Identification of a virulence tal gene in the cotton pathogen, Xanthomonas citri pv. malvacearum strain Xss-V2-18

Two TAL effectors of Xanthomonas citri promote pustule formation by directly repressing the expression of GRAS transcription factor in citrus

Citrus bacterial canker (CBC), caused by Xanthomonas citri subsp. citri (Xcc), poses a significant threat to the citrus industry. Xcc employs the transcription activator-like effector (TALE) PthA4 to target the major susceptibility (S) gene CsLOB1 in citrus, promoting host susceptibility to bacterial canker. However, the contribution of other Xcc TALEs, aside from PthA4, to virulence remains underexplored. In this study, we characterized two PthA1 variants, designated PthA5 and PthA6, which facilitate Xcc infection in susceptible citrus species by promoting the formation of hypertrophy and hyperplasia symptoms. Both PthA5 and PthA6 bind directly to effector-binding elements (EBEs) in the promoter of CsGRAS9, suppressing its expression. CsGRAS9 negatively regulates Xcc growth in citrus and contributes to CBC resistance. Notably, natural variations in the EBEs of the FhGRAS9 promoter, a homolog of CsGRAS9 in Hong Kong kumquat, prevent Xcc from affecting FhGRAS9 expression. Using the PTG/Cas9 system, we generated proCsGRAS9-edited sweet orange lines #18-2 and #23, which contain 86-bp and 62-bp deletions in the EBE regions of the CsGRAS9 promoter. These mutant lines showed enhanced CsGRAS9 expression and increased resistance to CBC during Xcc infection. Several GA-related genes and CsTAC1, regulated by CsGRAS9, were also identified. This is the first report that TALEs act as repressors of a resistance gene to confer host susceptibility.

Thymol as a Novel Plant-Derived Antibacterial Agent for Suppressing Xanthomonas citri pv. malvacearum in Cotton

Xanthomonas citri pv. malvacearum (Xcm) associated with bacterial blight disease is a significant and widespread pathogen affecting cotton worldwide. The excessive use of harmful chemicals to control plant pathogens has exerted a negative impact on environmental safety. Thymol is a monoterpene phenol present in the essential oils of plants belonging to Lamiaceae family. In this study the antimicrobial activity of thymol was evaluated against Xcm. The minimum inhibitory concentration (MIC) and 99.9% bactericidal concentration (MBC) of thymol against Xcm were 2 and 4 mg/mL, respectively. The effect of MIC and MBC of thymol against Xcm was assessed on the Luria-Bertani medium. The effect of thymol on intercellular ATP levels, membrane potential, and motility in Xcm was assessed using fluorescence spectrometry for membrane potential and firefly luciferase-based assay for ATP levels. Thymol ruptured the cellular membrane of Xcm, resulting in decreased intracellular ATP concentrations, intracellular leakage of genetic material, and changes in membrane potential. Scanning electron microscopy images supported the impact of thymol on the cell membrane of Xcm. Moreover, thymol inhibited the swimming motility and biofilm formation of Xcm at concentrations equal to or above the MIC and MBC. In contrast, sub-MIC concentrations of thymol had little to no impact on the virulence of Xcm. In conclusion, thymol demonstrated the potential as a strong bactericidal compound against Xcm.

Activation of three targets by a TAL effector confers susceptibility to bacterial blight of cotton

Bacterial transcription activator-like effectors (TALEs) promote pathogenicity by activating host susceptibility (S) genes. To understand the pathogenicity and host adaptation of Xanthomonas citri pv. malvacearum (Xcm), we assemble the genome and the TALE repertoire of three recent Xcm Texas isolates. A newly evolved TALE, Tal7b, activates GhSWEET14a and GhSWEET14b, different from GhSWEET10 targeted by a TALE in an early Xcm isolate. Activation of GhSWEET14a and GhSWEET14b results in water-soaked lesions. Transcriptome profiling coupled with TALE-binding element prediction identify a pectin lyase gene as an additional Tal7b target, quantitatively contributing to Xcm virulence alongside GhSWEET14a/b. CRISPR-Cas9 gene editing supports the function of GhSWEETs in cotton bacterial blight and the promise of disrupting the TALE-binding site in S genes for disease management. Collectively, our findings elucidate the rapid evolution of TALEs in Xanthomonas field isolates and highlight the virulence mechanism wherein TALEs induce multiple S genes to promote pathogenicity.

Comparative Genomics Identifies Conserved and Variable TAL Effectors in African Strains of the Cotton Pathogen Xanthomonas citri pv. malvacearum

Strains of Xanthomonas citri pv. malvacearum cause bacterial blight of cotton, a potentially serious threat to cotton production worldwide, including in sub-Saharan countries. Development of disease symptoms, such as water soaking, has been linked to the activity of a class of type 3 effectors, called transcription activator-like (TAL) effectors, which induce susceptibility genes in the host's cells. To gain further insight into the global diversity of the pathogen, to elucidate their repertoires of TAL effector genes, and to better understand the evolution of these genes in the cotton-pathogenic xanthomonads, we sequenced the genomes of three African strains of X. citri pv. malvacearum using nanopore technology. We show that the cotton-pathogenic pathovar of X. citri is a monophyletic lineage containing at least three distinct genetic subclades, which appear to be mirrored by their repertoires of TAL effectors. We observed an atypical level of TAL effector gene pseudogenization, which might be related to resistance genes that are deployed to control the disease. Our work thus contributes to a better understanding of the conservation and importance of TAL effectors in the interaction with the host plant, which can inform strategies for improving resistance against bacterial blight in cotton.

A Xanthomonas transcription activator-like effector is trapped in nonhost plants for immunity

Xanthomonas oryzae pv. oryzae (Xoo), the causal agent of bacterial leaf blight in rice, delivers transcription activator-like effector (TALE) proteins into host cells to activate susceptibility or resistance (R) genes that promote disease or immunity, respectively. Nonhost plants serve as potential reservoirs of R genes; consequently, nonhost R genes may trap TALEs to trigger an immune response. In this study, we screened 17 Xoo TALEs for their ability to induce a hypersensitive response (HR) in the nonhost plant Nicotiana benthamiana (Nb); only AvrXa10 elicited an HR when transiently expressed in Nb. The HR generated by AvrXa10 required both the central repeat region and the activation domain, suggesting a specific interaction between AvrXa10 and a potential R-like gene in nonhost plants. Evans blue staining and ion leakage measurements confirmed that the AvrXa10-triggered HR was a form of cell death, and the transient expression of AvrXa10 in Nb induced immune responses. Genes targeted by AvrXa10 in the Nb genome were identified by transcriptome profiling and prediction of effector binding sites. Using several approaches (in vivo reporter assays, electrophoretic mobility-shift assays, targeted designer TALEs, and on-spot gene silencing), we confirmed that AvrXa10 targets NbZnFP1, a C2H2-type zinc finger protein that resides in the nucleus. Functional analysis indicated that overexpression of NbZnFP1 and its rice orthologs triggered cell death in rice protoplasts. An NbZnFP1 ortholog was also identified in tomato and was specifically activated by AvrXa10. These results demonstrate that NbZnFP1 is a nonhost R gene that traps AvrXa10 to promote plant immunity in Nb.