Lilac Tasselflower (Emilia sonchifolia) is a New Host for Peanut Witches' Broom Phytoplasma, a 16SrII-V Subgroup Strain in Taiwan

Detection, Identification and Molecular Characterization of the 16SrII-V Subgroup Phytoplasma Strain Associated with Pisum sativum and Parthenium hysterophorus L

Two unrelated plant species, green pea and parthenium weed, harboring typical phytoplasma symptoms, were discovered in Yunlin, Taiwan. Green pea (Pisum sativum.) and parthenium weed (Parthenium hysterophorus L.) are both herbaceous annual plants belonging to the Fabaceae and Asteraceae families, respectively. Displayed symptoms were witches' broom, phyllody and virescence, which are typical indications of phytoplasma infection. Pleomorphic phytoplasma-like bodies were observed under the transmission electron microscope in the sieve elements of symptomatic green pea and parthenium weed. The iPhyClassifier-based virtual RFLP study demonstrated that the phytoplasma associated with the diseased plants belongs to the 16SrII-V subgroup. The disease symptoms of both plants can be explained by the identification of PHYL1 and SAP11 effectors, identical to those of peanut witches' broom phytoplasma. The phytoplasma strains identified in this study present a very close phylogenetic relationship with other 16SrII-V subgroup phytoplasma strains discovered in Taiwan. These results not only convey the local status of the 16SrII-V subgroup phytoplasma strains but also encourage attention to be given to preventing the spread of this threat before it becomes pervasive.

Accelerating Complete Phytoplasma Genome Assembly by Immunoprecipitation-Based Enrichment and MinION-Based DNA Sequencing for Comparative Analyses

Phytoplasmas are uncultivated plant-pathogenic bacteria with agricultural importance. Those belonging to the 16SrII group, represented by 'Candidatus P. aurantifolia', have a wide range of plant hosts and cause significant yield losses in valuable crops, such as pear, sweet potato, peanut, and soybean. In this study, a method that combines immunoprecipitation-based enrichment and MinION long-read DNA sequencing was developed to solve the challenge of phytoplasma genome studies. This approach produced long reads with high mapping rates and high genomic coverage that can be combined with Illumina reads to produce complete genome assemblies with high accuracy. We applied this method to strain NCHU2014 and determined its complete genome sequence, which consists of one circular chromosome with 635,584 bp and one plasmid with 4,224 bp. Although 'Ca. P. aurantifolia' NCHU2014 has a small chromosome with only 471 protein-coding genes, it contains 33 transporter genes and 27 putative effector genes, which may contribute to obtaining nutrients from hosts and manipulating host developments for their survival and multiplication. Two effectors, the homologs of SAP11 and SAP54/PHYL1 identified in 'Ca. P. aurantifolia' NCHU2014, have the biochemical activities in destabilizing host transcription factors, which can explain the disease symptoms observed in infected plants. Taken together, this study provides the first complete genome available for the 16SrII phytoplasmas and contributes to the understanding of phytoplasma pathogenicity.

Identification of 16SrII-V Phytoplasma Associated with Mungbean Phyllody Disease in Taiwan

Mungbean (Vigna radiata (L.) R. Wilczek), an important legume crop in Asia, is primarily cultivated in the central-southern region of western Taiwan. In 2020, mungbean exhibiting typical phytoplasma-induced disease symptoms such as witches' broom, phyllody, virescence, and proliferation was observed in Yunlin County, Taiwan. Moreover, the seed harvested from diseased plants displayed premature germination. Transmission electron microscopy examination of leaf veins prepared from symptomatic mungbean demonstrated that the occlusion of sieve tubes resulted from the accumulation of phytoplasma-like bodies in sieve elements along with filament-like structures in sieve pores. The association of phytoplasma in symptomatic mungbean was confirmed by PCR analyses of the 16S ribosomal RNA (rRNA) and immunodominant membrane protein genes. Further analyses of the 16S rRNA-based phylogenetic tree and the iPhyClassifier-based virtual restriction fragment length polymorphism study demonstrated that the phytoplasma-associated mungbean phyllody disease identified in this study belongs to the 16SrII-V subgroup. BLAST analysis and the phylogenetic analysis indicated that the SAP11-like protein identified in mungbean phyllody disease is identical to peanut witches' broom phytoplasma SAP11, which explains the witches' broom phenotype observed in symptomatic mungbean. The results described in this report confirm that the 16SrII-V phytoplasma, a widely distributed phytoplasma associated with peanut witches' broom disease in Taiwan, has also infected mungbean. This is not only the first instance of mungbean phyllody disease found in Taiwan but also the first instance of mungbean phyllody disease caused by 16SrII-V subgroup phytoplasma.