Ophiobolin A, a sesterterpene metaboHte ofHelminthosporium maydis, Nisikado and Miyake, stimulates net leakage of electrolytes and glucose from maize (Zea mays L.) seedlng roots. Treatmnent of the roots with ophiobolin A at a concentration of 10 ag/nd (25 jAm) inhbits uptake of 10 mM 2-deoxyglucose by 50% and of 0.5 mM 2-deoxyglucose by 85%. Compartmental (15) showed that ophn A at concentrations as low as 30 ,g/ml inhibited the elongation of rice coleoptiles and seedling roots and of oat coleoptiles. Treatment of beet root slices with ophn A caused leakage of anthocyanin. In a later report (16), leakage of phosphate and organic compounds from corn seedling roots and from potato tuber discs was mentioned. The inhibition of rice seedling growth was confirmed by Ohkawa and Tamura (13) and it was shown by Oku (14) that ophn A could be isolated from infected rice leaves, suggesting that ophiobolin toxicity may play a role in disease development.In this paper, we report that ophn A is an effective inhibitor, at low concentrations, of hexose uptake in maize and carrot roots and stimulates ion leakage from these tissues. (10) with the addition of 0.02 g FeSO4 7H20 and 1 g yeast extract/l; pH of the medium was 5.6. Spore suspensions were used to inoculate 200-ml portions of medium in 2,800-ml Fernbach flasks, and the flasks were incubated at 25 ± 1 C in a growth chamber with approximately 1200 ft-c of continuous light.After 10 days, the growth medium was separated from the mycelial mats by straining through four layers of cheesecloth and filtering through Whatman No. 1 filter paper. The combined filtrate (12 liters) was transferred to a liquid-liquid extractor and extracted with diethyl ether for a total of 12 hr. The ether extract was dried with sodium sulfate and taken to an orange, nondrying oil by evaporation under vacuum at 45 C. The oil was dissolved in methylene chloride and crystallized by the addition of heptane. Washing with cold diethyl ether yielded 283 mg of white, needle-shaped crystals. Progress of the purification was monitored by TLC on Silica Gel GF254 plates developed with chloroform-methanol (100:2, v/v), and visualized by spraying with 1 % phosphomolybdic acid in methanol followed by heating at 100 C for 2 min. After repeated recrystallization, TLC revealed a major component with an RF (0.5) identical to that of authentic ophn A and a small amount of a second component with higher mobility. Nuclear magnetic resonance and low resolution mass spectra of this material corresponded to published data (6, 12).Plant Material. Corn seeds (Zea mays L., W64A, N or T cytoplasm) were washed with water and placed in 20% H202 for 30 min. The seeds were washed three times with sterile distilled H20 and transferred aseptically into sterile Petri dishes lined with two filter paper discs and moistened with 5 ml of 0.1 mm CaCl2. Ten seeds were placed in each dish. The seeds were incubated for 3 days at 30 C in the dark. The root tips (approximately 2.5 cm) of the germinated seeds were removed for use in 90...