LAUSR

Phytophthora, which comprises nearly 80 pathogenic species, has caused devastating plant diseases worldwide and brings about serious economic losses every year [1]. Historically, P. infestans, the most notable species in the Phytophthora family, caused potato late blight and led to the great Irish famine in the mid-1840s [2]. P. ramorum, a new aggressive species, recently caused sudden oak death and infected large areas of forests across the United States and Europe [3]. However, few fungicides are effectively able to control Phytophthora. Thus, the discovery of effective methods to control Phytophthora has economic and social significance. Sexual reproduction and asexual reproduction are two crucial biological events in the life cycle of Phytophthora. To control the sexual reproduction of Phytophthora, we determined the chemical structures of two signaling molecules, hormones 1 and 2, which stimulate sexual reproduction in heterothallic species [4–6]. This strategy, which involves blocking the binding between hormones and their receptors to control the reproduction of Phytophthora, requires the identification of hormone receptors. The asexual sporangia of Phytophthora significantly affect the asexual reproduction of the organism and harm crops. A sporangium releases motile zoospores which can travel several kilometers by wind or water. Zoospores not only infect plants directly but also develop to hypha and then complete the cycle of asexual production. Thus, inhibiting the sporangium formation of P. capsici may potentially suppress the development of plant diseases. Although many chemical fungicides of Phytophthora, such as metalaxyl and dimethomorph, are available in the market nowadays, the long-term use of these agents has led to some resistance and environmental issues. Therefore, the discovery of new resources with a mechanism of action that differs from that of commercial agents to inhibit sporangium formation may considerably influence the control of Phytophthora. The MeOH extract of Kalimeris indica showed potential inhibitory activity against sporangium formation of Phytophthora capsici. Guided by bioassay, the extract of K. indica (3 kg, dry wt.) was separated by open columns, followed by HPLC purification to give five active compounds 1 (45 mg), 2 (54 mg), 3 (60 mg), 4 (30 mg) and 5 (53 mg). The structures of these active compounds were identified on the basis of spectral data and by comparing with the data reported in the literature. They were elucidated as nicotiflorin (1) [7, 8], narcissin (2) [9, 10] friedelinol (3) [11], friedelin (4) [11], and aurantiamide acetate (5) [12, 13]. Among them, compounds 1 and 2 were isolated from K. indica for the first time. The inhibition ratios of compounds 1–5 in comparison with those of the positive (metalaxyl) and negative control (0.2% DMSO) were studied. At the concentration of 30 g/mL, these compounds significantly inhibited the sporangium formation of P. capsici. Almost no sporangium was observed on the hypha of P. capsici after treatment with these compounds. Compounds 1–5 showed significant inhibitory rates of 99.1%, 100%, 99.5%, 100.0% and 94.3%, respectively, at the concentration of 30 g/mL 24 h after treatment. They inhibited the sporangium formation of P. capsici with EC50 values of 1.8, 2.4, 6.2, 5.4, and 3.5 g/mL, respectively. These compounds showed inhibitory activities equivalent to those observed in the positive control at the concentration of 30 g/mL.