplasma luffae’ strain NCHU2019 that is associated with witches’ broom disease of loofah (Luffa aegyptiaca) in Taiwan. The fully assembled circular chromosome is 769 kb in size and is the first representative genome sequence of group 16SrVIII phytoplasmas. Comparative analysis with other phytoplasmas revealed that NCHU2019 has a remarkably repetitive genome, possessing a pair of 75 kb repeats and at least 13 potential mobile units (PMUs) that account for ∼25% of its chromosome. This level of genome repetitiveness is exceptional for bacteria, particularly among obligate pathogens with reduced genomes. Our genus-level analysis of PMUs demonstrated that these phytoplasma-specific mobile genetic elements can be classified into three major types that differ in gene organization and phylogenetic distribution. Notably, PMU abundance explains nearly 80% of the variance in phytoplasma genome sizes, a finding that provides a quantitative estimate for the importance of PMUs in phytoplasma genome variability. Finally, our investigation found that in addition to horizontal gene transfer, PMUs also contribute to intra-genomic duplications of effector genes, which may provide redundancy for subfunctionalization or neofunctionalization. Taken together, this work improves the taxon sampling for phytoplasma genome research and provides novel information regarding the roles of mobile genetic elements in phytoplasma evolution.