Occurrence of a 16SrII-V Subgroup Phytoplasma Associated with Witches'-Broom Disease in Melochia corchorifolia in China.

Abstract

Melochia corchorifolia L. is a plant belonging to the family Sterculiaceae, extracts from this plant have been reported to inhibit melanogenesis (Yuan et al., 2020). During September to November 2020, the plants showing abnormal symptoms including witches'-broom, leaf chlorosis, leaflet and internode shortening (Fig.1), were found in Dingan county of Hainan province, China, with about 50% infection rates in the field. The disease symptoms were suspected to be caused by the phytoplasma, a plant pathogenic prokaryotes that could not be cultured in vitro. Aiming to confirm the pathogen causing the symptoms, total DNA of the symptomatic or asymptomatic Melochia corchorifolia samples were extracted by CTAB method (Doyle and Doyle, 1990) using 0.10 g fresh plant leaves using the rapid extraction kit for plant genomic DNA (CTAB Plant Genome DNA Rapid Extraction Kit, Aidlab Biotechnologies Co., Ltd, Beijing, China). PCR reactions were performed using primers R16mF2/R16mR1 (Gundersen and Lee, 1996) specific for phytoplasma 16S rRNA gene fragments. PCR products of phytoplasma 16S rRNA gene sequences were obtained from the ten symptomatic plant samples but not from the DNA of the asymptomatic plant samples. The PCR products were cloned and sequenced by Biotechnology (Shanghai) Co., Ltd. (Shanghai, China) and the data were deposited in GenBank. The sequences of 16S rRNA gene fragments amplified from the DNA extracted from the disease plant samples were all identical, with a length of 1336 bp for the 16S rRNA (GenBank accession: MZ353520). Nucleotide Blast search based on the 16S rRNA gene fragment of the phytoplasma strain showed 100% sequence identities with that of 16SrII peanut witches'-broom group members, such as Cassava witches'-broom phytoplasma (KM280679), Cleome sp. phytoplasma (KM280677), Tephrosia purpurea witches'-broom phytoplasma (MW616560), Desmodium triflorum little leaf phytoplasma (MT452308) and Peanut witches'-broom phytoplasma (JX403944). Analysis of the 16S rRNA gene sequence of McWB-hnda strain by interactive online phytoplasma classification tool iPhyClassifier (Zhao et al., 2009) indicated that the phytoplasma strain is a member of 16SrII-V subgroup. The phytoplasma strain was named as Melochia corchorifolia witches'-broom (McWB) phytoplasma, McWB-hnda strain. Phylogenetic analysis performed by MEGA 7.0 employing neighbor-joining (NJ) method with 1000 bootstrap value (Kumar et al., 2016) indicated that the McWB-hnda phytoplasma strain was clustered into one clade with the phytoplasma strains of Tephrosia purpurea witches'-broom, Cleome sp., Peanut witches'-broom, Cassava witches'-broom and Desmodium triflorum little leaf with 97 % bootstrap value (Fig.2); McWB-hnda phytoplasma strain identified in the study and Melochia corchorifolia phyllody phytoplasma strain (KX150461) belonging to 16SrI-B subgroup previously identified in the Hainan Island of China by Chen et al. (2017) are in two independent clades(Fig.2). To our knowledge, this is the first report of a 16SrII-V subgroup phytoplasma associated with Melochia corchorifolia witches'-broom disease in Hainan Province, a tropical island of China. The phytoplasma strain identified in the study was relatively close to 16SrII peanut witches'-broom group phytoplasma strains associated with witches'-broom or little leaf diseases in the plants like Peanut, Tephrosia purpurea, Cassava and Desmodium triflorum. Our finding in the study indicated that Melochia corchorifolia may act as an alternative natural host not only for 16SrI-B subgroup phytoplasma but also for 16SrII-V subgroup phytoplasma, which would contribute to the spreading of the related phytoplasma diseases.