Publications

4858

Abstract Huanglongbing (HLB), primarily induced by the phloem-restricted bacterium Candidatus Liberibacter asiaticus (CLas), poses a significant threaten to citrus production worldwide. CLas deploys Sec-dependent effectors (SDEs) to manipulate host cellular processes, yet the underlying virulence mechanisms remain incompletely understood. Here, we identify SDE4580, a conserved CLas effector localized to the plant plasma membrane, as a key virulence factor that promotes pathogen proliferation and suppresses host immunity across citrus and Arabidopsis. Transcriptomic analysis reveals that SDE4580 reprograms membrane-associated biological processes and defense responses. Through yeast two-hybrid screening and biochemical validation, we demonstrate that SDE4580 physically interacts with the citrus vesicle trafficking regulator CsPRA1.E, a Prenylated Rab Acceptor 1 family protein. Functional assays establish CsPRA1.E as a positive regulator of immunity that promotes apoplastic secretion of defense-related proteins and enhances resistance to Xanthomonas citri subsp. citri. Crucially, SDE4580 antagonizes CsPRA1.E-mediated resistance by suppressing defense protein secretion without inducing target degradation. These findings uncover a novel pathogenic strategy whereby CLas subverts host endomembrane trafficking to disarm plant immunity. Targeting the SDE4580-CsPRA1.E interface may offer a promising avenue for developing durable HLB control strategies.

‘Candidatus Liberibacter solanacearum’ (Lso) haplotype D, transmitted by the carrot psyllid Bactericera trigonica, is a major threat to carrot production, particularly in the Mediterranean Basin and the Middle East. We investigated the impact of plant age at the time of inoculation on symptom development and yield under greenhouse and outdoor conditions. In greenhouse trials, 1- and 2-month-old plants were inoculated with psyllids harboring Lso. Both age groups showed reduced taproot weight compared with controls, but younger plants were more severely affected, exhibiting 90% yield reduction, while 2-month-old plants showed a 30% reduction. Witches’ broom symptoms appeared earlier in plants inoculated at 1 month (5 weeks postinoculation); however, within 2 weeks, plants inoculated at 2 months reached comparable levels of symptom incidence and severity. Additional greenhouse experiments using Lso-free psyllids confirmed that yield losses and disease symptoms were specifically due to Lso infection and not psyllid feeding. In outdoor trials, we tested the effects of inoculation at 7, 11, 15, and 19 weeks postsowing (wps) on disease incidence and yield. Significant yield losses occurred only in plants inoculated at 7 and 11 wps, with up to 45% reduction by harvest time. Symptoms began to appear at 20 wps in plants inoculated at 7 and 11 wps. At 23 wps, symptoms also appeared in plants inoculated at 15 wps, with a similar incidence to that observed in the earlier inoculation groups. Molecular detection of Lso at harvest (23 wps) confirmed that all plants inoculated at 19 wps or earlier were infected. These results demonstrate that carrots are highly susceptible to Lso-induced yield losses at the earlier stages of growth and highlight the importance of managing early season vector pressure to minimize economic damage.

The spread of the reed planthopper Pentastiridius leporinus and its associated pathogens Candidatus Arsenophonus phytopathogenicus (ARSEPH) and Candidatus Phytoplasma solani (PHYPSO) increasingly threatens sugar beet production in Central Europe. Current surveillance systems rely mainly on vector monitoring, visual field assessments and molecular diagnostics of individual vectors and plants, all of which provide valuable but spatially limited information. This study evaluated sugar beet pulp samples generated during routine factory quality assessment as a large- scale surveillance resource for PHYPSO. During the 2025/26 campaign, 4887 pulp samples representing 1954 delivery field locations were collected, of which 1237 samples of individual field locations with consistent beet quality parameters were analyzed by direct PCR. PHYPSO was detected in 214 samples (17.3%). The molecular workflow was not subgroup-specific and did not include genotyping; therefore, PHYPSO-positive samples could not be assigned to individual 16SrXII subgroups. Comparison with visual field assessments showed that 85.7% of positive molecular detections originated from fields without visible symptoms. A carry-over experiment demonstrated that pathogen-derived DNA may persist across consecutive samples under worst-case conditions and must therefore be considered during data interpretation. In contrast, ARSEPH was detected by direct PCR in only six of 1237 samples (0.5%), all originating from the Euskirchen growing region. Comparison with qPCR, single-beet diagnostics and the carry-over experiment indicated insufficient sensitivity of the current ARSEPH direct-PCR assay; therefore, the observed detection rate should not be interpreted as true pathogen prevalence. Nevertheless, the observed spatial distribution patterns of monitoring sites indicate that industrial pulp sample monitoring substantially expands surveillance coverage beyond conventional symptom-based approaches and can help identify regions warranting further epidemiological investigation.

ABSTRACT Phytoplasmas significantly reduce crop yields across different plant species, as exemplified by coconut lethal yellowing disease (CLY), which affects coconut palms in the American continent. These pathogens encode in the genome the ability to produce effectors that target important proteins in the plant host, inducing morphology alterations, plant sterility and attractiveness to insect vectors. This attractiveness is crucial for the transmission and spread of phytoplasmas to other plant species. Understanding how these effectors function is essential for comprehending the mechanisms of action of phytoplasmas. However, only a few effectors coding genes have been identified in phytoplasmas, and no effector responsible for CLY has been reported. In this study, an in silico identification of two effectors of ‘ Candidatus Phytoplasma palmae’ associated with CLY in the Americas was conducted. The predicted effectors encoded two proteins of 124 aa (secretory protein 7) and 128 aa (secretory protein 10) with 73% identity between them. These proteins were classified as classical effectors, characterised by small size, signal peptide, nuclear localisation signal and absence of a transmembrane domain. Furthermore, gene expression of SP10 was detected in samples from leaves and stems of infected palms.