The Exploitation of Microbial Antagonists against Postharvest Plant Pathogens

Production-optimized fermentation of antifungal compounds by bacillus velezensis LZN01 and transcriptome analysis

Fusarium wilt is one of the major constraints on global watermelon production, and Fusarium oxysporum f. sp. niveum (Fon) is the causative agent of Fusarium wilt in watermelon and results in severe yield and quality losses worldwide. The enhancement of antifungal activity from antagonistic bacteria against Fon is highly practical for managing Fusarium wilt in watermelon. The aim of this study was to maximize the antifungal activity of Bacillus velezensis LZN01 by optimizing fermentation conditions and analysing its regulatory mechanism via transcriptome sequencing. The culture and fermentation conditions for strain LZN01 were optimized by single-factor and response surface experiments. The optimum culture conditions for this strain were as follows: the addition of D-fructose at 35 g/L and NH4Cl at 5 g/L in LB medium, pH 7, and incubation at 30°C for 72 h. The fungal inhibition rate for strain LZN01 reached 71.1%. The improvement of inhibition rate for strain LZN01 in optimization fermentation was supported by transcriptomic analysis; a total of 491 genes were upregulated, while 736 genes were downregulated. Transcriptome analysis revealed that some differentially expressed genes involved in carbon and nitrogen metabolism, oxidation-reduction, fatty acid and secondary metabolism; This optimization process could potentially lead to significant alterations in the production levels and types of antimicrobial compounds by the strain. Metabolomics and UPLC/Q-Exactive Orbitrap MS analysis revealed that the production yields of antimicrobial compounds, such as surfactin, fengycin, shikimic acid, and myriocin, increased or were detected in the cell-free supernatant (CFS) after the fermentation optimization process. Our results indicate that fermentation optimization enhances the antifungal activity of the LZN01 strain by influencing the expression of genes responsible for the synthesis of antimicrobial compounds.

Microbial Biofungicides as a Substitute for Chemical Fungicides in the Control of Phytopathogens: Current Perspectives and Research Directions

These days, two important issues are causing concern in the global community: the alarmingly growing trend of the human population and the issue of food security. To this end, people around the world have been searching for solutions that could feed the needy in a sustainable way. In response to this urgent call, scientists from around the world started working on increasing crop production and productivity by controlling crop pathogens that could harm the productivity of crops. Synthetic fungicides have been in use for controlling crop diseases for several decades, but later, due to the evidenced side effects of the fungicides, there have been attempts to shift towards a less cost-effective and eco-friendly method of controlling crop diseases, and so far, many remarkable results have been achieved. However, due to the less effective and shorter shelf life of microbial biofungicides, as well as the less accessibility of these microbial biofungicides to growers around the world, it became difficult to remove the fungicides totally from the market. To minimize this problem, researchers suggested an integrated approach: the combination of microbial biofungicides with a reduced dose of synthetic fungicides. Hence, this review explored the status as well as the merits and demerits of microbial biofungicides as compared to synthetic fungicides.

Identification and Aggressiveness of Fusarium Species Associated with Onion Bulb (Allium cepa L.) during Storage

Plant pathogens present a major challenge to crop production, leading to decreased yield and quality during growth and storage. During long-term storage, healthy onions can develop diseases from latent pathogen infections. This poses a challenge for onion growers because infected bulbs without visible symptoms can lead to significant crop losses during the growing season. In this study, we aimed to isolate and identify Fusarium species from yellow onion bulbs (Allium cepa L.) that developed disease symptoms during storage. The aggressiveness of these strains against onion bulbs and seedlings was also evaluated. The isolated strains were further subjected to morphological and molecular differentiation. The results revealed that all 16 isolated strains belonged to the Fusarium complex species incarnatum-equiseti and Fusarium fujikuroi, namely, F. proliferatum (98%), F. oxysporum (1%), and Fusarium sp. (1%). Koch's postulate analysis of isolated strains revealed varying aggressiveness on onion bulbs and plants depending on fungal species. Disease symptoms developed more slowly on plants than on onion bulb plants according to Koch's postulates. Moreover, the results revealed that Fusarium strains that can infect onion plants were less pathogenic to onion bulbs and vice versa. In addition, three isolates were found to be non-pathogenic to onions. Furthermore, the in vitro control of Fusarium species through Bacillus velezensis KS04-AU and Streptomyces albidoflavus MGMM6 showed high potential for controlling the growth of these pathogenic fungi. These results may contribute to the development of environmentally friendly approaches for controlling onion spoilage caused by pathogens during storage.