Momordica charantia Is a Novel Host of 'Candidatus Phytoplasma malaysianum'-Related Strains Associated with Bitter Melon Stem Fasciation Disease in China.

Abstract

Momordica charantia, also known as bitter melon, bitter gourd, and bitter squash, is a member of the Cucurbitaceae family and is widely grown in tropical and subtropical regions for its edible fruit and medicinal properties (Alves et al. 2017). In April 2022, bitter melon plants exhibiting stem fasciation and excessive tendril symptoms were observed in a 50-acre vegetable farm in Yijia Village, Weishan Yizu Huizu Autonomous County, Dali, Yunnan Province, China (Fig. 1). The farm primarily grew tomatoes, but around 400 bitter melon plants were planted in spots where tomatoes failed to establish. One plot had a 40% incidence rate, with four out of ten bitter melon plants showing symptoms. Scattered cases were observed in other plots, leading to an overall disease incidence rate of around 2% for the entire farm. Phytoplasma infection was suspected due to symptomatic plants growing in the same province as previously reported cases of phytoplasma diseases, such as happy tree (Camptotheca accuminata) witches'-broom disease, and the presence of phytoplasma-transmitting leafhoppers (Qiao et al. 2023). DNA was extracted from four symptomatic samples and two healthy controls collected from the abovementioned plot with a 40% disease incidence using Bioteke's Plant Genomic DNA Extraction Kit and then tested for phytoplasma infection. A nested PCR assay was conducted using primer pair P1/16S-SR followed by P1A/16S-SR to amplify the near full-length phytoplasma 16S rDNA (about 1.5kb) as previously described (Lee et al. 2004). None of the healthy controls tested positive for phytoplasma infection, while three out of four symptomatic plants showed positive results. The amplicons from the nested PCR were cloned into the pCRII-TOPO vector as previously described (Lee et al. 2004). The resulting clones were sequenced, and the representative sequence was deposited into GenBank (accession number PP489216). The iPhyClassifier (Zhao et al. 2009) was employed to determine the phytoplasma species and group/subgroup associated with the bitter melon stem fasciation (BMSF) disease. The results indicated that the diseased bitter melon plants were infected with a strain related to 'Candidatus Phytoplasma malaysianum' (EU371934), with a 98.07% sequence identity. The similarity coefficient was 1.00 compared to the reference strain of 16SrXXXII-D (GenBank accession: MW138004). The phytoplasma strain associated with BMSF disease was designated as BMSF1. In addition, the same DNA samples underwent further characterization of the BMSF strains. A nested PCR was conducted using primer pair rpL2F3/rpIR1A, followed by rp(III)-FN/rpIR1A to amplify a phytoplasma-specific rp gene segment (about 1.2 kb) (Martini et al. 2007; Davis et al. 2013). Three out of four samples tested positive, consistent with the 16S rRNA gene amplification results. Similarly, a primer pair L15F1/MapR1 followed by secYF1(III)/secYR1(III) was used to amplify a phytoplasma-specific partial spc operon (about 1.7 kb) that includes the complete secY gene and partial rpl15 and map genes, as previously described (Lee et al. 2010). The obtained rp and partial spc amplicons were cloned and sequenced (GenBank accession numbers PP464295 and PP464296). The rp and secY gene sequences were searched against the non-redundant nucleotide collection in the NCBI database using BLASTN. The top hit for the rp gene was 'Ca. Phytoplasma luffae' (CP054393), with 83.24% identity (1068/1283 base-matching). The top hit for the secY gene was also 'Ca. Phytoplasma luffae' (CP054393), with 72.53% identity (1294/1784 base-matching). The percent identity of the BMSF sequences compared to the top hit is low since no other group 16SrXXXII rp and secY gene sequences are available for comparison. A subgroup 16SrXXXII-D phytoplasma strain has been previously reported associated with Camptotheca acuminata witches'-broom (Qiao et al. 2023) and Trema tomentosa witches'-broom (Yu et al. 2021) in China. To our knowledge, bitter melon represents a new host of 'Candidatus Phytoplasma malaysianum'-related strains, and this is the first report of BMSF disease in China. The findings suggest that 'Candidatus Phytoplasma malaysianum'-related strains infect not only ornamental plants but also crops.