Murraya exotica L., commonly known as orange jasmine, is an evergreen shrub belonging to the Rutaceae family. It has long been used as traditional Chinese medicine for treating abdominal pain, toothache, scabies, and other disorders (Liu et al. 2018). M. exotica is widely grown as a garden bush in Taiwan. A prokaryotic pathogen, 'Candidatus Liberibacter asiaticus' (Damsteegt et al. 2010), reportedly could infect M. exotica, but there is no reported phytoplasma disease in M. exotica. In June 2020, M. exotica plants exhibiting witches'-broom (WB), leaf yellowing, and small leaves (Fig. s1) were observed in a horticultural landscaping field in Taichung City, Taiwan. It was estimated that more than 70% of M. exotica plants within a single area were affected. DNA was extracted separately from petioles of five symptomatic and one asymptomatic plants using a modified CTAB method (Echevarría-Machado et al. 2005) and used for nested PCR with two universal primers, P1 (Deng and Hiruki 1991)/P7 (Schneider et al. 1995) followed by R16F2n/R16R2 (Gundersen and Lee 1996) to amplify a 1.2-kb 16S rRNA fragment. PCR was also conducted by primers, rp(I)F1A/rp(I)R1A to amplify a partial ribosomal protein S3 and L22 (rplV-rpsC) fragment (Lee et al. 2004). Expected 1.2-kb bands were amplified from DNA extracted from all symptomatic plants, whereas no bands were amplified from that of the asymptomatic plant. The amplicons were cloned, sequenced with an ABI 3730 automatic sequencer (Applied Biosystems, Hammonton, NJ, USA) in Biotechnology Centre DNA-sequencing facility at National Chung Hsing University (NCHU) and deposited in GenBank. BLAST analysis revealed that 16S rDNA sequences (MZ373297 and MZ373298) shared 100% identity to each other and both shared 99.4% identity with those of several phytoplasma strains, e.g., rapeseed phyllody phytoplasma (CP055264), Brassica sp. phyllody phytoplasma (MN877914), Plumbago auriculata leaf yellowing phytoplasma (MN239504), and aster yellows phytoplasma (MK992774), which all belonging to the 16SrI group, by using the CLUSTAL W Methods of MegAlign program (DNASTAR, Inc., Madison, WI, USA). Further analysis using iPhyClassifier tool (https://plantpathology.ba.ars.usda.gov) indicated that the virtual restriction fragment length polymorphism (RFLP) patterns derived from the 16S rDNA F2nR2 fragment of the M. exotica WB phytoplasma was most similar to the reference pattern of the 16SrI-B subgroup, with a pattern similarity coefficient of 0.97 and shared 99.3% sequence identity to 'Candidatus Phytoplasma asteris' (M30790). The partial rplV-rpsC gene sequence (OM275408) showed 99.7% of sequence identities to those of rapeseed phyllody phytoplasma (CP055264), plum witches'-broom phytoplasma (MH061366) and oilseed rape phytoplasma (KX551965), by using the CLUSTAL W Methods of MegAlign program. Taken together, we concluded that the phytoplasma strain associated with M. exotica WB disease was a strain belonging to a 16SrI. To the best of our knowledge, this is the first report of M. exotica being infected by a phytoplasma in the aster yellows group, and M. exotica may also serve as an intermediate reservoir host to other plants, e.g., wax apple, periwinkle and roselle, of 16SrI phytoplasma.
Keywords: Causal Agent; Crop Type; Murraya exotica; Phytoplasma; Prokaryotes; Tropical plants; Witches’-broom; orange jasmine.