Evidence for tryptophan being a signal molecule that inhibits conidial anastomosis tube fusion during colony initiation in Neurospora crassa

► Low pH and tryptophan inhibit conidial anastomosis tube (CAT) fusion in Neurospora crassa. ► Tryptophan-mediated inhibition of CAT fusion is mitigated by other amino acids. ► Tryptophan may act as a signal molecule that regulates CAT fusion. ► Tryptophan acts intracellularly via amino acid transporter uptake into cells. ► CAT fusion and hyphal fusion in the mature colony are differently regulated.