Multi-Omics Analysis Reveals the Resistance Mechanism and the Pathogens Causing Root Rot of Coptis chinensis.

Coptis chinensis is a traditional Chinese medicinal herb used for more than 2,000 years. Root rot in C. chinensis can cause brown discoloration (necrosis) in the fibrous roots and rhizomes, leading to plants wilting and dying. However, little information exists about the resistance mechanism and the potential pathogens of the root rot of C. chinensis plants. As a result, in order to investigate the relationship between the underlying molecular processes and the pathogenesis of root rot, transcriptome and microbiome analyses were performed on healthy and diseased C. chinensis rhizomes. This study found that root rot can lead to the significant reduction of medicinal components of Coptis, including thaliotrine, columbamine, epiberberin, coptisine, palmatine chloride, and berberine, affecting its efficacy quality. In the present study, Diaporthe eres, Fusarium avenaceum, and Fusarium solani were identified as the main pathogens causing root rot in C. chinensis. At the same time, the genes in phenylpropanoid biosynthesis, plant hormone signal transduction, plant-pathogen interaction, and alkaloid synthesis pathways were involved in the regulation of root rot resistance and medicinal component synthesis. In addition, harmful pathogens (D. eres, F. avenaceum and F. solani) also induce the expression of related genes in C. chinensis root tissues to reduce active medicinal ingredients. These results provide insights into the root rot tolerance study and pave the way for process disease resistance breeding and quality production of C. chinensis. IMPORTANCE Root rot disease significantly reduces the medicinal quality of Coptis chinensis. In the present study, results found that the C. chinensis fibrous and taproot have different tactics in response to rot pathogen infection. Diaporthe eres, Fusarium avenaceum, and Fusarium solani were isolated and identified to cause different degrees of C. chinensis root rot. These results are helpful for researchers to further explore the mechanism of resistance to rhizoma Coptis root rot.

First Report of Root Rot Caused by Fusarium avenaceum on Coptis chinensis Franchet in Chongqing, China.

Coptis chinensis Franchet, is a perennial herb used as a traditional Chinese medicine. Annual production of Coptis is about 3000 tons in Shizhu, Chongqing. In recent years, root rot has become a serious and widespread disease on Coptis in Shizhu with an average incidence of 40%, and yield losses up to 67%. Infected plants were easy to pull from the soil, and most of the fibrous roots and main roots were brown or black compared to healthy roots that were yellow. Severely infected plants were wilted and necrotic. In October 2019, 33 diseased roots were collected from Shizhu (30°18'N, 108°30'E), and small samples (0.5 cm in length) were cut from the border between diseased and healthy tissue, successively sterilized with 75% ethanol and 2% sodium hypochlorite, rinsed 3 times with sterilized water, dried on sterilized filter paper, and transferred onto PDA, and incubated at 25°C for 7 days in dark. Eighteen distinct fungal isolates (H1-H18) were isolated and Koch's postulates were conducted to verify the pathogenicity of individual isolates. The rhizosphere soil of healthy 2-year-old Coptis plants was inoculated by pouring 5 mL of conidial suspension (106 conidia/mL) scraped from a culture of each isolate on PDA. Sterilized water was used to mock inoculate. For each isolate, 6 plants were inoculated. After 20 days, the roots of all plants inoculated with H15 or H18 were dark brown and rotten, while mock inoculated plants were healthy. The isolates H15 and H18 were re-isolated from symptomatic plants. Isolate H15 formed abundant white mycelium on PDA and produced rose pigment in the agar. Conidia were long and slender, straight to slightly curved, with 1-3 septate. The apical cells were tapering and bent, and the foot cells were distinctly notched. Conidiogenous cells were monophialides and polyphialides. No chlamydospores were observed (Figure S1). Isolate H18 formed white sparse mycelium on PDA and produced no pigment in the agar. Conidia were relatively wide, straight and stout, with 3-5 septate. The apical cells were blunt and rounded, and the foot cells were barely notched. Conidiogenous cells were long monophialides. Chlamydospores were formed intercalary in the hyphae (Figure S2). For further identification, the internal transcribed spacer (ITS), ß-tubulin, translation elongation factor 1ɑ (EF1ɑ) and RNA polymerase second largest subunit (RPB2) gene regions were amplified with ITS1/ITS4, Bt2a/Bt2b, EF1/EF2 and 5f2/7cr (White et al. 1990; Glass and Donaldson, 1995; O'Donnell et al. 2010). GenBank accession numbers of H15 and H18 were MT463390 and MT463389 for the ITS region, MT465656 and MT465654 for ß-tubulin, MT653321 and MT465651 for EF1ɑ, and MT653323 and MT653322 for RPB2. BLAST results showed the ITS, ß-tubulin, EF1ɑ, and RPB2 sequences revealed 100% (533/533 base pairs), 100% (265/265 base pairs), 98% (622/632 base pairs), and 99% (936/947 base pairs) homology respectively with those of Fusarium avenaceum (MN186746.1, MH791368.1, KU238140.1, and MK185027.1), and 100% (537/537 base pairs), 100% (227/227 base pairs), 100% (688/688 base pairs), and 99.03% (918/927 base pairs) with F. solani in GenBank (MH857319.1, MN692929.1, KP674211.1, and MH300549.1), respectively. Thus, H15 and H18 were identified as F. avenaceum and F. solani based on its morphological and molecular characteristics. To our knowledge, F. solani has been previously reported as a pathogen on Coptis (Luo et al. 2014), and this is the first report of root rot on Coptis caused by F. avenaceum in the world. Identification of the pathogens is important for effective disease management and control.