Toth, Rafael

Sugar beet cultivation in Europe is threatened by two vector-borne diseases: syndrome “basses richesses”, caused by the phloem-limited pathogen ‘Candidatus Arsenophonus phytopathogenicus’, and phytoplasmoses associated with ‘Ca. Phytoplasma solani’ subgroup 16SrXII-A and the related subgroup 16SrXII-P. Infections lead to reduced sugar yield, biomass and growth abnormalities. In Germany, Pentastiridius leporinus represents the main vector. Despite their importance, genetic diversity remains poorly understood. During a two-year survey, barcoded amplicons were generated from infected sugar beet samples from Germany and neighbouring countries using the phytoplasma markers 16S rRNA-ITS-23S rRNA, tuf, and groEL-stamp-nadE, as well as rplO-secY-rpmJ and groEL for ‘Ca. A. phytopathogenicus’. Amplicon pools underwent single-molecule real-time sequencing and amplicon-sequence-variant inference. Additionally, planthopper samples from sugar beet in Germany were analysed and compared to sugar beet data for ‘Ca. A. phytopathogenicus’. No genetic diversity of ‘Ca. A. phytopathogenicus’ was detected, whereas 16SrXII-A and -P showed variation below the subgroup level. 16SrXII-A exhibited higher diversity than 16SrXII-P. In Germany, 16SrXII-A formed a single cluster, while 16SrXII-P comprised two clusters based on 16S rRNA-ITS-23S rRNA. In neighbouring countries, only 16SrXII-A showed diversity, resolving up to four clusters by groEL-stamp-nadE. These results provide a basis for the identification of dominant strains supporting comparative variety evaluation for tolerance.

ABSTRACT The complete genome of “ Candidatus Phytoplasma solani” GOE was obtained from the infected vector Pentastiridius leporinus by single-molecule real-time sequencing. This 16SrXII-P phytoplasma is associated with the economically important sugar beet disease “syndrome basses richesses.” The genome sequence is an essential resource for diagnosis and understanding pathogen–host interaction.

Phytoplasmas are linked to diseases in hundreds of economically important crops, including carrots. In carrots, phytoplasmosis is associated with leaf chlorosis and necrosis, coupled with inhibited root system development, ultimately leading to significant economic losses. During a field study conducted in Baden-Württemberg (Germany), two strains of the provisional taxon ‘Candidatus Phytoplasma asteris’ were identified within a carrot plot. For further analysis, strains M8 and M33 underwent shotgun sequencing, utilising single-molecule-real-time (SMRT) long-read sequencing and sequencing-by-synthesis (SBS) paired-end short-read sequencing techniques. Hybrid assemblies resulted in complete de novo assemblies of two genomes harboring circular chromosomes and two plasmids. Analyses, including average nucleotide identity and sequence comparisons of established marker genes, confirmed the phylogenetic divergence of ‘Ca. P. asteris’ and a different assignment of strains to the 16S rRNA subgroup I-A for M33 and I-B for M8. These groups exhibited unique features, encompassing virulence factors and genes, associated with the mobilome. In contrast, pan-genome analysis revealed a highly conserved gene set related to metabolism across these strains. This analysis of the Aster Yellows (AY) group reaffirms the perception of phytoplasmas as bacteria that have undergone extensive genome reduction during their co-evolution with the host and an increase of genome size by mobilome.