Generation and Screening of T-DNA Insertion Mutants Mediated by Agrobacterium tumefaciens in the Garden Asparagus Stem Blight Pathogen Phomopsis asparagi

Screening of pathogenicity-deficient Penicillium italicum mutants established by Agrobacterium tumefaciens-mediated transformation

Blue mold, caused by Penicillium italicum, is one of the main postharvest diseases of citrus fruits during storage and marketing. The pathogenic mechanism remains largely unclear. To explore the potential pathogenesis-related genes of this pathogen, a T-DNA insertion library of P. italicum PI5 was established via Agrobacterium tumefaciens-mediated transformation (ATMT). The system yielded 200-250 transformants per million conidia, and the transformants were genetically stable after five generations of successive subcultures on hygromycin-free media. 2700 transformants were obtained to generate a T-DNA insertion library of P. italicum. Only a few of the 200 randomly selected mutants exhibited significantly weakened virulence on citrus fruits, with two mutants displaying attenuated sporulation. The T-DNA in the two mutants existed as a single copy. Moreover, the mutant genes PiBla (PITC_048370) and PiFTF1 (PITC_077280) identified may be involved in conidia production by regulating expressions of the key regulatory components for conidiogenesis. These results demonstrated that the ATMT system is useful to obtain mutants of P. italicum for further investigation of the molecular mechanisms of pathogenicity and the obtained two pathogenesis-related genes might be novel loci associated with pathogenesis and conidia production.

Improved production of β-glucan by a T-DNA-based mutant of Aureobasidium pullulans

To improve β-1,3-1,6-D-glucan (β-glucan) production by Aureobasidium pullulans, an Agrobacterium tumefaciens-mediated transformation method was developed to screen a mutant A. pullulans CGMCC 19650. Based on thermal asymmetric-interlaced PCR detection, DNA sequencing, BLAST analysis, and quantitative real-time PCR assay, the T-DNA was identified to be inserted in the coding region of mal31 gene, which encodes a sugar transporter involved in pullulan biosynthesis in the mutant. The maximal biomass and β-glucan production under batch fermentation were significantly increased by 47.6% and 78.6%, respectively, while pullulan production was decreased by 41.7% in the mutant, as compared to the parental strain A. pullulans CCTCC M 2012259. Analysis of the physiological mechanism of these changes revealed that mal31 gene disruption increased the transcriptional levels of pgm2, ugp, fks1, and kre6 genes; increased the amounts of key enzymes associated with UDPG and β-glucan biosynthesis; and improved intracellular UDPG contents and energy supply, all of which favored β-glucan production. However, the T-DNA insertion decreased the transcriptional levels of ags2 genes, and reduced the biosynthetic capability to form pullulan, resulting in the decrease in pullulan production. This study not only provides an effective approach for improved β-glucan production by A. pullulans, but also presents an accurate and useful gene for metabolic engineering of the producer for efficient polysaccharide production. KEY POINTS: • A mutant A. pullulans CGMCC 19650 was screened by using the ATMT method. • The mal31 gene encoding a sugar transporter was disrupted in the mutant. • β-Glucan produced by the mutant was significantly improved.

Evaluating the Sensitivities and Efficacies of Fungicides with Different Modes of Action Against Phomopsis asparagi

Asparagus stem blight caused by Phomopsis asparagi is a major hindrance to asparagus production worldwide. Currently, fungicides are used to manage the disease in commercial production, but resistance to common fungicides has emerged in the wild population. In the present study, 132 isolates of P. asparagi collected from different provinces in China were tested for sensitivities to pyraclostrobin, tebuconazole, and fluazinam. We also determined the efficacies of six fungicides against P. asparagi. The frequency distributions of EC₅₀ values of the isolates tested were unimodal, but the curves for pyraclostrobin and tebuconazole had long right-hand tails. The mean EC₅₀ values for pyraclostrobin, tebuconazole, and fluazinam were 0.0426 ± 0.0029, 0.6041 ± 0.0416, and 0.0314 ± 0.0013 μg/ml, respectively. In addition, the EC₅₀ values for pyraclostrobin were very similar with or without salicylhydroxamic acid (SHAM), 20 μg/ml, indicating that SHAM is not needed to determine the sensitivity of P. asparagi to pyraclostrobin when using the mycelial growth inhibition assay. In greenhouse assays, Merivon (42.4% fluxapyroxad plus pyraclostrobin SC), Frown-cide (500 g/liter fluazinam SC), Cabrio (250 g/liter pyraclostrobin EC), and Nativo (75% trifloxystrobin plus tebuconazole WG) showed excellent preventive efficacy against P. asparagi. And these fungicides were more effective before inoculation than when they were applied after inoculation (P < 0.05). Therefore, these fungicides should be applied prior to infection to control stem blight. In field trials, Frown-cide, Merivon, Nativo, and Cabrio also performed good control effects, ranging from 75.2 to 86.0% in 2017 and 75.4 to 87.1% in 2018. We demonstrated that Frown-cide, Merivon, Nativo, and Cabrio had considerable potential to manage asparagus stem blight. In addition, rotations of these fungicides are essential for precluding or delaying the development of resistance and for controlling the disease.