When ancestral plants colonized the land 450 million years ago, they needed to adapt to harsh environmental conditions when giving up their aquatic lifestyle. Perhaps this was one of the most impactful events during the evolutionary history of plants. We hypothesize that during this water-to-land transition, the volatile plant hormone ethylene became an important growth regulator to face terrestrial stressors. In fact, modern-day crops use ethylene to regulate stress responses, and perhaps ethylene served this role in pioneering land plants to cope with harsh conditions coinciding with this habitat transition. We previously showed that ethylene signaling was functionally assembled in ancestral Charophyte green algae, prior to land colonization. Now we question why and how early land plants produced ethylene. It is known that seed plants make ethylene using ACC as precursor. However, non-seed plants produce ethylene via a different unknown ethylene biosynthesis pathway, which we want to reveal using the liverwort Marchantia polymorpha, a model species representing early life on earth. We also question why non-seed plants make ACC, but not use it for ethylene synthesis. New studies revealed that ACC itself can act as a signaling molecule, independent from ethylene, by an unknown signaling pathway to regulate plant development. We also postulate that both the alternative ethylene biosynthesis and ACC signaling pathway might have an origin in ancient algae, prior to land colonization, and might be conserved in seed plants, possibly exerting important functions yet to be uncovered. Using functional genetics in representative species of algae and crops, we will unravel the importance and role of ACC and ethylene that allowed plants to thrive on earth.
