In terrestrial ecosystems, mycorrhizal fungi form extensive symbiotic relationships with plant roots. Their hyphal networks, the common mycorrhizal networks (CMNs), have long been considered a key channel for the exchange of nutrients such as nitrogen and phosphorus between plants. Previous studies have mainly focused on the role of CMNs in the transport of nutrients. However, there is still much uncertainty about the role that CMNs play when plants are under stress. So, when plants are under stress such as being attacked by diseases and pests or suffering from mechanical damage, can these underground networks help plants transmit defense signals to neighboring individuals?
A team led by Professor Erik Limpens from Wageningen University in the Netherlands has answered this question through a review study. The relevant research was published in the journal Frontiers of Agricultural Science and Engineering (DOI: 10.15302/J-FASE-2024578).
Scientists have found that when tomato plants are infected with the late blight pathogen, neighboring healthy plants connected via the arbuscular mycorrhizal (AM) network can activate the defense genes related to jasmonic acid (JA) and salicylic acid (SA) within 24 hours, and this process is completely dependent on the direct connection of the hyphae. Similar phenomena have also been verified in various plants such as citrus and alfalfa. Leguminous plants attacked by aphids can trigger neighboring plants to release compounds that attract the natural enemies of aphids through the AM network, and the signal transmission can be completed in as fast as 6 hours. When tomatoes suffer from mechanical damage, the stress-resistant enzyme activity of the connected plants significantly increases, forming a group defense response.
Further investigation into the signaling mechanisms of CMNs has identified six potential pathways for signal transmission: diffusion through the liquid film on the surface of the hyphae, diffusion through the cell wall or extracellular matrix, diffusion within the hyphae, active transport by fungi, secretion and absorption cycle, and electrical signal conduction. It is worth noting that during the process of signal transmission, plant roots are not passive participants. They can actively regulate the composition and quantity of secretions, and these secretions may spread in CMNs through passive diffusion or active transport by fungi.
This signal transmission is not only beneficial to plants but also holds significant importance for the survival and symbiotic relationships of fungi. From the perspective of ecological mutualism, once plants receive signals transmitted by the mycorrhizal network, they can activate their own defense mechanisms in advance, greatly reducing the risk of being attacked by diseases and pests. At the same time, by assisting plants in the process of signal transmission, fungi can maintain the symbiotic relationship with plants, thereby ensuring a stable supply of carbon sources and maintaining the balance of the entire symbiotic system.
In conclusion, when plants are infected by pathogens, attacked by herbivores, or suffer from mechanical damage, CMNs can effectively transmit warning signals. In the future, researchers need to further determine the specific signal molecules transmitted through CMNs and conduct in-depth investigations into the signal transmission mechanisms and their dynamic changes.