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S U M M A R YThe activities of L-threonine dehydrogenase (I), 2-amino-3-oxybutyrate : CoA ligase (11), malate synthetase (111), isocitrate lyase (IV), glyoxylate dehydrogenase (V), glycine decarboxylase (VI), L-serine hydroxymethyltransferase (VII), glucan synthetase (VIII), glucose 6-phosphate dehydrogenase (IX) and succinic dehydrogenase (X) were detected in cell-free extracts prepared from the mycelium of the fungus Sclerotium rolfsii type R. Transfer of S. rolfsii to a threonine-containing medium resulted in a significant increase in the intracellular concentrations of L-threonine, glycine, serine and glyoxylate, and a decrease in oxalate. Incubation with 14C-labelled L-threonine resulted in an immediate output of 14C02, and an accumulation of labelled glycine and serine in the mycelium. L-Threonine ( I O -~ M) increased branching, favoured formation of sclerotia, and induced the formation of enzymes I to VIII, but not IX and X. Sodium oxalate (1.5 x I O -~ M) inhibited branching, sclerotium formation and the activity of enzymes 111 and IV. Glycine ( 10-1 M) inhibited branching, sclerotium formation and activity of I and IT. Ammonium chloride (10-1 to I O -~ M) inhibited formation of sclerotia, threonine uptake and activity of TIT. Acetyl-CoA inhibited V and L-cysteine inhibited I as well as sclerotium formation and branching. It is suggested that hyphal morphogenesis and formation of sclerotia in S. rovsii require an increased supply of carbohydrate intermediates and energy and that these are mainly supplied by the glyoxylate pathway. I N T R O D U C T I O NThe effect of amino acids on morphogenesis has been observed in several fungi, and includes control of lateral hypha frequency in Mucor hiemalis (Plunkett, 1966) and sclerotium formation in Sclerotium rolfsii (Chet, Henis & Mitchell, 1966;. 1971) reported that L-threonine induced sclerotium formation in S. rolfsii, butImade no attempt to explain the mechanism of this effect. The metabolism of L-threonine, given as the sole carbon and nitrogen source, was studied in detail in penicillia and fusaria by Willetts (1972a, b) and by Willetts & Turner (1971) who showed that in these fungal groups, L-threonine could be metabolized via the glyoxylate pathway. Although the presence of this metabolic pathway in S. rolfsii has been demonstrated (Maxwell & Bateman, 1968a), its role in L-threonine metabolism in this fungus has not yet been studied.The purpose of this work was to reveal the possible links between the effect of L-threonine on the morphogenesis of S. rolfsii and its metabolic pathway in this fungus.

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2 %' (v/v) in air enhanced the growth rate and inhibited sclerotium formation in the fungus Sclerotium rolfii Sacc. A CO, concentration of 10 % inhibited growth. Similar growth patterns were observed when the fungus was grown on a medium supplemented with the fungicide carboxin, which inhibits succinate dehydrogenase. A high CO, concentration (I to 10 %) or growth on carboxin-supplemented medium caused a decrease in succinate dehydrogenase activity and significant increases in isocitrate lyase, isocitrate dehydrogenase, malate synthase and malate dehydrogenase activities. Mycelium of S. roysii grown at a high CO, concentration contained less glyoxylate, lipids and glycogen than mycelium grown in air. It is suggested that sclerotium formation in S. rolfii requires a balanced supply of carbohydrate intermediates and energy.