Metabolism of L-Threonine and its Relationship to Sclerotium Formation in Sclerotium rolfsii

Abstract

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.

Effect of Carbon Dioxide on Growth and Carbohydrate Metabolism in Sclerotium rolfsii

“…Increasing the amount of C 0 2 also induced more branched aerial mycelium. The presence of I o mM-L-threonine increases branching of lateral hyphae and enhances sclerotium formation (Henis, Okon & Chet, I 973 ;Kritzman et al, 1976). Our present findings confirm earlier observations on the lack of correlation between hyphal growth rate and sclerotium formation (Chet & Henis, 1968).…”

“…In Blastocladiella, glyoxylate is further transaminated to glycine (Cantino,I 966). Another pathway of glyoxylate metabolism involves its conversion to malate by malate synthase (Willetts, 1972a, b ;Kritzman et al, 1976). The glyoxylate shunt, however, only operates when there is a shortage of succinate (Krebs & Lowenstein, 1 960).…”

Changes in Fatty Acids During Morphogenesis in Sclerotium rolfsii

“…rolfsii in a solid medium (Kritzman et al, 1976), it was interesting to see whether this compound had any effect on lipid metabolism. Liquid media containing 0-5 % glucose as carbon source were supplemented with Lthreonine (1 O-* or 1 O-M).…”

“…Synchronization of sclerotium formation could be induced by the addition to the growth medium of iodoacetic acid (Chet et al, 1966), lactose (Okon et al, 1972), or L-threonine (Kritzman et al, 1976). Synchronous formation of sclerotia could also be triggered by changing the proportions of aerial and submerged mycelium (Hadar et al, 1981).…”

Induction of morphogenesis, branching and conidiogenesis in Fomes annosus

“…KRITZMAN et al (1976) found another effect of threonine, the activation of malate synthetase and the concomitant reduction of oxalate secretion in Sclerotium rolfsii. This specific reaction was confirmed ioi Fomes annosus in the meantime, too (HUTTERMANN et al 1980).…”