Making Fungal-Photobiont Symbioses in the Lab: Past, Present, and Future of the Elusive In Vitro Lichen.

The ability to synthesize lichen symbioses in vitro from pure cultures of transformable symbionts would be a game changer for experiments to identify the metabolic interplay that underpins the success of lichens. However, despite multiple reports of successful lichen resynthesis, no lichen lab model system exists today. We reviewed 150 years of in vitro lichen studies and found that the term resynthesis is applied to many types of fungal-photobiont cocultures that do not resemble lichens. Some of the most lichen-like results, for their part, were obtained from nonaxenic tissue culture. Only a few studies reported obtaining natural-looking lichens from axenic input cultures, but all appear to have been isolated successes obtained against the background of extensive contamination. We suggest revisiting resynthesis experiments in light of recent advances in our understanding of lichen microbial composition to test whether in vitro lichen morphogenesis requires microbial inputs beyond those of the canonical fungal and algal symbionts.