Biology subjects

Darrasse, A.

Publications and source records attributed to Darrasse, A..

4 recordsLinked to original sources

bioRxiv2026

Insights into the transmission of seedborne Pseudomonas syringae strains that cause zucchini diseases.

Vein clearing of zucchini (VCZ) and bacterial leaf spot (BLS) are caused by various strains of the Pseudomonas syringae species complex that infect zucchini (Cucurbita pepo) seeds. VCZ strains have a narrow host range of cucurbits and affect only seedlings, whereas BLS strains have a broader host range and cause symptoms on adult plants. A multiplex qPCR test showed that VCZ strains predominated in infected seed lots produced in different countries. We surveyed hybrid seed crops grown in parallel in two French regions to address inoculum sources. According to DNA-based approach, parental seed lots were positive to BLS strains, although no culturable bacteria were recovered. Hybrid seed lots produced in the Rhone Valley (southeastern France) showed higher infection rates than those produced in Limagne (central France), and VCZ strains were recovered only from the Rhone Valley. Two representative strains of VCZ and BLS colonized seeds through the vascular and floral pathways, whereas only the BLS strain was transmitted through the pericarp. These findings suggest that floral transmission, potentially mediated by pollinators, could explain the predominance of VCZ strains under favorable regional conditions, and that pericarp transmission in BLS strains could be linked to their capacity to cause disease on adult plants. Furthermore, some infections undetected in seeds became apparent after germination, indicating that testing germinated seeds rather than seeds could help seed industry to take in account only bacterial infections transmitted to the seedling. Together, these results provide valuable insights into the epidemiology of P. syringae transmission to zucchini seeds.

pathology↗

bioRxiv2026

The efficiency of different transmission routes of Xanthomonas citri pv. fuscans and other seed-borne bacteria to bean seeds.

Seed transmission is a critical pathway for the dispersal of phytopathogenic bacteria. This transmission can occur through three main routes: floral, internal, and external. Yet the relative contribution of individual transmission routes remains poorly characterized. Using a pathosystem based on Xanthomonas citri pv. fuscans (Xcf) and common bean (Phaseolus vulgaris cv. Flavert), we quantified the efficiency of each route. Under our experimental conditions, the vascular route was the most efficient with 25% of contaminated seeds and population sizes averaging 107 CFU per contaminated seed. Deploying this experimental framework to ten seed-borne bacterial strains isolated from bean revealed that almost none transmitted to seeds through any route, or at best at low efficiency. However, most of the strains were capable of surviving and disseminating within the vascular system. A major bottleneck for seed transmission was identified for pod vascular organs colonization and the similar behavior of an Xcf mutant, deficient in the T3SS, suggested that plant immunity could be involved at this step. Co-inoculation of a consortium composed of the seed-borne strains with Xcf reduced the number of seeds contaminated by Xcf at the highest inoculum concentration, although other consortia members were never recovered from seeds. This suggests that the strains are recognized by the plant and trigger defense responses. These findings also raise questions about the mechanisms used by seed-associated bacteria to colonize seeds in situ.

microbiology↗

bioRxiv2024

Comparative transcriptomics reveal a highly polymorphic Xanthomonas HrpG virulence regulon

Bacteria of the genus Xanthomonas cause economically significant diseases in various crops. Their virulence is dependent on the translocation of type III effectors (T3Es) into plant cells by the type III secretion system (T3SS), a process regulated by the master response regulator HrpG. Although HrpG has been studied for over two decades, its regulon across diverse Xanthomonas species, particularly beyond type III secretion, remains understudied. In this study, we conducted transcriptome sequencing to explore the HrpG regulons of 17 Xanthomonas strains, encompassing six species and nine pathovars, each exhibiting distinct host and tissue specificities. We employed constitutive expression of plasmid-borne hrpG*, which encodes a constitutively active form of HrpG, to induce the regulon. Our findings reveal substantial inter- and intra-specific diversity in the HrpG* regulons across the strains. Besides 21 genes directly involved in the biosynthesis of the T3SS, the core HrpG* regulon is limited to only five additional genes encoding the transcriptional activator HrpX, the two T3E proteins XopR and XopL, a major facility superfamily (MFS) transporter, and the phosphatase PhoC. Interestingly, genes involved in chemotaxis and genes encoding enzymes with carbohydrate-active and proteolytic activities are variably regulated by HrpG*. The diversity in the HrpG* regulon suggests that HrpG-dependent virulence in Xanthomonas might be achieved through several distinct strain-specific strategies, potentially reflecting adaptation to diverse ecological niches. These findings enhance our understanding of the complex role of HrpG in regulating various virulence and adaptive pathways, extending beyond T3Es and the T3SS. IMPORTANCEIn the decades since its discovery, HrpG and its role in the regulation of the type III secretion system (T3SS) and its associated type III effectors (T3Es) in Xanthomonas has been the subject of extensive research. Despite notable progress in understanding its molecular regulatory mechanisms, the full spectrum of processes under control of HrpG, particularly beyond the T3SS and T3Es, and the degree of regulatory conservation across plant-pathogenic Xanthomonas species, remained unclear. To address this knowledge gap, we systematically compared the transcriptomes of 17 Xanthomonas strains, expressing a constitutively active form of HrpG, called HrpG*. We showed that HrpG* regulates different physiological processes other than the T3SS and T3Es and that this regulation shows substantial variation across the different strains. Taken together, our results provide new insights into Xanthomonas-plant interactions through the regulation of different metabolic and virulence pathways by the master response regulator HrpG.

microbiology↗

bioRxiv2023

A novel "ceasefire" model to explain efficient seed transmission of Xanthomonas citri pv. fuscans to common bean.

O_LIAlthough seed represents an important means of plant pathogen dispersion, the seed-pathogen dialogue remains largely unexplored. C_LIO_LIA multi-omic approach (i.e. dual RNAseq, plant small RNAs and methylome) was performed at different seed developmental stages of common bean (Phaseolus vulgaris L.) during asymptomatic colonization by Xanthomonas citri pv. fuscans (Xcf). C_LIO_LIIn this condition, Xcf did not produce disease symptoms, neither affect seed development. Although, an intense molecular dialogue, via important transcriptional changes, was observed at the early seed developmental stages with down-regulation of plant defense signal transduction, via action of plant miR, and upregulation of the bacterial Type 3 Secretion System. At later seed maturation stages, molecular dialogue between host and pathogen was reduced to few transcriptome changes, but marked by changes in DNA methylation of plant defense and germination genes, in response to Xcf colonization, potentially acting as defense priming to prepare the host for the post-germination battle. This distinct response of infected seeds during maturation, with a more active role at early stages refutes the widely diffused assumption considering seeds as passive carriers of microbes. C_LIO_LIFinally, our data support a novel plant-pathogen interaction model, specific to the seed tissues, which differs from others by the existence of distinct phases during seed-pathogen interaction with seeds first actively interacting with colonizing pathogens, then both belligerents switch to more passive mode at later stages. We contextualized this observed scenario in a novel hypothetical model that we called "ceasefire", where both the pathogen and the host benefit from temporarily laying down their weapons until the moment of germination. C_LI

plant biology↗