Most land plants establish symbiotic relationships with arbuscular mycorrhizal (AM) fungi from the Glomeromycotina group, enhancing their access to mineral nutrients. These fungi are especially effective at facilitating phosphorus uptake, which is pivotal for plant growth. A recent study reveals the role of the plant hormone ethylene as a significant inhibitor of AM development, providing new insights not only on ethylene signaling but also on how it integrates with broader plant physiological processes.
This research, conducted on the model legume Lotus japonicus and the AM fungi Rhizophagus irregularis, investigates the underlying mechanisms by which ethylene disrupts AM symbiosis. Ethylene, known for its roles in various plant growth and response pathways, was found to reduce AM development by enhancing the accumulation of the transcriptional regulator SUPPRESSOR OF MAX2 1 (SMAX1). SMAX1 regulates the expression of numerous genes required for AM formation.
Ethylene’s impact appears to involve suppression of strigolactone biosynthesis genes, which are necessary for fungal activation, alongside genes needed for fungal entry, such as the Common Symbiosis genes. These disruptions were evidenced by reduced root colonization when plants were treated with ethylene precursors such as 1-aminocyclopropane-1-carboxylic acid (ACC) and ethephon. For example, treatment with 200 µM ACC reduced root colonization from approximately 70% to 22% and caused later developmental issues within the AM fungi, demonstrated by aborted fungal structures on treated roots.
Interestingly, the study suggests the phenomenon is not merely due to ethylene-induced defense responses but rather due to direct suppression of genes linked to AM development. The authors of the article noted, ‘we conclude from our data...that ethylene suppresses the expression of many important genes involved in AM development.’ Such findings have historical significance, as the negative impact of ethylene on AM colonization has been acknowledged for decades, yet the molecular mechanisms remained elusive until now.
Through careful experimentation, the researchers confirmed ethylene’s inhibitory effects and precisely traced the role of SMAX1 as the locus of control. When SMAX1 is elevated, the necessary signals for effective AM establishment are blocked, underscoring SMAX1 as a key player integrating both ethylene and karrikin signaling—another pathway involved with plant responses to fire-induced germination cues.
What does this mean for plant science and agriculture? Understanding how ethylene signaling functions as a brake on AM development could pave the way for new agricultural strategies aimed at maximizing the benefits of AM fungi. By manipulating ethylene pathways, or managing SMAX1 levels, farmers could potentially increase crop yields through enhanced nutrient uptake.
This research not only deepens our grasp of hormonal regulation within plants but also shows promise for practical applications, shedding light on how to potentially optimize the nutrient uptake mechanisms inherent to most land plants. Insights gained from the ethylene-SMAX1-AM fungi relationship could aid in developing crops more adept at nutrient absorption, particularly important as global food demands rise.
Exploring SMAX1’s role could lead to innovative agricultural practices and enhanced plant breeding strategies focusing on improving plant health and resilience. Future research may well seek to identify specific genetic modifications or treatments to modify SMAX1 and other relevant pathways to boost AM symbiosis and overall crop performance.
To summarize, this study reveals ethylene’s regulatory role as not just detrimental but rather as part of sophisticated signaling mechanisms shaping plant interactions with fungi. By elucidated how ethylene promotes SMAX1 accumulation, inhibiting the expression of AM conducive genes, it opens invaluable scientific inquiry paths on how plants navigate their complex relationships with the soil biome.