Effect of ornithine decarboxylase and norspermidine in modulating cell division in the green alga Chlamydomonas reinhardtii

The extensive genetic resources of Chlamydomonas has led to its widespread use as a model system for understanding fundamental processes in plant cells, including rates of cell division potentially modulated through polyamines. Putrescine was the major polyamine in both free (88%) and membrane-bound fractions (93%) while norspermidine was the next most abundant in these fractions accounting for 11% and 6%, respectively. Low levels of diaminopropane, spermidine and spermine were also observed although no cadaverine or norspermine were detected. Ornithine decarboxylase (ODC, EC 4.1.1.17) activity was almost five times higher than arginine decarboxylase (ADC, EC 4.1.1.19) and is the major route of putrescine synthesis. The fluoride analogue of ornithine (α-DFMO) inhibited membrane associated ODC activity whilst simultaneously stimulating cell division in a dose dependent manner. Following exposure to α-DFMO the putrescine content in the cells declined while the norspermidine content increased over two fold. Addition of norspermidine to cultures stimulated cell division mimicking the effects observed using DFMO and also reversed the inhibitory effects of cyclohexylamine on growth. The results reveal that ODC is the major route to polyamine formation in the Chlamydomonas CC-406 cell-wall mutant, in contrast to the preferential ADC route reported for Chlorella vulgaris, suggesting that significant species differences exist in biosynthetic pathways which modulate endogenous polyamine levels in green algae.