Polyamines and Ornithine Decarboxylase Activity during Growth and Differentiation in Sclerotium rolfsii

Abstract

Changes in polyamine content and in the activity of ornithine decarboxylase (ODC) were followed during growth and differentiation of the plant pathogenic fungus Sclerotium rolfsii, in submerged mycelium liquid cultures and on solid agar cultures. Mycelial growth in submerged cultures was characterized by high putrescine content and ODC activity. Growth cessation, resulting from glucose exhaustion in the medium, was accompanied by a sharp decrease in putrescine content and ODC activity. Spermine, the level of which was initially low, was detected in high amounts after all of the glucose was consumed and when the fungus developed the potential for sclerotium formation. A decrease in spermidine, and especially putrescine content, and an increase in spermine content, were observed during the transition from mycelium to mature sclerotia on solid agar medium. Addition of spermine to solid agar medium increased the number of sclerotia by 40%. The changes in the content of the three polyamines were reversed when sclerotia were allowed to germinate. Moreover, α-difluoromethylornithine, the enzyme-activated inhibitor of ODC, greatly inhibited mycelium growth, sclerotium germination and ODC activity, and this inhibition was completely reversed by the addition of putrescine. Cycloheximide delayed sclerotium germination and initially inhibited ODC activity, but ODC inhibition was relieved as soon as sclerotia began to germinate. The data indicate that specific changes in polyamines are linked with two distinct developmental events in S. rolfsii. Mycelial growth and sclerotium germination are positively correlated, and possibly causally linked, with a marked increase in putrescine content and biosynthesis (while spermine cannot be detected). Differentiation (sclerotium formation), however, is accompanied by a major increase in spermine content.