Ampelopsis grossedentata Represents a New Host of the 16SrI Group of Phytoplasma Associated with Yellow Leaf Symptoms in China.

Abstract

Ampelopsis grossedentata, commonly known as "Vine Tea" and well-recognized for its rich flavonoid content, is mainly distributed in the southern regions of the Yangtze River basin in China. These regions include Hunan, Hubei, Jiangxi, and Guizhou provinces. Vine Tea is mainly consumed as an herbal tea and has garnered attention for its reported health benefits, including antioxidant, anti-inflammatory, anti-tumor, anti-diabetic, and neuroprotective properties. It has been used to alleviate coughs and sore throats (Zhang et al., 2021; Wang et al., 2017; Gao et al., 2009). In the Zhangjiajie region of Hunan province alone, the Vine Tea planting area reached 7,670.5 hectares and produced commercial goods worth 1.417 billion RMB in 2022. In May 2021, leaf margins and veins fading to yellowing mottling, and crumpling of leaf blades in the shape of a boat symptoms were found in ~16% of Vine Tea plants in the Sanjiakuan Township, Yongding District, Zhangjiajie region (29°15'E, 110°30' N) (Figure 1a, b, c). (Figure 1a, b, c). Phytoplasma-like microbial cells (small oval shaped bacterial cells, around 1000 nm in size) were observed in sieve tube cells in the phloem of diseased leaves using transmission electron microscopy. No such cell was observed in the phloem of healthy leaves (Figure 2a, b). To investigate the potential association between phytoplasma and the observed symptoms of the diseased plants, total DNA was isolated from ten diseasedeaves and compared with ten healthy leaves from the same field using SteadyPure Plant Genomic DNA Extraction Kit. The isolated DNAs were analyzed first in a direct PCR using universal phytoplasma primer pair R16mF2/R16mR1 targeting the 16S rRNA gene (Gundersen and Lee 1996) and specific pair rpF1/rpR1 (Lee et al. 1998) targeting the DNA fragment encoding partial ribosomal proteins (rp) L22 (complete) and S3 and S19 (partial). The initial amplified products were used as templates and further amplified by nested PCR respectively with primer pair R16F2n/R16R2 for the 16S rRNA gene (Lee et al. 1998) and the rpF2/rpR2 primer pair for the rp gene (Martini et al. 2007). No amplification was obtained with DNA from healthy leaf samples using any of the four primer pairs. The amplified fragments from diseased leaves by nested PCR were cloned and sequenced (Qingke Biotech, China). The obtained sequences have been deposited in GenBank with accession numbers OR282806 for the 16S rRNA gene and GenBank OR353012 for the rp gene. BLASTn analysis revealed that the partial 16S rRNA gene sequence in our sample shared 99.4% nucleotide sequence identity with 'Candidatus Phytoplasma sp.' (MW364378) and 'Peony yellows phytoplasma' (KY814723) of the 16SrI group. Similarly, our rp gene sequence shared 99.6% nucleotide identity with the rpI group of phytoplasma such as the 'Balsamine virescence phytoplasma' (JN572890) and 'Paulownia witches'-broom phytoplasma' (HM146079). Phylogenetic analysis of the 16S rRNA and rp sequences using MEGA version 7.0 revealed that the phytoplasma strain associated with A. grossedentata yellow leaf syndrome in our study site belonged to the 16SrI (Candidatus Phytoplasma asteris) group of phytoplasma (Figure 3a, b). Using the interactive online phytoplasma classification tool iPhyClassifier (Zhao et al., 2009), virtual restriction fragment length polymorphism (RFLP) analysis of the 16S rRNA gene sequences showed our strain having a distinct RFLP map but was closest to that of the onion yellow phytoplasma 16SrI-B subgroup (GenBank accession number: AP006628), with a similarity coefficient of 0.94 (Figure 4a, b). To confirm phytoplasma transmission, healthy plants were inoculated with three scions of infected plants of A. grossedentata. After 16 days, the new leaves of the inoculated A. grossedentata showed yellow leaf symptoms (Figure 5a, b, c), akin to the symptoms originally observed in the field, and the outer contour of the leaf margin appeared chlorotic. After 26 days, primer pairs R16mF2/R16R1 and R16F2n/R16R2 were used for nested PCR detection of phytoplasma in symptomatic A. grossedentata leaves. Phytoplasma was detected in the first and second leaves of symptomatic branches and leaves while negative control showed no amplification. Sequencing of the amplified fragments showed 100% nucleotide identity to the strain from the grafting source. Our results indicated that the pathogen and the disease can be transmitted by tissue grafting, consistent with the biological characteristics of phytoplasma, and further confirmed that the phytoplasma was the pathogen of yellow leaf syndrome of A. grossedentata. Toour knowledge, this is the first report of phytoplasma of group 16SrI affecting A. grossedentata.