Evaluation of the novel endophytic fungus Chaetomium ascotrichoides 1-24-2 from Pinus massoniana as a biocontrol agent against pine wilt disease caused by Bursaphelenchus xylophilus.

BACKGROUND: Bursaphelenchus xylophilus, the causative agent of pine wilt disease (PWD), is an ever-increasing threat to Pinus forests worldwide. This study aimed to develop biological control of PWD by the application of endophytic fungi isolated from healthy pine trees. RESULTS: We successfully isolated a novel endophytic fungal strain 1-24-2 from branches of healthy Pinus massoniana. The culture filtrates (CFs) of strain 1-24-2 exhibited strong nematicidal activity against Bursaphelenchus xylophilus, with a corrected mortality rate of 99.00%. Based on the morphological and molecular characteristics, the isolated strain 1-24-2 was identified as Chaetomium ascotrichoides. In the in-planta assay, pine seedlings (2-years-old) treated with 1-24-2 CFs + pine wood nematode (T2) showed a significant control effect of 80%. A total of 24 toxic compounds were first identified from 1-24-2 CFs through gas chromatography-mass spectrometry (GC-MS) analysis, from which O-methylisourea, 2-chlorobenzothiazole, and 4,5,6-trihydroxy-7-methylphthalide showed robust binding sites at Tyr119 against phosphoethanolamine methyltransferase (PMT) protein of Bursaphelenchus xylophilus by molecular docking approach and could be used as potential compounds for developing effective nematicides. Interestingly, strain 1-24-2 produces toxic volatile organic compounds (VOCs), which disturb the natural development process of B. xylophilus, whose total number decreased by up to 83.32% in the treatment group as compared to control and also reduced Botrytis cinerea growth by up to 71.01%. CONCLUSION: Our results highlight the potential of C. ascotrichoides 1-24-2 as a promising biocontrol agent with solid nematicidal activity against B. xylophilus. This is the first report of C. ascotrichoides isolated from P. massoniana exhibiting strong biocontrol potential against B. xylophilus in the world. © 2024 Society of Chemical Industry.

Ralstonia solanacearum differentially modulates soil physicochemical properties and rhizospheric bacteriome of resistant and susceptible tobacco cultivars.

Ralstonia solanacearum is a devastating soilborne pathogen which poses significant yield and economic losses to tobacco production globally. The impact of R. solanacearum on rhizosphere bacteriome and soil physicochemical characteristics in resistant and susceptible tobacco cultivars is poorly understood. This study aims to determine the effect of R. solanacearum on soil physicochemical parameters and rhizosphere bacteriome of resistant (K326) and susceptible (Hongda) tobacco cultivars at various growth stages. Results demonstrated that the contents of available potassium and phosphorus, as well as soil pH were significantly increased in K326 soils (CK and T2) compared with Hongda (T1) after 21, 42, and 63 days post-inoculation (dpi) of R. solanacearum except for available nitrogen which showed an opposite trend. The qPCR results showed a significant decrease in R. solanacearum population in rhizosphere of K326 (T2) compared to the Hongda (T1) at 21 and 63 dpi than that after 42 dpi. The rhizosphere bacteriome analysis through 16S rRNA amplicon sequencing revealed that rhizosphere bacterial community composition was significantly different between two tobacco cultivars (Hongda and K326) and this effect was more prominent after 63 dpi (93 days after post-transplantation), suggesting that each cultivar recruits a unique set of bacterial communities. There was no obvious difference observed in the rhizosphere bacteriome of CK (K326) and T2 (K326), which might be attributed to the same genetic makeup and inherent resistance of K326 to bacterial wilt infection. Analysis of co-occurrence networks revealed that the microbial network in T1 (Hongda) was more complex than those in T2 (K326) and CK (K326), while the networks in CK and T2 were almost identical. The present research highlights the time-course relationship between environmental factors and rhizosphere bacteriome of tobacco cultivars showing different levels of resistance against R. solanacearum. Conclusively, studying the plant-soil-microbe interaction system in susceptible and resistant tobacco cultivars may enable us to develop effective integrated disease control plans for the healthy production of tobacco crops.

Promotion of tomato growth by the volatiles produced by the hypovirulent strain QT5-19 of the plant gray mold fungus Botrytis cinerea.

Our previous study identified a hypovirulent strain QT5-19 of Botrytis cinerea, the causal agent of the plant gray mold disease, and found that QT5-19 can produce volatile organic compounds (VOCs) with high antifungal activity and high control efficacy against B. cinerea. However, impact of the QT5-19 VOCs on plant growth remains unknown. This study was conducted to investigate the impact of the QT5-19 VOCs on tomato growth, and to elucidate the mechanisms for the plant growth-promoting (PGP) activity of the QT5-19 VOCs. Results showed that compared to the control treatment, the QT5-19 VOCs significantly (P < 0.05) promoted tomato growth, and the PGP activity of the QT5-19 VOCs acted in dose- and time-dependent manners. Results also showed that the values of photosynthetic assimilation, stomatal conductance and transpiration, water use efficiency and chlorophyll content in the treatments of the QT5-19 VOCs were significantly (P < 0.05) higher than the corresponding values in the control treatment. The QT5-19 VOCs up-regulated expression of the genes for expansins (EXP2, EXP9 and EXP18), IAA (SlIAA1, SlIAA3 and SlIAA9), cytokinins (SlCKX1) and gibberellins in leaves and/or roots, whereas down-regulated expression of the gene ACO1 for ethylene in both organs. Moreover, enhanced accumulation of auxins and decreased accumulation of ethylene were observed in tomato roots in the treatment of the QT5-19 VOCs, compared to the control treatment. These results suggest that the QT5-19 VOCs probably promote tomato growth through improving photosynthesis and biosynthesis of expansins and IAA, and reducing ethylene biosynthesis. This study suggests that QT5-19 is a versatile biocontrol control agent.

A Novel Partitivirus in the Hypovirulent Isolate QT5-19 of the Plant Pathogenic Fungus Botrytis cinerea.

A pink isolate (QT5-19) of Botrytis cinerea was compared with three gray isolates of B. cinerea for growth and morphogenesis on potato dextrose agar (PDA), and for pathogenicity on tobacco. A double-stranded (ds) RNA mycovirus infecting QT5-19 was identified based on its genome feature and morphology of the virus particles. The results showed that QT5-19 grew rapidly and established flourishing colonies as the gray isolates did. However, it is different from the gray isolates, as it failed to produce conidia and sclerotia asthe gray isolates did. QT5-19 hardly infected tobacco, whereas the gray isolates aggressively infected tobacco. Two dsRNAs were detected in QT5-19, dsRNA 1 and dsRNA 2, were deduced to encode two polypepetides with homology to viral RNA-dependent RNA polymerase (RdRp) and coat protein (CP), respectively. Phylogenetic analysis of the amino acid sequences of RdRp and CP indicated that the two dsRNAs represent the genome of a novel partitivirus in the genus Alphapartitivirus, designated here as Botrytis cinerea partitivirus 2 (BcPV2). BcPV2 in QT5-19 was successfully transmitted to the three gray isolates through hyphal contact. The resulting BcPV2-infected derivatives showed rapid growth on PDA with defects in conidiogenesis and sclerogenesis, and hypovirulence on tobacco. This study suggests that BcPV2 is closely associated with hypovirulence of B. cinerea.