Phytopathogenic fungal infections have become a major threat to agricultural production, food security, and human health globally, and novel antifungal agents with simple chemical scaffolds and high efficiency are needed. In this study, we designed and synthesized 38 8-hydroxyquinoline metal complexes and evaluated their antifungal activities. The results showed that most of the tested compounds possessed remarkable in vitro antifungal activity. Especially, compound 1e exhibited the highest antifungal potency among all target compounds, with EC50 values of 0.0940, 0.125, 2.95, and 5.96 μg/mL, respectively, against Sclerotinia sclerotiorum, Botrytis cinerea, Fusarium graminearum, and Magnaporthe oryzae. Preliminary mechanistic studies had shown that compound 1e might cause mycelial abnormalities of S. sclerotiorum, cell membrane permeability changes, leakage of cell contents, and inhibition of sclerotia formation and germination. Moreover, the results of in vivo antifungal activity of compound 1e against S. sclerotiorum showed that 1e possessed higher curative effects than that of the positive control azoxystrobin. Therefore, compound 1e is expected to be a novel leading structure for the development of new antifungal agents.

Plant pathogenic fungi seriously affect agricultural production and are difficult to control. The discovery of new leads based on natural products is an important way to innovate fungicides. In this study, 30 natural-product-based magnolol derivatives were synthesized and characterized on the basis of NMR and mass spectroscopy. Bioactivity tests on phytopathogenic fungi (Rhizoctonia solani, Fusarium graminearum, Botrytis cinerea, and Sclerotinia sclerotiorum) in vitro of these compounds were performed systematically. The results showed that 11 compounds were active against four kinds of phytopathogenic fungi with EC50 values in the range of 1.40–20.00 μg/mL, especially compound L5 that exhibited excellent antifungal properties against B. cinerea with an EC50 value of 2.86 μg/mL, approximately 2.8-fold more potent than magnolol (EC50 = 8.13 μg/mL). Moreover, compound L6 showed the highest antifungal activity against F. graminearum and Rhophitulus solani with EC50 values of 4.39 and 1.40 μg/mL, respectively, and compound L7 showed good antifungal activity against S. sclerotiorum. Then, an in vivo experiment of compound L5 against B. cinerea was further investigated in vivo using infected tomatoes (curative effect, 50/200 and 36%/100 μg/mL). The physiological and biochemical studies illustrated that the primary action mechanism of compound L5 on B. cinerea might change the mycelium morphology, increase cell membrane permeability, and destroy the function of mitochondria. Furthermore, structure–activity relationship (SAR) studies revealed that hydroxyl groups play a key role in antifungal activity. To sum up, this study provides a reference for understanding the application of magnolol-based antifungal agents in crop protection.

Inspired by the widely antiphytopathogenic application of diversified derivatives from natural sources, cryptolepine and its derivatives were subsequently designed, synthesized, and evaluated for their antifungal activities against four agriculturally important fungi Rhizoctonia solani, Botrytis cinerea, Fusarium graminearum, and Sclerotinia sclerotiorum. The results obtained from in vitro assay indicated that compounds a1–a24 showed great fungicidal property against B. cinerea (EC50 < 4 μg/mL); especially, a3 presented significantly prominent inhibitory activity with an EC50 of 0.027 μg/mL. In the pursuit of further expanding the antifungal spectrum of cryptolepine, ring-opened compound f1 produced better activity with an EC50 of 3.632 μg/mL against R. solani and an EC50 of 5.599 μg/mL against F. graminearum. Furthermore, a3 was selected to be a candidate to investigate its preliminary antifungal mechanism to B. cinerea, revealing that not only spore germination was effectively inhibited and the normal physiological structure of mycelium was severely undermined but also detrimental reactive oxygen was obviously accumulated and the normal function of the nucleus was fairly disordered. Besides, in vivo curative experiment against B. cinerea found that the therapeutic action of a3 was comparable to that of the positive control azoxystrobin. These results suggested that compound a3 could be regarded as a novel and promising agent against B. cinerea for its valuable potency.

Enlightened from our previous work of structural simplification of quinine and innovative application of natural products against phytopathogenic fungi, lead structure 2,8-bis­(trifluoromethyl)-4-quinolinol (3) was selected to be a candidate and its diversified design, synthesis, and antifungal evaluation were carried out. All of the synthesized compounds Aa1–Db1 were evaluated for their antifungal activity against four agriculturally important fungi, Botrytis cinerea, Fusarium graminearum, Rhizoctonia solani, and Sclerotinia sclerotiorum. Results showed that compounds Ac3, Ac4, Ac7, Ac9, Ac12, Bb1, Bb10, Bb11, Bb13, Cb1. and Cb3 exhibited a good antifungal effect, especially Ac12 had the most potent activity with EC50 values of 0.52 and 0.50 μg/mL against S. sclerotiorum and B. cinerea, respectively, which were more potent than those of the lead compound 3 (1.72 and 1.89 μg/mL) and commercial fungicides azoxystrobin (both >30 μg/mL) and 8-hydroxyquinoline (2.12 and 5.28 μg/mL). Moreover, compound Ac12 displayed excellent in vivo antifungal activity, which was comparable in activity to the commercial fungicide boscalid. The preliminary mechanism revealed that compound Ac12 might cause an abnormal morphology of cell membranes, an increase in membrane permeability, and release of cellular contents. These results indicated that compound Ac12 displayed superior in vitro and in vivo fungicidal activities and could be a potential fungicidal candidate against plant fungal diseases.

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