Publications

4416

Citrus Huanglongbing (HLB), a devastating disease caused by the Gram-negative bacterium ‘ Candidatus Liberibacter asiaticus’ ( C Las), poses serious threats to global citrus production and lacks effective control strategies. Previously, SDE2470 (CLIBASIA_02470) was identified as a Sec-dependent effector that contributes to C Las pathogenesis, although its underlying molecular mechanisms were not fully elucidated. In this study, SDE2470 was found to target a citrus vascular one-zinc-finger transcription factor CsVOZ2. CsVOZ2 overexpression ( CsVOZ2- OE) in transgenic citrus plants significantly suppressed C Las colonization, whereas its RNA interference (RNAi) in citrus hairy roots enhanced susceptibility to C Las. Additionally, CsVOZ2 -OE significantly increased reactive oxygen species (ROS) and abscisic acid (ABA) contents accumulation and activated related genes expression. Further investigation revealed that the E3 ligase CsBTS1 directly interacts with CsVOZ2 and promotes its degradation via the 26S proteasome pathway. CsBTS1E3 -OE in citrus hairy roots markedly enhanced C Las proliferation. Importantly, SDE2470 directly interacts with CsBTS1E3 and strengthen CsBTS1E3-CsVOZ2 interaction. Meanwhile, SDE2470 strengthened the E3 ligase activity of CsBTS1, promoting CsVOZ2 degradation. Taken together, these findings support a model in which SDE2470 hijacks CsBTS1 to destabilize CsVOZ2, thereby disrupting ROS- and ABA-dependent immunity and promoting C Las infection in citrus.

Pear decline (PD), associated with ‘Candidatus Phytoplasma pyri’, is a major disease of pear in Europe and the United States. Several psyllid species are involved in the tritrophic system of PD as vectors of phytoplasmas belonging to the 16SrX group. Four years after the first detection of PD in Sicily, an integrated approach was applied to investigate the epidemic in a major pear-growing area. Visual surveys and molecular analyses were conducted over two years in eight orchards. A total of 115 plant samples and 101 Cacopsylla spp. specimens selected from a total of 1,435 collected individuals were analysed, confirming ‘Ca. P. pyri’ in 69% of symptomatic plants and in 4.6% of C. pyri individuals. Multilocus sequence typing (MLST) revealed high genetic similarity among 16SrX isolates. Remote sensing analyses since 2018, combined with vector monitoring, confirmed the epidemic nature of PD and the persistence of a risk of further pathogen spread within the region, proving, inter alia, to be a valid method for identifying the syndrome even on a large scale.

ABSTRACT Citrus Huanglongbing (HLB), a devastating disease caused by the unculturable bacterium ‘ Candidatus Liberibacter asiaticus’ ( C Las), poses a severe threat to global citrus production. C Las secretes effectors to suppress host immune responses and facilitate its colonisation. Previously, the C Las effector SECP8 (CLIBASIA_05330) has been identified as an immune inhibitor. However, its molecular mechanisms on host immune suppression remain unclear. This study identifies the citrus transcription factor CsTCP15 as a target of SECP8. Transgenic citrus plants overexpressing CsTCP15 enhanced resistance to C Las, whereas CsTCP15 ‐RNAi interference plants became more susceptible, confirming its role as a positive immune regulator. Meanwhile, CsTCP15 was demonstrated to directly bind to cis ‐elements of salicylic acid (SA)‐responsive genes CsPR5 and CsWRKY22 , and overexpression of either gene strengthened citrus hairy roots' resistance against C Las. However, SECP8 directly interacts with CsTCP15 and inhibits its homodimerization. Concurrently, mSECP8 facilitates CsBRG3‐mediated degradation and further prevents the dimerization of CsTCP15. This two‐pronged interference eventually impairs the transcriptional activation of CsPR5 and CsWRKY22 , thereby compromising salicylic acid‐mediated immunity and promoting C Las infection. Our findings reveal a virulence strategy whereby a C Las effector manipulates a key host immune regulator to establish pathogenesis.

From 2016, witches’ broom and stunting symptoms were observed in Lolium rigidum grown in some fruit tree nurseries in Faragheh (Abarkouh, Yazd province, Iran). Total DNAs were extracted from symptomatic and asymptomatic plants and assessed for phytoplasma presence using direct and nested PCR to detect the 16S ribosomal RNA gene. From all symptomatic L. rigidum plant samples, expected length PCR amplicons were obtained. RFLP analysis with informative restriction enzymes showed identical profiles in all the samples resulted positive, that were also consistent with those of one of the subgroups of the aster yellows phytoplasmas (16SrI). The 16S rRNA gene sequence of Faragheh L. rigidum bushy stunt strain was 100% identical to some ‘Candidatus Phytoplasma asteris’ related strains, and 99.12% similar to the reference ‘Ca. P. asteris’ strain. The virtual RFLP pattern was identical (similarity coefficient 1.00) to the pattern of phytoplasmas in subgroup 16SrI-F. This is the first report of occurrence and molecular identification of this phytoplasma strain in L. rigidum and indicates a potential phytoplasma reservoir for trees in fruit tree nurseries where insect vectors may be present. This phytoplasma strain has been reported in symptomatic stone fruits in Spain and in potato in Ecuador. Further research on the epidemiology of witches’ broom and stunting in L. rigidum is required to develop elimination the phytoplasma from areas surrounding agricultural crops and avoid the risks of epidemics.

ABSTRACT Five draft genome sequences of ‘ Candidatus Liberibacter solanacearum’ haplotype E from five different Bactericera trigonica vector insects sampled in France are reported. Each genome has a size of about 1.20 Mbp, a G + C content of 35% and approximately 1,000 predicted coding DNA sequences.