Movements of Plant Organs: From Root Skototropism to Leaf Mimicking

Abstract

Plants colonized land around 450 million years ago, facilitated by a symbiotic relationship with fungi, particularly through root-fungal co-evolution. This symbiosis was pivotal in developing complex root systems in modern flowering plants. Charles and Francis Darwin later proposed that the root apex acts like a brain of lower animals, receiving information and directing growth. Connecting this historical context to modern understandings, the root apex, especially the transition zone, is considered the plant-specific "root brain", as it primarily focuses on sensory tasks, supported by high sucrose levels and nutrient demands similar to neurons. Roots communicate through fast electrical signaling, volatile chemical compounds and root exudates. Plant roots, typically underground, can be affected by light exposure, as seen in laboratory-grown Arabidopsis thaliana, leading to stress responses, and altered growth ratios. Their ability to sense and respond to environmental changes is evidence of their complex nature. For example, roots can detect gravity, moisture, nutrients, and even the presence of other roots. This adaptability is essential for plant survival, especially in challenging environments where competition for resources is intense. Based on plants' sensory abilities, Boquila trifoliolata, a climber native to South American temperate rainforests, exhibits a unique ability to mimic the leaves of various host plants, a phenomenon first documented in 2014. This mimicry helps Boquila avoid herbivory. The exact mechanism of mimicry remains uncertain, with initial hypotheses suggesting volatile chemical signals or genetic material transfer. However, an experiment showing Boquila mimicking artificial plastic leaves suggested a new hypothesis: Boquila might have some form of vision, allowing it to observe and mimic its surroundings. This hypothesis of plant vision aligns with Gottlieb Haberlandt early 20th-century theory of plant ocelli, which proposed that plant epidermal cells could function like lenses, focusing light onto underlying photosensitive cells. Plants are responding to light through specialized photoreceptor proteins like phytochromes, cryptochromes, UV-B photoreceptors, and phototropins. These receptors allow plants to perceive different light wavelengths and intensities, triggering physiological responses. Supporting the notion of plant vision further, light-sensitive cells are also found in lower photosynthetic organisms, like green algae, dinoflagellates, and cyanobacteria. If there is any kind of vision in lower photosynthetic life forms, it can probably be found in plants. This comprehensive understanding of plant behavior and sensory capabilities highlights their evolutionary success and the intricate ways they interact with their surroundings. In conclusion, the study of plant sensory abilities, from the Darwinian root apex "brains" to potential plant vision, reveals a level of sophistication previously unappreciated. The case of B. trifoliolata, with its extraordinary leaf mimicry, opens new avenues of research into plant perception and adaptation mechanisms. These findings challenge traditional views of plants as passive organisms, highlighting their dynamic interactions with the environment. As research progresses, our understanding of plant intelligence and sensory capabilities will likely expand, offering deeper insights into the remarkable ways plants navigate and adapt to their world.