The Coprinopsis cinerea septin Cc.Cdc3 is involved in stipe cell elongation

We have identified and characterized a Coprinopsis cinerea mutant defective in stipe elongation during fruiting body development. In the wild-type, stipe cells elongate at the maturation stage of fruiting, resulting in very slender cells. In the mutant, the stipe cells fail to elongate, but become rather globular at the maturation stage. We found that the mutant phenotype is rescued by a gene encoding a homolog of Saccharomyces cerevisiae CDC3 septin, Cc.Cdc3. The C. cinerea genome includes 6 septin genes, 5 of which, including Cc.cdc3, are highly transcribed during stipe elongation in the wild type. In the mutant, the level of Cc.cdc3 transcription in the stipe cells remains the same as that in the mycelium, and the level of Cc.cdc10 transcription is approximately 100 times lower than that in the wild-type stipe cells. No increase in transcription of Cc.cdc3 in the mutant may be due to the fact that the Cc.cdc3 gene has a 4-base pair insertion in its promoter and/or that the promoter region is methylated in the mutant. Overexpressed EGFP-Cc.Cdc3 fusion protein rescues the stipe elongation in the transformants, localizes to the cell cortex and assembles into abundant thin filaments in the elongating stipe cells. In contrast, in vegetative hyphae, EGFP-Cc.Cdc3 is localized to the hyphal tips of the apical cells of hyphae. Cellular defects in the mutant, combined with the localization of EGFP-Cc.Cdc3, suggest that septin filaments in the cell cortex provide the localized rigidity to the plasma membrane and allow cells to elongate cylindrically.