It’s hard to imagine Earth without land plants. Yet, terrestrial environments were plant-less for the first 3.5 billion years of Earth’s history. Plants transitioned to terrestrial life as early as 432 million years ago. The benefits of moving to land for plants probably included more light and carbon dioxide for photosynthesis. The first land plants faced challenges inherent to their new lifestyle; they had to adapt to drying out, and lower nutrient conditions and higher UV radiation than they encountered underwater. However, primordial plants had some help in overcoming these new challenges.
Today, many terrestrial plant species grow associated with mycorrhizal fungi, which attach to plant roots and facilitate plant water and nutrient uptake. One study found that 80 percent of plants surveyed had these fungi. Many plants depend on fungal partners for successful growth. In turn, the plant provides its fungi with sugars for food and lipids which form the structure of its cell membranes. According to DNA and fossil evidence, these symbiotic relationships are ancient, and they are hypothesized to have been essential in allowing ancestral plants to colonize land. Now, a new study published in Science provides the first experimental evidence that supports this hypothesis.
The scientists reasoned that if the symbiosis between mycorrhizal fungi and land plants is an ancestral (simply, very old) trait, extant land plants should share a response to mycorrhizal fungi that was inherited from their common ancestor. Conversely, if the relationship with mycorrhizal fungi developed more recently, different types of plants should have different responses to mycorrhizal fungi. The researchers compared how a species of liverwort, a primitive terrestrial plant group, and flowering plants responded to a mycorrhizal fungus.
Their experiments revealed that liverworts and flowering plants share genes that respond similarly to mycorrhizal fungi. These genes are involved in the production of plant hormones, infection responses, and the transfer of lipids, which were identified as essential in driving this symbiosis. The results show that primitive land plants respond to mycorrhizal fungi in the same way as flowering plants, supporting the hypothesis that symbiotic relationships between plants and mycorrhizal fungi are ancestral traits that facilitated colonization of land.