A Conserved Basal Transcription Factor Is Required for the Function of Diverse TAL Effectors in Multiple Plant Hosts

Comprehensive Characterization of the C3HC4 RING Finger Gene Family in Potato (Solanum tuberosum L.): Insights into Their Involvement in Anthocyanin Biosynthesis

The C3HC4 RING finger gene (RING-HC) family is a zinc finger protein crucial to plant growth. However, there have been no studies on the RING-HC gene family in potato. In this study, 77 putative StRING-HCs were identified in the potato genome and grouped into three clusters based on phylogenetic relationships, the chromosome distribution, gene structure, conserved motif, gene duplication events, and synteny relationships, and cis-acting elements were systematically analyzed. By analyzing RNA-seq data of potato cultivars, the candidate StRING-HC genes that might participate in tissue development, abiotic stress, especially drought stress, and anthocyanin biosynthesis were further determined. Finally, a StRING-HC gene (Soltu.DM.09G017280 annotated as StRNF4-like), which was highly expressed in pigmented potato tubers was focused on. StRNF4-like localized in the nucleus, and Y2H assays showed that it could interact with the anthocyanin-regulating transcription factors (TFs) StbHLH1 of potato tubers, which is localized in the nucleus and membrane. Transient assays showed that StRNF4-like repressed anthocyanin accumulation in the leaves of Nicotiana tabacum and Nicotiana benthamiana by directly suppressing the activity of the dihydroflavonol reductase (DFR) promoter activated by StAN1 and StbHLH1. The results suggest that StRNF4-like might repress anthocyanin accumulation in potato tubers by interacting with StbHLH1. Our comprehensive analysis of the potato StRING-HCs family contributes valuable knowledge to the understanding of their functions in potato development, abiotic stress, hormone signaling, and anthocyanin biosynthesis.

TALEs as double-edged swords in plant-pathogen interactions: Progress, challenges, and perspectives

Xanthomonas species colonize many host plants and cause huge losses worldwide. Transcription activator-like effectors (TALEs) are secreted by Xanthomonas and translocated into host cells to manipulate the expression of target genes, especially by Xanthomonas oryzae pv. oryzae and Xanthomonas oryzae pv. oryzicola, which cause bacterial blight and bacterial leaf streak, respectively, in rice. In this review, we summarize the progress of studies on the interaction between Xanthomonas and hosts, covering both rice and other plants. TALEs are not only key factors that make plants susceptible but are also essential components of plant resistance. Characterization of TALEs and TALE-like proteins has improved our understanding of TALE evolution and promoted the development of gene editing tools. In addition, the interactions between TALEs and hosts have also provided strategies and possibilities for genetic engineering in crop improvement.

TAL effectors and the predicted host targets of pomegranate bacterial blight pathogen Xanthomonas citri pv. punicae

The molecular mechanism of pomegranate susceptibility to bacterial blight, a serious threat to pomegranate production in India, is largely unknown. In the current study, we have used PacBio and Illumina sequencing of Xanthomonas citri pv. punicae (Xcp) strain 119 genome to identify tal genes and RNA-Seq analysis to identify putative host targets in the susceptible pomegranate variety Bhagwa challenged with Xcp119. Xcp119 genome encodes seven transcription activator-like effectors (TALEs), three of which are harbored by a plasmid. RVD-based phylogenetic analysis of TALEs of Xanthomonas citri pathovars indicate the TALEs of Xcp as evolutionarily and functionally close to Xanthomonas citri pv. malvacearum and Xanthomonas citri pv. glycines. Comparative RNA-Seq of Xcp and mock-inoculated leaf tissues revealed Xcp-induced pomegranate transcription modulation. The prediction of TALE binding elements (EBEs) in the promoters of up-regulated genes identified a set of TALE-targeted candidate genes in pomegranate-Xcp interaction. The predicted candidate susceptibility genes include two oxoglutarate-dependent dioxygenase gene, ethylene-responsive transcription factor and flavanone 3-hydroxylase-like gene, and the further characterization of these would enable blight resistance engineering in pomegranate.

Base Editors for Citrus Gene Editing

Base editors, such as adenine base editors (ABE) and cytosine base editors (CBE), provide alternatives for precise genome editing without generating double-strand breaks (DSBs), thus avoiding the risk of genome instability and unpredictable outcomes caused by DNA repair. Precise gene editing mediated by base editors in citrus has not been reported. Here, we have successfully adapted the ABE to edit the TATA box in the promoter region of the canker susceptibility gene LOB1 from TATA to CACA in grapefruit (Citrus paradise) and sweet orange (Citrus sinensis). TATA-edited plants are resistant to the canker pathogen Xanthomonas citri subsp. citri (Xcc). In addition, CBE was successfully used to edit the acetolactate synthase (ALS) gene in citrus. ALS-edited plants were resistant to the herbicide chlorsulfuron. Two ALS-edited plants did not show green fluorescence although the starting construct for transformation contains a GFP expression cassette. The Cas9 gene was undetectable in the herbicide-resistant citrus plants. This indicates that the ALS edited plants are transgene-free, representing the first transgene-free gene-edited citrus using the CRISPR technology. In summary, we have successfully adapted the base editors for precise citrus gene editing. The CBE base editor has been used to generate transgene-free citrus via transient expression.

TALE-triggered and iTALE-suppressed Xa1-mediated resistance to bacterial blight is independent of rice transcription factor subunits OsTFIIAγ1 or OsTFIIAγ5

Xa1-mediated resistance to rice bacterial blight, caused by Xanthomonas oryzae pv. oryzae (Xoo), is triggered by transcription activator-like effectors (TALEs) and suppressed by interfering TALEs (iTALEs). TALEs interact with the rice transcription factor OsTFIIAγ1 or OsTFIIAγ5 (Xa5) to activate expression of target resistance and/or susceptibility genes. However, it is not clear whether OsTFIIAγ is involved in TALE-triggered and iTALE-suppressed Xa1-mediated resistance. In this study, genome-edited mutations in OsTFIIAγ5 or OsTFIIAγ1 of Xa1-containing rice 'IRBB1' and Xa1-transgenic plants of xa5-containing rice 'IRBB5' did not impair the activation or suppression of Xa1-mediated resistance. Correspondingly, the expression pattern of Xa1 in mutated OsTFIIAγ5 and OsTFIIAγ1 rice lines and 'IRBB1' rice was similar. In contrast, the expression of OsSWEET11 was repressed in rice lines mutated in OsTFIIAγ5 and OsTFIIAγ1. Bimolecular fluorescence complementation (BiFC) and co-immunoprecipitation assays showed that both TALE PthXo1 and iTALE Tal3a interacted with OsTFIIAγ1 and OsTFIIAγ5 in plant nuclei. These results indicated that TALE-triggered and iTALE-suppressed Xa1-mediated resistance to bacterial blight is independent of OsTFIIAγ1 or OsTFIIAγ5 in rice, and suggest that an unknown factor is potentially involved in the interaction of Xa1, TALEs and iTALEs.

Targeted Delivery for Neurodegenerative Disorders Using Gene Therapy Vectors: Gene Next Therapeutic Goals

The technique of gene therapy, ever since its advent nearly fifty years ago, has been utilized by scientists as a potential treatment option for various disorders. This review discusses some of the major neurodegenerative diseases (NDDs) like Alzheimer's disease (AD), Parkinson's Disease (PD), Motor neuron diseases (MND), Spinal Muscular Atrophy (SMA), Huntington's Disease (HD), Multiple Sclerosis (MS), etc. and their underlying genetic mechanisms along with the role that gene therapy can play in combating them. The pathogenesis and the molecular mechanisms specifying the altered gene expression of each of these NDDs have also been discussed in elaboration. The use of gene therapy vectors can prove to be an effective tool in the field of curative modern medicine for the generations to come. Therefore, consistent efforts and progressive research towards its implementation can provide us with powerful treatment options for disease conditions that have so far been considered as incurable.

Natural variations of TFIIAγ gene and LOB1 promoter contribute to citrus canker disease resistance in Atalantia buxifolia

Citrus canker caused by Xanthomonas citri subsp. citri (Xcc) is one of the most devastating diseases in citrus industry worldwide. Most citrus cultivars such as sweet orange are susceptible to canker disease. Here, we utilized wild citrus to identify canker-resistant germplasms, and found that Atalantia buxifolia, a primitive (distant-wild) citrus, exhibited remarkable resistance to canker disease. Although the susceptibility gene LATERAL ORGAN BOUNDARIES 1 (LOB1) could also be induced in Atalantia after canker infection, the induction extent was far lower than that in sweet orange. In addition, three of amino acids encoded by transcription factor TFIIAγ in Atalantia (AbTFIIAγ) exhibited difference from those in sweet orange (CsTFIIAγ) which could stabilize the interaction between effector PthA4 and effector binding element (EBE) of LOB1 promoter. The mutation of AbTFIIAγ did not change its interaction with transcription factor binding motifs (TFBs). However, the AbTFIIAγ could hardly support the LOB1 expression induced by the PthA4. In addition, the activity of AbLOB1 promoter was significantly lower than that of CsLOB1 under the induction by PthA4. Our results demonstrate that natural variations of AbTFIIAγ and effector binding element (EBE) in the AbLOB1 promoter are crucial for the canker disease resistance of Atalantia. The natural mutations of AbTFIIAγ gene and AbLOB1 promoter in Atalantia provide candidate targets for improving the resistance to citrus canker disease.

It has been well documented that most citrus cultivars are susceptible to canker disease, while little is known about the resistance or susceptibility of primitive or wild citrus to canker disease. This study reveals that primitive citrus (Atalantia buxifolia) is highly resistant to citrus canker. Transcriptome data demonstrated that Atalantia had an active resistance response to the infection of Xcc, compared with susceptible sweet orange. Our results indicated that natural variations of AbTFIIAγ gene and AbLOB1 promoter contributed to the resistance. Hence, we propose that the natural mutations of AbTFIIAγ gene and AbLOB1 promoter could provide candidate targets for breeding canker resistant citrus.

The genetic arms race between plant and Xanthomonas: lessons learned from TALE biology

The pathogenic bacterial genus Xanthomonas infects a wide variety of host plants and causes devastating diseases in many crops. Transcription activator-like effectors (TALEs) are important virulence factors secreted by Xanthomonas with the ability to directly bind to the promoters of target genes in plant hosts and activate their expression, which often facilitates the proliferation of pathogens. Understanding how plants cope with TALEs will provide mechanistic insights into crop breeding for Xanthomonas defense. Over the past 30 years, numerous studies have revealed the modes of action of TALEs in plant cells and plant defense strategies to overcome TALE attack. Based on these findings, new technologies were adopted for disease management to optimize crop production. In this article, we will review the most recent advances in the evolutionary arms race between plant resistance and TALEs from Xanthomonas, with a specific focus on TALE applications in the development of novel breeding strategies for durable and broad-spectrum resistance.

Detection and identification of Xanthomonas pathotypes associated with citrus diseases using comparative genomics and multiplex PCR

Background

In Citrus cultures, three species of Xanthomonas are known to cause distinct diseases. X. citri subsp. citri patothype A, X. fuscans subsp. aurantifolii pathotypes B and C, and X. alfalfae subsp. citrumelonis, are the causative agents of cancrosis A, B, C, and citrus bacterial spots, respectively. Although these species exhibit different levels of virulence and aggressiveness, only limited alternatives are currently available for proper and early detection of these diseases in the fields. The present study aimed to develop a new molecular diagnostic method based on genomic sequences derived from the four species of Xanthomonas.

Results

Using comparative genomics approaches, primers were synthesized for the identification of the four causative agents of citrus diseases. These primers were validated for their specificity to their target DNA by both conventional and multiplex PCR. Upon evaluation, their sensitivity was found to be 0.02 ng/µl in vitro and 1.5 × 10⁴ CFU ml⁻¹ in infected leaves. Additionally, none of the primers were able to generate amplicons in 19 other genomes of Xanthomonas not associated with Citrus and one species of Xylella, the causal agent of citrus variegated chlorosis (CVC). This denotes strong specificity of the primers for the different species of Xanthomonas investigated in this study.

Conclusions

We demonstrated that these markers can be used as potential candidates for performing in vivo molecular diagnosis exclusively for citrus-associated Xanthomonas. The bioinformatics pipeline developed in this study to design specific genomic regions is capable of generating specific primers. It is freely available and can be utilized for any other model organism.

The host basal transcription factor IIA subunits coordinate for facilitating infection of TALEs-carrying bacterial pathogens in rice

Many plant-pathogenic Xanthomonas rely largely on secreting virulence transcription activator-like effectors (TALEs) proteins into plant nucleus to activate host susceptibility genes to cause disease, the process is dependent on pathogen TALEs association with host plants basal transcription factor IIA small subunit TFIIAγ. TFIIAγ together with large subunit TFIIAαβ constitute as a key component of RNA polymerase II complex for transcriptome initiation. However, whether TFIIAαβ coordinates or competes with pathogen TALEs for interaction with TFIIAγ to activate transcript of TALEs-targeting genes is unclear. Here, we showed that TALE-carrying bacterial pathogens Xanthomonas oryzae pv. oryzae (Xoo) and Xanthomonas oryzae pv. oryzicola (Xoc), the causal agent for bacterial leaf blight and bacterial leaf streak in rice, using their major virulence TALEs to physically associate with N-terminal of OsTFIIAγ5. OsTFIIAα and OsTFIIAβ which are post-translationally mature proteins of OsTFIIAαβ separately bound to N- and C-terminal of OsTFIIAγ5. OsTFIIAα coordinated with TALEs for binding with OsTFIIAγ5 to upregulate rice susceptibility genes to cause disease. Conversely, suppression of OsTFIIAαβ attenuated TALEs-targeting genes transcription, thus improved broad-spectrum disease resistance of rice to Xoo and Xoc. These results provide an applicable strategy for improving resistance to TALE-carrying pathogens in rice by appropriate suppression of plant basal transcription factors expression.

TALE-carrying bacterial pathogens trap host nuclear import receptors for facilitation of infection of rice

Many plant-pathogenic Xanthomonas rely on the secretion of virulence transcription activator-like effector (TALE) proteins into plant cells to activate plant susceptibility genes to cause disease. The process is dependent on the binding of TALEs to specific elements of host target gene promoters in the plant nucleus. However, it is unclear how TALEs, after injection into host cells, are transferred from the plant cytoplasm into the plant nucleus, which is the key step of successful pathogen infection. Here, we show that the host plant cytoplasm/nuclear shuttle proteins OsImpα1a and OsImpα1b are key components for infection by the TALE-carrying bacterial pathogens Xanthomonas oryzae pv. oryzae (Xoo) and Xanthomonas oryzae pv. oryzicola (Xoc), the causal agents of bacterial leaf blight and bacterial leaf streak, respectively, in rice. Direct interaction between the second nuclear localization signal of TALEs of Xoo or Xoc and OsImpα1a or OsImpα1b is required for the transportation of TALEs into the nucleus. Conversely, suppression of the expression of OsImpα1a and OsImpα1b genes attenuates the shuttling of TALEs from the cytoplasm into the nucleus and the induction of susceptibility genes, thus improving the broad-spectrum disease resistance of rice to Xoo and Xoc. These results provide an applicable strategy for the improvement of resistance to TALE-carrying pathogens in rice by moderate suppression of the expression of plant nuclear import receptor proteins.

Transcriptional Reprogramming of Rice Cells by Xanthomonas oryzae TALEs

Rice-pathogenic Xanthomonas oryzae bacteria cause severe harvest loss and challenge a stable food supply. The pathogen virulence relies strongly on bacterial TALE (transcription activator-like effector) proteins that function as transcriptional activators inside the plant cell. To understand the plant targets of TALEs, we determined the genome sequences of the Indian X. oryzae pv. oryzae (Xoo) type strain ICMP 3125T and the strain PXO142 from the Philippines. Their complete TALE repertoire was analyzed and genome-wide TALE targets in rice were characterized. Integrating computational target predictions and rice transcriptomics data, we were able to verify 12 specifically induced target rice genes. The TALEs of the Xoo strains were reconstructed and expressed in a TALE-free Xoo strain to attribute specific induced genes to individual TALEs. Using reporter assays, we could show that individual TALEs act directly on their target promoters. In particular, we show that TALE classes assigned by AnnoTALE reflect common target genes, and that TALE classes of Xoo and the related pathogen X. oryzae pv. oryzicola share more common target genes than previously believed. Taken together, we establish a detailed picture of TALE-induced plant processes that significantly expands our understanding of X. oryzae virulence strategies and will facilitate the development of novel resistances to overcome this important rice disease.

Xanthomonas oryzae pv. oryzae TALE proteins recruit OsTFIIAγ1 to compensate for the absence of OsTFIIAγ5 in bacterial blight in rice

Xanthomonas oryzae pv. oryzae (Xoo), the causal agent of bacterial blight (BB) of rice, uses transcription activator-like effectors (TALEs) to interact with the basal transcription factor gamma subunit OsTFIIAγ5 (Xa5) and activates the transcription of host genes. However, how OsTFIIAγ1, the other OsTFIIAγ protein, functions in the presence of TALEs remains unclear. In this study, we show that OsTFIIAγ1 plays a compensatory role in the absence of Xa5. The expression of OsTFIIAγ1, which is activated by TALE PthXo7, increases the expression of host genes targeted by avirulent and virulent TALEs. Defective OsTFIIAγ1 rice lines show reduced expression of the TALE-targeted susceptibility (S) genes, OsSWEET11 and OsSWEET14, which results in increased BB resistance. Selected TALEs (PthXo1, AvrXa7 and AvrXa27) were evaluated for interactions with OsTFIIAγ1, Xa5 and xa5 (naturally occurring mutant form of Xa5) using biomolecular fluorescence complementation (BiFC) and microscale thermophoresis (MST). BiFC and MST demonstrated that the three TALEs bind Xa5 and OsTFIIAγ1 with a stronger affinity than xa5. These results provide insights into the complex roles of OsTFIIAγ1 and OsTFIIAγ5 in TALE-mediated host gene transcription.

Xanthomonas TAL effectors hijack host basal transcription factor IIA α and γ subunits for invasion

The Xanthomonas genus includes Gram-negative plant-pathogenic bacteria, which infect a broad range of crops and wild plant species, cause symptoms with leaf blights, streaks, spots, stripes, necrosis, wilt, cankers and gummosis on leaves, stems and fruits in a wide variety of plants via injecting their effector proteins into the host cell during infection. Among these virulent effectors, transcription activator-like effectors (TALEs) interact with the γ subunit of host transcription factor IIA (TFIIAγ) to activate the transcription of host disease susceptibility genes. Functional TFIIA is a ternary complex comprising α, β and γ subunits. However, whether TALEs recruit TFIIAα, TFIIAβ, or both remains unknown. The underlying molecular mechanisms by which TALEs mediate host susceptibility gene activation require full elucidation. Here, we show that TALEs interact with the α+γ binary subcomplex but not the α+β+γ ternary complex of rice TFIIA (holo-OsTFIIA). The transcription factor binding (TFB) regions of TALEs, which are highly conserved in Xanthomonas species, have a dominant role in these interactions. Furthermore, the interaction between TALEs and the α+γ complex exhibits robust DNA binding activity in vitro. These results collectively demonstrate that TALE-carrying pathogens hijack the host basal transcription factors TFIIAα and TFIIAγ, but not TFIIAβ, to enhance host susceptibility during pathogen infection. The uncovered mechanism widens new insights on host-microbe interaction and provide an applicable strategy to breed high-resistance crop varieties.