First Report of Leaf Spot on Ipomoea nil Caused by Alternaria alternata in China.

Ipomoea nil (Linnaeus) Roth, belonging to the Convolvulaceae family, is an ornamental and medicinal plant in China, which has the function of diuretic and expectorant, and it is also a common weed in the field. In October 2021, a leaf spot disease was observed on I. nil in a field as weed in Jingzhou (N 30° 21', E 112° 19'), Hubei Province, China. Symptoms began as small brown blotches, then developed into oval or irregularly shaped brown necrotic lesions. In severe cases, the leaves were completely necrotic and detached. In the surveyed area, the incidence was between 30% - 40%. To isolate the pathogen, twenty-one leaf pieces (5×5 mm) were cut from the lesion edges of seven symptomatic leaves, disinfected with 70% ethanol and 2% sodium hypochlorite (NaOCl), rinsed with sterile water five times, then placed on three potato dextrose agar (PDA) modified with 50 µg/mL kanamycin, and incubated at 25 °C in dark for 5 days. The isolates were subcultured by transferring mycelium tips. Sixteen fungal strains were isolated from the tissues, and nine of them showed similar morphological characteristics. After cultured 7 days on PDA at 25 °C, the nine colonies were initially white, then turned greenish brown to black in the center and had abundant fine villous aerial mycelia up to 61.5 mm in average diameter. To examine its conidial morphology, the fungi were cultured for 7 days on potato carrot agar (PCA) at 22°C with a light/dark period of 8/16 h. On PCA, conidia were brown or olive-brown, obclavate to obpyriform, with a short beak, one to five transverse and zero to three longitudinal septa. They formed chains of 1 - 8 conidia, with branches. Conidia were 16 - 46 µm long and 8 - 14 µm wide (n=50). These morphological features were similar to those described in Alternaria spp. (Simmons 2007). A single isolate "Q2" was selected for molecular identification because it was the most aggressive in preliminary leaf pathogenicity assays. The internal transcribed spacer (ITS) region of rDNA and histone 3 (H3) gene were amplified and sequenced using primers ITS1/ITS4 (White et al. 1990) and H3-1a/H3-1b (Zheng et al. 2015). BLAST analysis revealed that the sequences (ITS, ON360984; H3, ON375577) were 100% identical to Alternaria alternata (ITS, MK396607; H3, MN840996), respectively. Maximum likelihood analysis based on combined two gene sequences was conducted with an evolutionary model of GTR+I+G under 1000 bootstrap replicates. Phylogenetic tree showed that Q2 and Alternaria alternata 21-5 and BLH-YB-11 located in one clade supported with 99% bootstrap values. The pathogen was identified as A. alternata. To fulfill Koch's postulate, 10 ml conidia (106 spores/ml) of Q2 was sprayed on five healthy seedlings, with sterile distilled water as a control. All leaves were rinsed three times with sterile water before inoculation. All seedlings were placed in sealed plastic bags with air valves, and grown in a greenhouse (25 ± 2 ˚C, RH 65%). The test was repeated twice. After 10 days, symptoms typical of brown blotches similar to those observed in the field were observed on leaves of inoculated plants, while control remained healthy. A. alternata was re-isolated from the inoculated symptomatic leaves with a frequency of 100% based on morphological and molecular characters, thus Koch's postulate was confirmed. To the best of our knowledge, this is the first report of A. alternata causing leaf spot on I. nil in China. Our findings extended the host range of the pathogen A. alternata on characteristic plants.

AvrPm2 encodes an RNase-like avirulence effector which is conserved in the two different specialized forms of wheat and rye powdery mildew fungus.

There is a large diversity of genetically defined resistance genes in bread wheat against the powdery mildew pathogen Blumeria graminis (B. g.) f. sp. tritici. Many confer race-specific resistance to this pathogen, but until now only the mildew avirulence gene AvrPm3a2/f2 that is recognized by Pm3a/f was known molecularly. We performed map-based cloning and genome-wide association studies to isolate a candidate for the mildew avirulence gene AvrPm2. We then used transient expression assays in Nicotiana benthamiana to demonstrate specific and strong recognition of AvrPm2 by Pm2. The virulent AvrPm2 allele arose from a conserved 12 kb deletion, while there is no protein sequence diversity in the gene pool of avirulent B. g. tritici isolates. We found one polymorphic AvrPm2 allele in B. g. triticale and one orthologue in B. g. secalis and both are recognized by Pm2. AvrPm2 belongs to a small gene family encoding structurally conserved RNase-like effectors, including Avra13 from B. g. hordei, the cognate Avr of the barley resistance gene Mla13. These results demonstrate the conservation of functional avirulence genes in two cereal powdery mildews specialized on different hosts, thus providing a possible explanation for successful introgression of resistance genes from rye or other grass relatives to wheat.

Indole-3-Carboxylic Acid From the Endophytic Fungus Lasiodiplodia pseudotheobromae LPS-1 as a Synergist Enhancing the Antagonism of Jasmonic Acid Against Blumeria graminis on Wheat.

The discovery of natural bioactive compounds from endophytes or medicinal plants against plant diseases is an attractive option for reducing the use of chemical fungicides. In this study, three compounds, indole-3-carbaldehyde, indole-3-carboxylic acid (3-ICA), and jasmonic acid (JA), were isolated from the EtOAc extract of the culture filtrate of the endophytic fungus Lasiodiplodia pseudotheobromae LPS-1, which was previously isolated from the medicinal plant, Ilex cornuta. Some experiments were conducted to further determine the antifungal activity of these compounds on wheat powdery mildew. The results showed that JA was much more bioactive than indole-3-carbaldehyde and 3-ICA against Blumeria graminis, and the disease severity caused by B. graminis decreased significantly with the concentration increase of JA treatment. The assay of the interaction of 3-ICA and JA indicated that there was a significant synergistic effect between the two compounds on B. graminis in each of the ratios of 3-ICA to JA (3-ICA:JA) ranging from 1:9 to 9:1. When the compound ratio of 3-ICA to JA was 2:8, the synergistic coefficient was the highest as 22.95. Meanwhile, a histological investigation indicated that, under the treatment of JA at 500 µg/ml or 3-ICA:JA (2:8) at 40 µg/ml, the appressorium development and haustorium formation of B. graminis were significantly inhibited. Taken together, we concluded that JA plays an important role in the infection process of B. graminis and that 3-ICA as a synergist of JA enhances the antagonism against wheat powdery mildew.

Complete Genome Sequence of Bacillus amyloliquefaciens EA19, an Endophytic Bacterium with Biocontrol Potential Isolated from Erigeron annuus.

Bacillus amyloliquefaciens strain EA19 is an endophyte isolated from Erigeron annuus with antifungal activity against Blumeria graminis f. sp. tritici, Magnaporthe oryzae, and Fusarium graminearum. The genome sequence of this strain is 3.96 Mb and contains 3,421 coding sequences, which will facilitate an understanding of the mechanisms of biocontrol.

Transcriptome Analyses Shed New Insights into Primary Metabolism and Regulation of Blumeria graminis f. sp. tritici during Conidiation.

Conidia of the obligate biotrophic fungal pathogen Blumeria graminis f. sp. tritici (Bgt) play a vital role in its survival and rapid dispersal. However, little is known about the genetic basis for its asexual reproduction. To uncover the primary metabolic and regulatory events during conidiation, we sequenced the transcriptome of Bgt epiphytic structures at 3 (vegetative hyphae growth), 4 (foot cells initiation), and 5 (conidiophore erection) days post-inoculation (dpi). RNA-seq analyses identified 556 and 404 (combined 685) differentially expressed genes (DEGs) at 4 and 5 dpi compared with their expression levels at 3 dpi, respectively. We found that several genes involved in the conversion from a variety of sugars to glucose, glycolysis, the tricarboxylic acid cycle (TAC), the electron transport chain (ETC), and unsaturated fatty acid oxidation were activated during conidiation, suggesting that more energy supply is required during this process. Moreover, we found that glucose was converted into glycogen, which was accumulated in developing conidiophores, indicating that it could be the primary energy storage molecule in Bgt conidia. Clustering for the expression profiles of 91 regulatory genes showed that calcium (Ca2+), H2O2, and phosphoinositide (PIP) signaling were involved in Bgt conidiation. Furthermore, a strong accumulation of H2O2 in developing conidiophores was detected. Application of EGTA, a Ca2+ chelator, and trifluoperazine dihydrochloride (TFP), a calmodulin (CaM) antagonist, markedly suppressed the generation of H2O2, affected foot cell and conidiophore development and reduced conidia production significantly. These results suggest that Ca2+ and H2O2 signaling play important roles in conidiogenesis and a crosslink between them is present. In addition to some conidiation-related orthologs known in other fungi, such as the velvet complex components, we identified several other novel B. graminis-specific genes that have not been previously found to be implicated in fungal conidiation, reflecting a unique molecular mechanism underlying asexual development of cereal powdery mildews.

Identification and evaluation of resistance to powdery mildew and yellow rust in a wheat mapping population.

Deployment of cultivars with genetic resistance is an effective approach to control the diseases of powdery mildew (PM) and yellow rust (YR). Chinese wheat cultivar XK0106 exhibits high levels of resistance to both diseases, while cultivar E07901 has partial, adult plant resistance (APR). The aim of this study was to map resistance loci derived from the two cultivars and analyze their effects against PM and YR in a range of environments. A doubled haploid population (388 lines) was used to develop a framework map consisting of 117 SSR markers, while a much higher density map using the 90K Illumina iSelect SNP array was produced with a subset of 80 randomly selected lines. Seedling resistance was characterized against a range of PM and YR isolates, while field scores in multiple environments were used to characterize APR. Composite interval mapping (CIM) of seedling PM scores identified two QTLs (QPm.haas-6A and QPm.haas-2A), the former being located at the Pm21 locus. These QTLs were also significant in field scores, as were Qpm.haas-3A and QPm.haas-5A. QYr.haas-1B-1 and QYr.haas-2A were identified in field scores of YR and were located at the Yr24/26 and Yr17 chromosomal regions respectively. A second 1B QTL, QYr.haas-1B-2 was also identified. QPm.haas-2A and QYr.haas-1B-2 are likely to be new QTLs that have not been previously identified. Effects of the QTLs were further investigated in multiple environments through the testing of selected lines predicted to contain various QTL combinations. Significant additive interactions between the PM QTLs highlighted the ability to pyramid these loci to provide higher level of resistance. Interactions between the YR QTLs gave insights into the pathogen populations in the different locations as well as showing genetic interactions between these loci.

Physcion, a natural anthraquinone derivative, enhances the gene expression of leaf-specific thionin of barley against Blumeria graminis.

BACKGROUND: Physcion is a key active ingredient of the ethanol extract from roots of Chinese rhubarb (Rheum officinale Baill.) that has been commercialised in China for controlling powdery mildews. The biological mechanism of action of physcion against the barley powdery mildew pathogen was studied using bioassay and microarray methods. RESULTS: Bioassay indicated that physcion did not directly affect conidial germination of Blumeria graminis Speer f. sp. hordei Marchal, but significantly inhibited conidial germination in vivo. Challenge inoculation indicated that physcion induced localised resistance rather than systemic resistance against powdery mildew. Gene expression profiling of physcion-treated barley leaves detected four upregulated and five downregulated genes (ratio >or= 2.0 and P-value < 0.05) by using an Affymetrix Barley GeneChip. The five upregulated probe sequences blasted to the same barley leaf-specific thionin gene, with significant changes varying from 4.26 to 19.91-fold. All downregulated genes were defence-related, linked to peroxidase, oxalate oxidase, bsi1 protein and a pathogenesis-related protein. These changes varied from - 2.34 to - 2.96. Quantitative real-time PCR data confirmed that physcion enhanced the gene expression of leaf-specific thionin of barley. CONCLUSION: Results indicated that physcion controls powdery mildew mainly through changing the expression of defence-related genes, and especially enhancing expression of leaf-specific thionin in barley leaves.