Heterocycle-Substituted α-Methylene-γ-Butyrolactones Derivatives Synthesis, Antifungal Activity, and 3D-QSAR.

Developing fungicides from active botanical skeletons is one of the effective methods to tackle the resistance of plant pathogens. Based on our previous discoveries, a series of novel α-methylene-γ-butyrolactone (MBL) derivatives containing heterocycles and phenyl rings were designed according to the antifungal molecule carabrone first discovered in plant Carpesium macrocephalum. The target compounds were synthesized, and the inhibitory activity against pathogenic fungi as well as the mechanism of action were then systematically investigated. Several compounds showed promising inhibitory activities against a variety of fungi. The most potent compound 38 exhibited the EC50 value of 0.50 mg/L against Valsa mali (V. mali), which was more effective than that of commercial fungicide famoxadone. The protective effect of compound 38 against V. mali on apple twigs was superior to that of famoxadone, with an inhibition rate of 47.9% at 50 mg/L. The physiological and biochemical results showed that compound 38 inhibits V. mali by causing cell deformation and contraction, reducing the number of intracellular mitochondria, thickening the cell wall, as well as increasing the permeability of the cell membrane. Based on three-dimensional quantitative structure-activity relationship (3D-QSAR) analyses, it was shown that the introduction of the bulky and negatively charged groups favored the antifungal activity of the novel MBL derivatives. These findings suggest that compound 38 can be a potential candidate for novel fungicides worthy of further investigation further.

Plant Growth-Promoting Rhizobacteria HN6 Induced the Change and Reorganization of Fusarium Microflora in the Rhizosphere of Banana Seedlings to Construct a Healthy Banana Microflora.

Streptomyces aureoverticillatus HN6 was isolated in our previous study and effectively controlled banana Fusarium wilt. We explored the role of HN6 in constructing a healthy rhizosphere microflora of banana seedlings. The method of antibiotic resistance was used to determine the colonization ability of HN6. The effect of HN6 on the rhizosphere microbial communities was assessed using culture-dependent and high-throughput sequencing. The effect of HN6 on the infection process of the pathogen was evaluated using a pot experiment and confocal laser scanning microscopy. The results showed that HN6 could prevent pathogen infection; it increased the nutrient content and diversity of the bacterial community in the rhizosphere, promoted plant growth, and decreased the mycotoxin fusaric acid content and abundance of pathogens in the banana rhizosphere. Thus, HN6 decreased the relative abundance of Fusarium species, increased the diversity of fungi, and increased the relative abundance of bacteria in the rhizosphere. HN6 induced the change and reorganization of the microbial community dominated by Fusarium in the rhizosphere of banana seedlings, and it evolved into a community dominated that was not conducive to the occurrence of diseases, shaping the rhizosphere microflora and promoting the growth of banana.

Aureoverticillactam, a Potent Antifungal Macrocyclic Lactam from Streptomyces aureoverticillatus HN6, Generates Calcium Dyshomeostasis-Induced Cell Apoptosis via the Phospholipase C Pathway in Fusarium oxysporum f. sp. cubense Race 4.

Extensive efforts have been made to discover new biofungicides of high efficiency for control of Fusarium oxysporum f. sp. cubense race 4, a catastrophic soilborne phytopathogen causing banana Fusarium wilt worldwide. We confirmed for the first time that aureoverticillactam (YY3) has potent antifungal activity against F. oxysporum f. sp. cubense race 4, with effective dose for 50% inhibition (EC50) of 20.80 µg/ml against hyphal growth and 12.62 µg/ml against spore germination. To investigate its mechanism of action, we observed the cellular ultrastructures of F. oxysporum f. sp. cubense race 4 with YY3 treatment and found that YY3 led to cell wall thinning, mitochondrial deformities, apoptotic degradation of the subcellular fractions, and entocyte leakage. Consistent with these variations, increased permeability of cell membrane and mitochondrial membrane also occurred after YY3 treatment. On the enzymatic level, the activity of mitochondrial complex III, as well as the ATP synthase, was significantly suppressed by YY3 at a concentration >12.50 µg/ml. Moreover, YY3 elevated the cytosolic Ca2+ level to promote mitochondrial reactive oxygen species (ROS) production. Cell apoptosis also occurred as expected. On the transcriptome level, key genes involved in the phosphatidylinositol signaling pathway were significantly affected, with the expression level of Plc1 increased approximately fourfold. The expression levels of two apoptotic genes, casA1 and casA2, were also significantly increased by YY3. Of note, phospholipase C activation was observed with YY3 treatment in F. oxysporum f. sp. cubense race 4. These findings indicate that YY3 exerts its antifungal activity by activating the phospholipase C calcium-dependent ROS signaling pathway, which makes it a promising biofungicide.