Search Results (9)

Citrus bacterial canker (CBC), caused by Xanthomonas citri subsp. citri (Xcc), threatens citrus production worldwide. Long-term dependence on copper-based bactericides not only poses environmental risks but also accelerates the emergence of copper-resistant Xcc strains. To develop safe and efficient alternative control strategies, 72 bacterial strains were isolated from the phyllosphere of citrus plants naturally infected by CBC and identified by 16S rRNA sequencing. Using an Xcc-GFP-based screening method, we systematically screened a highly effective strain, which was identified as Bacillus velezensis RF2 (Bv-RF2). Both inhibition zone assays and bioactivity tests of the crude methanolic extract of Bv-RF2 demonstrated stable antibacterial activity under UV irradiation, protease treatment, high temperature, and across a wide pH range. Whole-genome sequencing and antiSMASH analysis revealed multiple predicted NRPS/PKS-type biosynthetic gene clusters (BGCs). Together with metabolomic profiling, these data provide hypotheses for candidate metabolites that may contribute to antagonism. Bv-RF2 was associated with the induction of PR gene expression in immune-related pathways implicated in CBC responses. In sweet orange leaves, Bv-RF2 infiltration was associated with transient induction of defense-related (PR) genes, consistent with an ISR-like, priming-related response. In addition, Bv-RF2 inhibited the growth of fungal pathogens associated with citrus anthracnose and brown spot in vitro, indicating broad inhibitory potential under the tested conditions. Collectively, Bv-RF2 represents a promising candidate for developing environmentally friendly strategies against CBC and other citrus diseases. Full article

Citrus bacterial canker (CBC) is a severe bacterial infection caused by Xanthomonas citri subsp. citri (Xcc), which continues to adversely impact citrus production worldwide. Members of the GATA family are important regulators of plant development and regulate plant responses to particular stressors. This report aimed to systematically elucidate the Citrus sinensis genome to identify and annotate genes that encode GATAs and evaluate the functional importance of these CsGATAs as regulators of CBC resistance. In total, 24 CsGATAs were identified and classified into four subfamilies. Furthermore, the phylogenetic relationships, chromosomal locations, collinear relationships, gene structures, and conserved domains for each of these GATA family members were also evaluated. It was observed that Xcc infection induced some CsGATAs, among which CsGATA12 was chosen for further functional validation. CsGATA12 was found to be localized in the nucleus and was differentially upregulated in the CBC-resistant and CBC-sensitive Kumquat and Wanjincheng citrus varieties. When transiently overexpressed, CsGATA12 significantly reduced CBC resistance with a corresponding increase in abscisic acid, jasmonic acid, and antioxidant enzyme levels. These alterations were consistent with lower levels of salicylic acid, ethylene, and reactive oxygen species. Moreover, the bacteria-induced CsGATA12 gene silencing yielded the opposite phenotypic outcomes. This investigation highlights the important role of CsGATA12 in regulating CBC resistance, underscoring its potential utility as a target for breeding citrus varieties with superior phytopathogen resistance. Full article

Xanthomonas citri pv. citri (Xcc) (X. citri subsp. citri) type A is the causal agent of citrus bacterial canker (CBC) on most Citrus spp. and close relatives. Two narrow-host-range strains of Xcc, A^w and A*, from Florida and Southwest Asia, respectively, infect only Mexican lime (Citrus aurantifolia) and alemow (C. macrophylla). In the initial stage of infection, these xanthomonads enter via stomata to reach the apoplast. Herein, we investigated the differences in chemotactic responses for wide and narrow-host-range strains of Xcc A, X. euvesicatoria pv. citrumelonis (X. alfalfae subsp. citrumelonis), the causal agent of citrus bacterial spot, and X. campestris pv. campestris, the crucifer black rot pathogen. These strains of Xanthomonas were compared for carbon source use, the chemotactic responses toward carbon compounds, chemotaxis sensor content, and responses to apoplastic fluids from Citrus spp. and Chinese cabbage (Brassica pekinensis). Different chemotactic responses occurred for carbon sources and apoplastic fluids, depending on the Xanthomonas strain and the host plant from which the apoplastic fluid was derived. Differential chemotactic responses to carbon sources and citrus apoplasts suggest that these Xanthomonas strains sense host-specific signals that facilitate their location and entry of stomatal openings or wounds. Full article

Superoxide dismutases (SODs) play critical roles in plants, especially in the maintenance of redox homeostasis. The response of SODs in Citrus (Citrus sinensis (L.) Osbeck) to citrus bacterial canker (CBC) infection were investigated. The CsSODs were identified, and their gene structures, phylogeny, conserved domains and motifs, predicted interactions, and chromosomal distribution were analyzed. CsSOD expression in response to stress-related plant hormones (salicylic acid, SA; methyl jasmonate, MeJA; and abscisic acid, ABA) and Xanthomonas citri subsp. citri (Xcc) infection were also investigated. Thirteen CsSODs were identified in C. sinensis, including four Fe/MnSODs and nine Cu/ZnSODs with typical functional domains. The CsSODs were distributed on chromosomes 3, 5, 7, and 8. Specific hormone-response motifs were identified in the gene promoter regions. Ten genes were induced by MeJA treatment, as shown by qRT-PCR, and were upregulated in the CBC-susceptible Wanjincheng citrus variety, while CsSOD06 and CsSOD08 were upregulated by ABA in both the Wanjincheng and the CBC-resistant Kumquat varieties. Xcc infection significantly altered the levels of most CsSODs. The overexpression of CsSOD06 and CsSOD08 resulted in increased hydrogen peroxide levels and SOD activity. Our findings highlight the significance of SOD enzymes in the plant response to pathogen infection and have a potential application for breeding CBC-tolerant citrus varieties. Full article

Xanthomonas citri pv. citri (Xcc) and X. citri pv. aurantifolii (Xca) are causal agents of Citrus Bacterial Canker (CBC), a devastating disease that severely affects citrus plants. They are harmful organisms not reported in Europe or the Mediterranean Basin. Host plants are in the Rutaceae family, including the genera Citrus, Poncirus, and Fortunella, and their hybrids. In addition, other genera of ornamental interest are reported as susceptible, but results are not uniform and sometimes incongruent. We evaluated the susceptibility of 32 ornamental accessions of the Rutaceae family belonging to the genera Citrus, Fortunella, Atalantia, Clausena, Eremocitrus, Glycosmis, Microcitrus, Murraya, Casimiroa, Calodendrum, and Aegle, and three hybrids to seven strains of Xcc and Xca. Pathotyping evaluation was assessed by scoring the symptomatic reactions on detached leaves. High variability in symptoms and bacterial population was shown among the different strains in the different hosts, indicative of complex host–pathogen interactions. The results are mostly consistent with past findings, with the few discrepancies probably due to our more complete experimental approach using multiple strains of the pathogen and multiple hosts. Our work supports the need to regulate non-citrus Rutaceae plant introductions into areas, like the EU and Mediterranean, that are currently free of this economically important pathogen. Full article

The present study was designed to serve as a comprehensive analysis of Citrus sinensis (C. sinensis) pectin acetylesterases (CsPAEs), and to assess the roles of these PAEs involved in the development of citrus bacterial canker (CBC) caused by Xanthomonas citri subsp. citri (Xcc) infection. A total of six CsPAEs were identified in the genome of C. sinensis, with these genes being unevenly distributed across chromosomes 3, 6, and 9, and the unassembled scaffolds. A subset of CsPAEs were found to be involved in responses to Xcc infection. In particular, CsPAE2 was identified to be associated with such infections, as it was upregulated in CBC-susceptible variety Wanjincheng and inversely in CBC-resistant variety Calamondin. Transgenic citrus plants overexpressing CsPAE2 were found to be more susceptible to CBC, whereas the silencing of this gene was sufficient to confer CBC resistance. Together, these findings provide evolutionary insights into and functional information about the CsPAE family. This study also suggests that CsPAE2 is a potential candidate gene that negatively contributes to bacterial canker disease and can be used to breed CBC-resistant citrus plants. Full article

Citrus bacterial canker (CBC) is an important disease of citrus cultivars worldwide that causes blister-like lesions on host plants and leads to more severe symptoms such as plant defoliation and premature fruit drop. The causative agent, Xanthomonas citri pv. citri, exists as three pathotypes—A, A*, and A^w—which differ in their host range and elicited host response. To date, comparative analyses have been hampered by the lack of closed genomes for the A* pathotype. In this study, we sequenced and assembled six CBC isolates of pathotype A* using second- and third-generation sequencing technologies to produce complete, closed assemblies. Analysis of these genomes and reference A, A*, and A^w sequences revealed genetic groups within the A* pathotype. Investigation of accessory genomes revealed virulence factors, including type IV secretion systems and heavy metal resistance genes, differentiating the genetic groups. Genomic comparisons of closed genome assemblies also provided plasmid distribution information for the three genetic groups of A*. The genomes presented here complement existing closed genomes of A and A^w pathotypes that are publicly available and open opportunities to investigate the evolution of X. citri pv. citri and the virulence factors that contribute to this serious pathogen. Full article

Citrus bacterial canker (CBC), caused by the plant pathogenic bacterium Xanthomonas citri subsp. citri (Xcc), is a devastating disease in many commercial citrus cultivars. Every year, CBC causes a substantial reduction in fruit quality and quantity that corresponds to significant economic losses worldwide. Endophytic microorganisms produce numerous bioactive secondary metabolites that can control plant pathogens. We investigated the antagonistic activities of 66 endophytic bacteria isolated from nine citrus cultivars to control streptomycin-resistant Xcc. The suspension of Endophytic Bacteria-39 (EB-39), identified as Bacillus velezensis, exhibited the highest antibacterial activity against three wild-type and six streptomycin-resistant Xcc strains, with the inhibition zones between 39.47 ± 1.6 and 45.31 ± 1.6 mm. The ethyl acetate extract of EB-39 also controlled both wild-type and streptomycin-resistant Xcc strains, with the inhibition zones between 29.28 ± 0.6 and 33.88 ± 1.3 mm. Scanning electron microscopy indicated the ethyl acetate extract of EB-39-induced membrane damage and lysis. The experiments using the detached leaves of a susceptible Citrus species showed that EB-39 significantly reduced the incidence of canker on the infected leaves by 38%. These results strongly suggest that our newly isolated EB-39 is a novel biocontrol agent against CBC caused by wild-type and streptomycin-resistant Xcc strains. Full article

Citrus bacterial canker (CBC) caused by Xanthomonas citri subsp. citri (Xcc), is the most devastating of the citrus diseases worldwide. During our study, we found that Essential oils (EOs) of some citrus cultivars are effective on Xcc. Therefore, it prompted us to determine the plant metabolites responsible for the antibacterial properties. We obtained EOs from some locally cultivated citrus by using a Clevenger apparatus and their major constituents were identified by gas chromatography/mass spectrometry (GC-MS). The effect of Citrus aurantium, C. aurantifolia, Fortunella sp. EOs and their major constituents were evaluated against Xcc-KVXCC1 using a disk diffusion assay. Minimal inhibitory and bactericidal concentration of the EOs and their constituents were determined using the broth microdilution method. C. aurantium, C. aurantifolia Eos, and their major constituents including citral, linalool, citronellal, geraniol, α-terpineol, and linalyl acetate indicated antibacterial effects against Xcc. The C. aurantifolia EO and citral showed the highest antibacterial activity among the tested EOs and constituents with inhibition zones of 15 ± 0.33 mm and 16.67 ± 0.88 mm, respectively. Synergistic effects of the constituents were observed between α-terpineol-citral, citral-citronellal, citral-geraniol, and citronellal-geraniol by using a microdilution checkerboard assay. Transmission electron microscopy revealed that exposure of Xcc cells to citral caused cell wall damage and altered cytoplasmic density. We introduced C. aurantifolia and C. aurantium EOs, and their constituents citral, α-terpineol, citronellal, geraniol, and linalool as possible control agents for CBC. Full article