Root Rot of Codonopsis tangshen Caused by Ilyonectria robusta in Chongqing, China

DNA metabarcoding analysis of fungal community on surface of four root herbs

Objective

Angelicae Sinensis Radix (ASR, Danggui in Chinese), Cistanches Herba (CH, Roucongrong in Chinese), Ginseng Radix et Rhizoma (PG, Renshen in Chinese), and Panacis Quinquefolii Radix (PQ, Xiyangshen in Chinese), widely used as medicine and dietary supplement around the world, are susceptible to fungal and mycotoxin contamination. In this study, we aim to analyze their fungal community by DNA metabarcoding.

Methods

A total of 12 root samples were collected from three main production areas in China. The samples were divided into four groups based on herb species, including ASR, CH, PG, and PQ groups. The fungal community on the surface of four root groups was investigated through DNA metabarcoding via targeting the internal transcribed spacer 2 region (ITS2).

Results

All the 12 samples were detected with fungal contamination. Rhizopus (13.04%-74.03%), Aspergillus (1.76%-23.92%), and Fusarium (0.26%-15.27%) were the predominant genera. Ten important fungi were identified at the species level, including two potential toxigenic fungi (Penicillium citrinum and P. oxalicum) and eight human pathogenic fungi (Alternaria infectoria, Candida sake, Hyphopichia burtonii, Malassezia globosa, M. restricta, Rhizopus arrhizus, Rhodotorula mucilaginosa, and Ochroconis tshawytschae). Fungal community in ASR and CH groups was significantly different from other groups, while fungal community in PG and PQ groups was relatively similar.

Conclusion

DNA metabarcoding revealed the fungal community in four important root herbs. This study provided an important reference for preventing root herbs against fungal and mycotoxin contamination.

First Report of Root Rot Caused by Ilyonectria robusta in the Medicinal Herb Aconitum carmichaelii in China

Aconitum carmichaelii Debeaux is used as a traditional Chinese medicine with antiarrhythmic, antiinflammatory and other pharmacological functions. It is widely cultivated in China. According to our survey, about 60% of A. carmichaelii in Qingchuan, Sichuan, suffered from root rot, reducing yields by 30% in the past five years. Symptomatic plants exhibited stunted growth, dark brown roots, reduced root biomass, and fewer root hairs. The disease caused root rot and plant death in 50% of the infected plants. In October 2019, ten symptomatic 6-month-old plants were collected from fields in Qingchuan. Diseased pieces of the roots were surface sterilized with sodium hypochlorite solution (2%), rinsed three times in sterile water, plated on potato dextrose agar (PDA), and incubated at 25°C in the dark. Six single-spore isolates of a Cylindrocarpon-like anamorp were obtained. The colonies on PDA were 35 to 37 mm diam after seven days with regular margins. The plates were covered with felty aerial mycelium, white to buff, and the reverse side chestnut near center with a ochre to yellowish leading edge. On spezieller nährstoffarmer agar (SNA), macroconidia were 1 to 3 septate, straight or slightly curved, cylindrical, with rounded ends, and varied in size: 1-septate 15.1 to 33.5 × 3.7 to 7.3 μm (n=250), 2-septate 16.5 to 48.5 × 3.7 to 7.6 μm (n=85), and 3-septate 22.0 to 50.6 × 4.9 to 7.4 μm (n=115). Microconidia were ellipsoid to ovoid, and 0 to 1 septate; aseptate spores were 4.5 to 16.8 × 1.6 to 4.9 μm (n=200), and 1-septate spores were 7.4 to 20.0 × 2.4 to 5.1 μm (n=200). The chlamydospores were brown, thick-walled, globose to subglobose, 7.9 to 15.9 μm (n=50). The morphology of these isolates was consistent with the previous description of Ilyonectria robusta (Cabral et al. 2012). Isolate QW1901 was characterized by sequencing the ITS, TUB, H3, and tef1α loci using previously reported primer pairs: ITS1/ITS4 (White et al. 1990), T1/Bt-2b (O'Donnell and Cigelnik 1997), CYLH3F/CYLH3R (Crous et al. 2004), and EF1/EF2 (O'Donnell et al. 1998). A Blastn search of the sequences of ITS, TUB, H3, and tef1α showed that QW1901 shared 99.26, 97.89, 97.79, and 99.17 % identities, respectively, with the ex-type strain of I. robusta (CBS308.35). The ITS, TUB, H3, and tef1α sequences were deposited in GenBank under accession nos. MW534715, and MW880180 to MW880182, respectively. A phylogenetic tree was constructed from a neighbor-joining analysis on the alignment of the combined ITS, TUB, H3, and tef1α sequence. QW1901 was clustered with the ex-type strain of I. robusta. To confirm the pathogenicity of I. robusta, bare roots of healthy 6-month-old A. carmichaelii were inoculated with mycelial plugs of 7-day-old QW1901 colonies selected randomly (Lu et al. 2015). Five needle-wound lateral roots and five intact roots were inoculated as replicates with pathogen-free agar plugs as a control. Then, all plants were grown in sterile soil in a growth chamber at 20±1°C and watered regularly. Pathogenicity assays were repeated twice. After 20 days of cultivation, infected plants exhibited symptoms similar to those observed in the field. All control plants remained asymptomatic. Sequencing confirmed the re-isolation of I. robusta from the inoculated plants, satisfying Koch's hypothesis. Ilyonectria robusta has been reported to cause root rot of plants such as Codonopsis tangshen and Panax ginseng ( Lu et al. 2015; Zheng et al. 2021), and has also been reported to be isolated from Aconitum kongboense in China (Wang et al. 2015). However, this is the first report of the pathogen causing root rot of A. carmichaelii. Management measures, such as growing disease-free seedlings in sterile soil, should be used to minimize the risk of this pathogen.

First report of Ilyonectria robusta causing root rot on Coptis chinensis in Chongqing, China

Coptis chinensis belongs to the Ranunculaceae family and is a widely used traditional Chinese herb. Chongqing Municipality produces >60% of China's production. Root rot seriously reduced yield and quality (Mei et al. 2021). In May 2020, root rot of C. chinensis were observed on 3-year-old roots with an average incidence of 45.3% in three commercial fields (about 0.5 acre) in Fengmu Town, Shizhu County (30.24°N; 108.48°E) from Chongqing. Diseased plants were stunted and less vigorous with wilting and twisting leaves. Brown or black discoloration lesion was appeared in the vascular and cortical tissue of roots and rhizomes. Ten fresh symptomatic plants were randomly sampled from the fields. Root tissues were surface sterilized in 75% ethanol for 60s, rinsed thrice with sterile water, placed on potato dextrose agar (PDA), and incubated at 25°C for 7 days. A total of 11 isolates were obtained from the infected tissues. Pure colonies of all fungal isolates had similar characteristics, and five isolates (a2, a4, a9, a11, a12) were randomly selected for further study. Colonies of this fungus were aurantium and felty at first, and then became brownish grey. Macroconidia (n=50) were predominating, hyaline, cylindrical, predominantly straight with both ends broadly rounded, 1~3 septate; one septate, 18.8~25.5×5.9~6.8μm; two septate, 22.6~35.4×6.1~7.2μm; three septate, 26.1~42.5×7.2~8.0 μm. Microconidia (n=50) were hyaline, ellipsoid to ovoid, 0 to 1 septate; aseptate, 7.5~8.8×3.4~4.3μm. Chlamydospores (n=50) were hyaline at first, and becoming brown, globose to subglobose, smooth, 8.3~12.5×8.1~13.5μm, mostly occurring intercalary in chains. The DNA of isolates were extracted and the ITS, HIS, TEF, TUB2 genes were amplified and sequenced using the primers ITS1/ITS4, CYLH3F/CYLH3R, EF1/EF2, T1/CYLTUB1R, respectively (Cabral et al. 2012). The representative isolate a2 were deposited in GenBenk (OK105140, ITS; OM799544, HIS; OK493444, TEF; OK493445, TUB2). BLAST analysis showed the ITS, HIS, TEF, TUB2 sequences of a2 were 100% (417/417), 100% (472/472), 100% (762/762), and 99.7% (490/491) homology with those of Ilyonectria robusta (CBS 605.92) from Tilia petiolaris in Germany. Phylogenetic analysis using Maximum Likelihood and concatenated sequences (ITS+HIS+TEF+TUB2) with MEGA7 placed isolate a2 in I. robusta with 100% bootstrap support. The isolate was thus identified as I. robusta based on morphological and molecular characteristics (Cabral et al. 2012). Thirty healthy 6-month-old C. chinensis plants were used for the pathogenicity tests, and five plants were into each of 6 pots. 10ml of conidia suspension (1×106conidia/ml) of 10-day-old isolate a2 was gently applied to the soil in each of 6 pots. Sterile water (10ml) was applied to each of 6 pots as control. All 12 pots were placed in a greenhouse (25°C, 12h photoperiod). After 6 weeks inoculation, all inoculated plants showed twisting and wilting symptoms, and the roots showed light-brown to dark-brown lesions. No symptoms were observed on the controls. The pathogen was reisolated from all symptomatic roots and identified as I.robustaas previously described above. The test was repeated twice with similar results. Although this fungus was previously reported to cause root disease on many plants (Zheng et al. 2022; Qiao et al. 2019; Guggenheim et al. 2019), this is the first report of I. robusta causing root rot on C. chinensis in China, and will establish a foundation for controlling the disease.

RNA-Seq Provides Insights into the Mechanisms Underlying Ilyonectria robusta Responding to Secondary Metabolites of Bacillus methylotrophicus NJ13

(1) Background: Ilyonectria robusta can cause ginseng to suffer from rusty root rot. Secondary metabolites (SMs) produced by Bacillus methylotrophicus NJ13 can inhibit the mycelial growth of I. robusta. However, the molecular mechanism of the inhibition and response remains unclear. (2) Methods: Through an in vitro trial, the effect of B. methylotrophicus NJ13’s SMs on the hyphae and conidia of I. robusta was determined. The change in the physiological function of I. robusta was evaluated in response to NJ13’s SMs by measuring the electrical conductivity, malondialdehyde (MDA) content, and glucose content. The molecular interaction mechanism of I. robusta’s response to NJ13’s SMs was analyzed by using transcriptome sequencing. (3) Results: NJ13’s SMs exhibited antifungal activity against I. robusta: namely, the hyphae swelled and branched abnormally, and their inclusions leaked out due to changes in the cell membrane permeability and the peroxidation level; the EC50 value was 1.21% (v/v). In transcripts at 4 dpi and 7 dpi, the number of differentially expressed genes (DEGs) (|log2(fold change)| > 1, p adj ≤ 0.05) was 1960 and 354, respectively. NJ13’s SMs affected the glucose metabolism pathway, and the sugar-transporter-related genes were downregulated, which are utilized by I. robusta for energy production. The cell wall structure of I. robusta was disrupted, and chitin-synthase-related genes were downregulated. (4) Conclusions: A new dataset of functional responses of the ginseng pathogenic fungus I. robusta was obtained. The results will benefit the development of targeted biological fungicides for I. robusta and the study of the molecular mechanisms of interaction between biocontrol bacteria and phytopathogenic fungi.

Mapping of dynamic QTLs for resistance to Fusarium wilt (Fusarium oxysporum f. sp. vasinfectum) race 4 in a backcross inbred line population of Upland cotton

KEY MESSAGE

A backcross inbred line population of cotton was evaluated for Fusarium wilt race 4 resistance at different days after inoculation (DAI). Both constitutively expressed and developmentally regulated QTLs were detected. The soil-borne fungus Fusarium oxysporum f. sp. vasinfectum (FOV) race 4 (FOV4) causes Fusarium wilt including seedling mortality in cotton. A backcross inbred line (BIL) population of 181 lines, derived from a bi-parental cross of moderately resistant non-recurrent Hai 7124 (Gossypium barbadense) and recurrent parent CCRI 36 (G. hirsutum), was evaluated under temperature-controlled conditions for FOV4 resistance with artificial inoculations. Based on three replicated tests evaluated at 7, 14, 21, and 28 days after inoculation (DAI), only 2-5 BILs showed lower disease severity ratings (DSR) than the parents while 22-50 BILs were more susceptible, indicating transgressive segregation toward susceptibility. Although DSR were overall congruent between DAI, there were many BILs displaying different responses to FOV4 across DAI. Genetic mapping using 7709 SNP markers identified 42 unique QTLs for four evaluation parameters- disease incidence (DI), DSR, mortality rate (MR), and area under disease progress curve (AUDPC), including 26 for two or more parameters. All five QTLs for AUDPC were co-localized with QTLs for DI, DSR, and/or MR at one or two DAI, indicating the unnecessary use of AUDPC in QTL mapping for FOV4 resistance. Those common QTLs explained the significant positive associations between parameters observed. Ten common QTLs with negative or positive additive effects were detected between DAI. DAI-specific and consistent QTLs were detected between DAI in cotton for the first time, suggesting the existence of both constitutively expressed and developmentally regulated QTLs for FOV4 resistance and the importance of evaluating genetic populations for FOV4 resistance at different growth stages.