Resistance to citrus canker induced by a variant of Xanthomonas citri ssp. citri is associated with a hypersensitive cell death response involving autophagy-associated vacuolar processes

Botanical-Based Strategies for Controlling Xanthomonas spp. in Cotton and Citrus: In Vitro and In Vivo Evaluation

Citrus canker, caused by Xanthomonas citri subsp. citri, and bacterial blight, caused by Xanthomonas citri subsp. malvacearum, results in substantial economic losses worldwide, and searching for new antibacterial agents is a critical challenge. In this study, regional isolates AE28 and RQ3 were obtained from characteristic lesions on Citrus limon and Gossypium hirsutum, respectively. Essential oils extracted by steam distillation from the fresh aerial parts of Pelargonium graveolens and Schinus molle exhibited complete (100%) inhibition of bacterial growth in vitro at a concentration of 1000 ppm, as determined by diffusion tests. To evaluate the potential of these essential oils for controlling Xanthomonas-induced diseases, in vivo assays were conducted on lemon leaves and cotton cotyledons inoculated with the regional AE28 and RQ3 strains. Two treatment approaches were tested: preventive application (24 h before inoculation) and curative application (24 h after inoculation). Preventive and curative treatments with P. graveolens essential oil significantly reduced citrus canker severity, whereas S. molle essential oil did not show a significant reduction compared to the control. In contrast, regardless of the treatment's timing, both essential oils effectively reduced bacterial blight severity in cotton cotyledons by approximately 1.5-fold. Gas chromatography-mass spectrometry (GC-MS) analysis identified geraniol and citronellol as the major components of P. graveolens essential oil, while limonene and t-cadinol were predominant in S. molle. These findings highlight the promising potential of botanical products as bactericidal agents, warranting further research to optimize their application and efficacy.

Transcriptomic analysis reveals differential transcriptional regulation underlying Citrus Bacterial Canker (CBC) tolerance in Citrus sinensis

The sustainable development of the citrus industry is greatly affected by citrus canker, an important bacterial disease. To explore the transcriptional regulatory mechanism of citrus resistance to canker disease, this study used the susceptible Citrus sinensis cv. 'Newhall' and its citrus canker-resistant bud mutation variety 'Longhuitian' (LHT) as materials. Through analysing the variances in leaf phenotypes between Newhall and LHT, as well as the variations in their transcriptional expression under Xanthomonas citri subsp. citri (Xcc) inoculation, our study concluded that LHT displays markedly greater resistance to Xcc compared to Newhall. Additionally, the spongy parenchyma of LHT leaves is significantly thicker than that of Newhall, and the stomatal number is significantly higher in LHT leaves, while the length and width of individual stomata in LHT leaves are significantly smaller than those in Newhall. RNA-seq analysis indicates that the differentially expressed genes between LHT and Newhall are involved in biotic stress-related biological processes, secondary metabolite biosynthesis, as well as phytohormone signalling pathways. Furthermore, significant differences were observed in reactive oxygen metabolism and phenylalanine metabolism pathways. The findings of our study provide data support for a deeper understanding of the citrus-Xcc interactions and offer valuable clues for unravelling citrus resistance to citrus canker.

Designer TALEs enable discovery of cell death-inducer genes

Transcription activator-like effectors (TALEs) in plant-pathogenic Xanthomonas bacteria activate expression of plant genes and support infection or cause a resistance response. PthA4AT is a TALE with a particularly short DNA-binding domain harboring only 7.5 repeats which triggers cell death in Nicotiana benthamiana; however, the genetic basis for this remains unknown. To identify possible target genes of PthA4AT that mediate cell death in N. benthamiana, we exploited the modularity of TALEs to stepwise enhance their specificity and reduce potential target sites. Substitutions of individual repeats suggested that PthA4AT-dependent cell death is sequence specific. Stepwise addition of repeats to the C-terminal or N-terminal end of the repeat region narrowed the sequence requirements in promoters of target genes. Transcriptome profiling and in silico target prediction allowed the isolation of two cell death inducer genes, which encode a patatin-like protein and a bifunctional monodehydroascorbate reductase/carbonic anhydrase protein. These two proteins are not linked to known TALE-dependent resistance genes. Our results show that the aberrant expression of different endogenous plant genes can cause a cell death reaction, which supports the hypothesis that TALE-dependent executor resistance genes can originate from various plant processes. Our strategy further demonstrates the use of TALEs to scan genomes for genes triggering cell death and other relevant phenotypes.

Four novel Cit7GlcTs functional in flavonoid 7-O-glucoside biosynthesis are vital to flavonoid biosynthesis shunting in citrus

Citrus fruits have abundant flavonoid glycosides (FGs), an important class of natural functional and flavor components. However, there have been few reports about the modification of UDP-glycosyltransferases (UGTs) on flavonoids in citrus. Notably, in flavonoid biosynthesis, 7-O-glucosylation is the initial and essential step of glycosylation prior to the synthesis of flavanone disaccharides, the most abundant and iconic FGs in citrus fruits. Here, based on the accumulation of FGs observed at the very early fruit development stage of two pummelo varieties, we screened six novel flavonoid 7-O-glucosyltransferase genes (7GlcTs) via transcriptomic analysis and then characterized them in vitro. The results revealed that four Cg7GlcTs possess wide catalytic activities towards various flavonoid substrates, with CgUGT89AK1 exhibiting the highest catalytic efficiency. Transient overexpression of CgUGT90A31 and CgUGT89AK1 led to increases in FG synthesis in pummelo leaves. Interestingly, these two genes had conserved sequences and consistent functions across different germplasms. Moreover, CitUGT89AK1 was found to play a role in the response of citrus to Huanglongbing infection by promoting FG production. The findings improve our understanding of flavonoid 7-O-glucosylation by identifying the key genes, and may help improve the benefits of flavonoid biosynthesis for plants and humans in the future.

Plant structural and storage glucans trigger distinct transcriptional responses that modulate the motility of Xanthomonas pathogens

IMPORTANCE

Pathogenic Xanthomonas bacteria can affect a variety of economically relevant crops causing losses in productivity, limiting commercialization and requiring phytosanitary measures. These plant pathogens exhibit high level of host and tissue specificity through multiple molecular strategies including several secretion systems, effector proteins, and a broad repertoire of carbohydrate-active enzymes (CAZymes). Many of these CAZymes act on the plant cell wall and storage carbohydrates, such as cellulose and starch, releasing products used as nutrients and modulators of transcriptional responses to support host colonization by mechanisms yet poorly understood. Here, we reveal that structural and storage β-glucans from the plant cell function as spatial markers, providing distinct chemical stimuli that modulate the transition between higher and lower motility states in Xanthomonas citri, a key virulence trait for many bacterial pathogens.

Priming crop plants with rosemary (Salvia rosmarinus Spenn, syn Rosmarinus officinalis L.) extract triggers protective defense response against pathogens

Plant bioactive compounds provide novel straightforward approaches to control plant diseases. Rosemary (Salvia rosmarinus)-derived extracts carry many prominent pharmacological activities, including antimicrobial and antioxidant, mainly due to its phenolic compounds, rosmarinic acid (RA), carnosic acid and carnosol. However, the effects of these extracts on plant diseases are still unknown, which constrains its potential application as bioprotectant in the agricultural production. In this study we demonstrate the antiviral effect of the aqueous rosemary extract (ARE) against tobacco necrosis virus strain A (TNVA) in ARE-treated tobacco (Nicotiana tabacum) plants. Our results show that ARE-treatment enhances plant defense response, contributing to reduce virus replication and systemic movement in tobacco plants. RA, the main phenolic compound detected in this extract, is one of the main inducers of TNVA control. The ARE-induced protection in TNVA-infected plants was characterized by the expression of H₂O₂ scavengers and defense-related genes, involving salicylic acid- and jasmonic acid-regulated pathways. Furthermore, treatment with ARE in lemon (Citrus limon) and soybean (Glycine max) leaves protects the plants against Xanthomonas citri subsp. citri and Diaporthe phaseolorum var. meridionalis, respectively. Additionally, ARE treatment also promotes growth and development, suggesting a biostimulant activity in soybean. These results open the way for the potential use of ARE as a bioprotective agent in disease management.

Transient expression of an scFvG8 antibody in plants and characterization of its effects on the virulence factor pthA of Xanthomonas citri subsp. citri

Citrus bacterial canker, caused by Xanthomonas citri subsp. citri (Xcc), is a major disease of citrus plants, causing a significant loss in the citrus industry. The pthA is a bacterial effector protein mediates protein-protein and protein-DNA interactions and modulates host transcription. Injection of pthA effector protein into the host cell induces the expression of the susceptibility gene CsLOB1 which is required for citrus canker disease development. In this study, we described in planta expression of a specific anti-pthA single-chain variable fragment (scFv) recombinant antibody, scFvG8, and assessed its function using molecular docking, immunoblotting, and indirect enzyme-linked immunosorbent assay (ELISA). Based on the results, homology-based molecular docking suggested that at least eight intermolecular hydrogen bonds are involved in pthA-scFvG8 interactions. Immunoblotting and indirect ELISA results reconfirmed specific binding of scFvG8 to pthA protein. Moreover, gene fragment encoding scFvG8 was cloned into plant expression vector and transiently expressed in leaves of Nicotiana tabacum cv. Samson by agroinfiltration method. Transient expression of scFvG8 (at the expected size of 35 kDa) in N. tabacum leaves was confirmed by western blotting. Also, immunoblotting and indirect ELISA showed that the plant-derived scFvG8 had similar activity to purified scFvG8 antibody in detecting pthA. Additionally, in scFvG8-expressing tobacco leaves challenged with Xcc, a reduction (for up to 70%) of hypersensitive response (HR) possibly via direct interaction with pthA, was observed in the necrotic leaf area compared to control plants infected with empty vector. The results obtained in this study confirm that scFvG8 can suppress the function of pthA effector protein within plant cells, thus the induction of stable expression of scFvG8 in lime trees can be considered as an appropriate approach to confer resistance to Xcc.

Transcriptome-wide identification and characterization of microRNAs in diverse phases of wood formation in Populus trichocarpa

We applied miRNA expression profiling method to Populus trichocarpa stems of the three developmental stages, primary stem (PS), transitional stem (TS), and secondary stem (SS), to investigate miRNA species and their regulation on lignocellulosic synthesis and related processes. We obtained 892, 872, and 882 known miRNAs and 1727, 1723, and 1597 novel miRNAs, from PS, TS, and SS, respectively. Comparisons of these miRNA species among different developmental stages led to the identification of 114, 306, and 152 differentially expressed miRNAs (DE-miRNAs), which had 921, 2639, and 2042 candidate target genes (CTGs) in the three respective stages of the same order. Correlation analysis revealed 47, 439, and 71 DE-miRNA-CTG pairs of high negative correlation in PS, TS, and SS, respectively. Through biological process analysis, we finally identified 34, 6, and 76 miRNA-CTG pairs from PS, TS, and SS, respectively, and the miRNA target genes in these pairs regulate or participate lignocellulosic biosynthesis-related biological processes: cell division and differentiation, cell wall modification, secondary cell wall biosynthesis, lignification, and programmed cell death processes. This is the first report on an integrated analysis of genome-wide mRNA and miRNA profilings during multiple phases of poplar stem development. Our analysis results imply that individual miRNAs modulate secondary growth and lignocellulosic biosynthesis through regulating transcription factors and lignocellulosic biosynthetic pathway genes, resulting in more dynamic promotion, suppression, or regulatory circuits. This study advanced our understanding of many individual miRNAs and their essential, diversified roles in the dynamic regulation of secondary growth in woody tree species.

Different Responses in Mandarin Cultivars Uncover a Role of Cuticular Waxes in the Resistance to Citrus Canker

'Okitsu' is a mandarin cultivar showing substantial resistance to X. citri subsp. citri (X. citri). We have previously shown that this cultivar has significantly lower canker incidence and severity than 'Clemenules', particularly during early stages of leaf development in the field. This differential response is only seen when the leaves are inoculated by spraying, suggesting that leaf surface contributes to resistance. In this work, we have studied structural and chemical properties of leaf surface barriers of both cultivars. Ultrastructural analysis showed a thicker cuticle covering epidermal surface and guard cells in young 'Okitsu' leaves than in 'Clemenules'. This thicker cuticle was associated with a smaller stomatal aperture and reduced cuticle permeability. These findings correlated with an accumulation of cuticular wax components, including primary alcohols, alkanes, and fatty acids. None of these differences were observed in mature leaves, where both cultivars are equally resistant to the bacterium. Remarkably, mechanical alteration of cuticular thickness of young 'Okitsu' leaves allows canker development. Furthermore, cuticular waxes extracted from young 'Okitsu' leaves have higher antibacterial activity against X. citri than 'Clemenules'. Taken together, these data suggest that a faster development of epicuticular waxes in 'Okitsu' leaves play a central role in its resistance to X. citri.

PthA4AT , a 7.5-repeats transcription activator-like (TAL) effector from Xanthomonas citri ssp. citri, triggers citrus canker resistance

Transcription activator-like effectors (TALEs) are important effectors of Xanthomonas spp. that manipulate the transcriptome of the host plant, conferring susceptibility or resistance to bacterial infection. Xanthomonas citri ssp. citri variant AT (X. citri AT ) triggers a host-specific hypersensitive response (HR) that suppresses citrus canker development. However, the bacterial effector that elicits this process is unknown. In this study, we show that a 7.5-repeat TALE is responsible for triggering the HR. PthA4AT was identified within the pthA repertoire of X. citri AT followed by assay of the effects on different hosts. The mode of action of PthA4AT was characterized using protein-binding microarrays and testing the effects of deletion of the nuclear localization signals and activation domain on plant responses. PthA4AT is able to bind DNA and activate transcription in an effector binding element-dependent manner. Moreover, HR requires PthA4AT nuclear localization, suggesting the activation of executor resistance (R) genes in host and non-host plants. This is the first case where a TALE of unusually short length performs a biological function by means of its repeat domain, indicating that the action of these effectors to reprogramme the host transcriptome following nuclear localization is not limited to 'classical' TALEs.

Structure-function relationship of a citrus salicylate methylesterase and role of salicylic acid in citrus canker resistance

Salicylic acid (SA) and its methyl ester, methyl salicylate (MeSA) are well known inducers of local and systemic plant defense responses, respectively. MeSA is a major mediator of systemic acquired resistance (SAR) and its conversion back into SA is thought to be required for SAR. In many plant species, conversion of MeSA into SA is mediated by MeSA esterases of the SABP2 family. Here we show that the Citrus sinensis SABP2 homologue protein CsMES1 catalyzes the hydrolysis of MeSA into SA. Molecular modeling studies suggest that CsMES1 shares the same structure and SA-binding mode with tobacco SABP2. However, an amino acid polymorphism in the active site of CsMES1-related proteins suggested an important role in enzyme regulation. We present evidence that the side chain of this polymorphic residue directly influences enzyme activity and SA binding affinity in CsMES proteins. We also show that SA and CsMES1 transcripts preferentially accumulate during the incompatible interaction between Xanthomonas aurantifolii pathotype C and sweet orange plants. Moreover, we demonstrate that SA and MeSA inhibited citrus canker caused by Xanthomonas citri, whereas an inhibitor of CsMES1 enhanced canker formation, suggesting that CsMES1 and SA play a role in the local defense against citrus canker bacteria.

Plant responses underlying nonhost resistance of Citrus limon against Xanthomonas campestris pv. campestris

Citrus is an economically important fruit crop that is severely afflicted by citrus canker, a disease caused by Xanthomonas citri ssp. citri (X. citri); thus, new sustainable strategies to manage this disease are needed. Although all Citrus spp. are susceptible to this pathogen, they are resistant to other Xanthomonas species, exhibiting non-host resistance (NHR), for example, to the brassica pathogen X. campestris pv. campestris (Xcc) and a gene-for-gene host defence response (HDR) to the canker-causing X. fuscans ssp. aurantifolii (Xfa) strain C. Here, we examine the plant factors associated with the NHR of C. limon to Xcc. We show that Xcc induced asymptomatic type I NHR, allowing the bacterium to survive in a stationary phase in the non-host tissue. In C. limon, this NHR shared some similarities with HDR; both defence responses interfered with biofilm formation, and were associated with callose deposition, induction of the salicylic acid (SA) signalling pathway and the repression of abscisic acid (ABA) signalling. However, greater stomatal closure was seen during NHR than during HDR, together with different patterns of accumulation of reactive oxygen species and phenolic compounds and the expression of secondary metabolites. Overall, these differences, independent of Xcc type III effector proteins, could contribute to the higher protection elicited against canker development. We propose that Xcc may have the potential to steadily activate inducible defence responses. An understanding of these plant responses (and their triggers) may allow the development of a sustained and sustainable resistance to citrus canker.

Paenibacillus lentimorbus induces autophagy for protecting tomato from Sclerotium rolfsii infection

During biotic stress, plants use several mechanisms to protect themselves that include the production of reactive oxygen species (ROS), induction of pathogenesis-related proteins and cell death. Some plant growth promoting rhizobacteria (PGPR) are known to act as bio-control agents that protect crops against pathogens. The biocontrol activity of PGPR Paenibacillus lentimorbus (B-30488) against Sclerotium rolfsii showed previously where several defense-related genes were upregulated with ROS induction in tomato. We further evaluate the other possibility, i.e. role of autophagy in enhancing defense in tomato using PGPR. Confocal microscopy revealed the presence of an acidotropic dye Mono Dansyl Cadaverine (MDC) stained autophagosomes in B-30488 treated healthy and infected plants. These autophagosomes almost disappeared in plants treated with an autophagy inhibitor chloroquine. The results were also confirmed by ultrastructural analysis of leaf tissues using transmission electron microscopy. Enhanced expression of autophagy-related genes was also monitored in B-30488 primed fungal infected tissues as compared to control by qRT-PCR. Results of ROS accumulation, fluorescence, confocal and transmission electron microscopy and gene expression analysis revealed induction of autophagy using B-30488 as a biocontrol agent suggesting a role in enhancing disease resistance in tomato. Overall, the present study indicated a role of B-30488 as a biocontrol in enhancing disease resistance in tomato and also assists a better understanding of fungal pathogenesis that is expected to be useful in developing new strategies for disease control.