The Fungal Phytotoxin Alternariol 9‑Methyl Ether and Some of Its Synthetic Analogues Inhibit the Photosynthetic Electron Transport Chain Antonio Jacinto Demuner, † Luiz Cla ́ udio Almeida Barbosa,* ,†,‡ Ana Cristina Mendes Miranda, ‡ Guilherme Carvalho Geraldo, † Cleiton Moreira da Silva, † Samuele Giberti, § Michele Bertazzini, § and Giuseppe Forlani* ,§ † Department of Chemistry, Federal University of Viç osa, Av. P.H. Rolfs, Viç osa, Minas Geraís, 36570-000, Brazil ‡ Department of Chemistry, Federal University of Minas Geraís, Av. Pres. Antô nio Carlos 6627, Campus Pampulha, Belo Horizonte, Minas Geraís, 31270-901, Brazil § Department of Life Science and Biotechnology, University of Ferrara, Via L. Borsari 46, I-44121 Ferrara, Italy * S Supporting Information ABSTRACT: Alternariol and monomethylalternariol are natural phytotoxins produced by some fungal strains, such as Nimbya and Alternaria. These substances confer virulence to phytopathogens, yet no information is available concerning their mode of action. Here we show that in the micromolar range alternariol 9-methyl ether is able to inhibit the electron transport chain (IC 50 = 29.1 ± 6.5 μM) in isolated spinach chloroplasts. Since its effectiveness is limited by poor solubility in water, several alternariol analogues were synthesized using different aromatic aldehydes. The synthesized 6H-benzo[c]cromen-6-ones, 5H-chromene[4,3- b]pyridin-5-one, and 5H-chromene[4,3-c]pyridin-5-one also showed inhibitory properties, and three 6H-benzo[c]cromen-6-ones were more effective (IC 50 = 12.8-22.8 μM) than the lead compound. Their addition to the culture medium of a cyanobacterial model strain was found to inhibit algal growth, with a relative effectiveness that was consistent with their activity in vitro. In contrast, the growth of a nonphotosynthetic plant cell culture was poorly affected. These compounds may represent a novel lead for the development of new active principles targeting photosynthesis. M icrobial phytotoxins and their synthetic analogues are a potential source of new bioactive compounds for agriculture, medicine, and the food industry. 1-7 Since the discovery of paclitaxel production by the endophytic fungus Taxomyces andreanae in 1993, the interest of many scientists in fungal endophytes as potential producers of bioactive compounds has increased even more, and over the past two decades many valuable substances endowed with antimicrobial, insecticidal, cytotoxic, anticancer, and other activities have been isolated from these organisms. Such bioactive compounds include alkaloids, terpenoids, steroids, quinones, isocoumarins, lignans, phenylpropanoids, phenols, and lactones. 6,8,9 Endophytic fungi spend all or part of their life cycle inter- or intracellularly colonizing healthy tissues of their host plants. 10,11 The endophytic fungus Alternaria sp. causes lesions in young leaves, fruits, and stems. Depending on the intensity of the disease, the lesions can lead to a total defoliation of the plant. 12,13 All pathogenic species of Alternaria produce a series of mycotoxins 14,15 that have been found in many different foodstuffs including fruits, vegetables, cereals, nuts, and oil seeds. 2,16-21 Several of these metabolites have been identified as phenolic compounds with a coumarin-like structure. Considering the interest in bioactive metabolites produced by weed pathogens as sources of novel natural herbicides, the production of toxins by Alternaria species was thoroughly investigated, 22 leading to the discovery of alternariol (1), alternariol 9-methyl ether (2), and alternuisol (3) (Figure 1), 15,23,24 for which total syntheses have also been reported. 22-29 Among Alternaria species, A. alternata is the Received: July 19, 2013 Published: November 18, 2013 Article pubs.acs.org/jnp © 2013 American Chemical Society and American Society of Pharmacognosy 2234 dx.doi.org/10.1021/np4005882 | J. Nat. Prod. 2013, 76, 2234-2245