1. The Metabolic Characteristics of Plant Root Exudates
Plants produce an astonishingly large number of root exudates, which are a complex mixture of various compounds. These compounds can be broadly classified into two main categories: primary metabolites and secondary metabolites. Primary metabolites, such as sugars, amino acids, and organic acids, are essential for the basic physiological functions of the plant. They are involved in processes like energy production, growth, and reproduction. On the other hand, secondary metabolites, which include terpenoids, flavonoids, phenolic acids, siderophores, peptides, and enzymes, are not directly involved in these basic functions but play crucial roles in plant defense, communication, and adaptation to the environment.
2. Characteristics and Functions of Rhizosphere Microbial Metabolites
3. The Impact of Metabolite-Mediated Plant-Microbe Interactions on Environmental Adaptation and Ecosystem Function
By releasing specific exudates, plants establish a close relationship with the surrounding rhizosphere microbial community, initiating plant-microbe interactions that help alleviate various adverse conditions. Root exudates mainly consist of labile carbon compounds, which promote the reproduction of microorganisms and facilitate the interaction between plants and microbes in the rhizosphere. Although rhizosphere symbiotic microorganisms are mostly neutral in most cases, some symbiotic microorganisms can form symbiotic relationships with host plants. Among the interactions between plants and microbes, the most well-known processes are the symbiosis between plants and symbiotic mycorrhizal fungi and rhizobia. Once a symbiotic relationship is established, the host plant provides lipids to AM fungi, and in return, the fungi provide the host with a large amount of phosphorus and nitrogen (Jiang et al., 2017). Leguminous plants can obtain ammonia from root nodules and provide carbohydrates to rhizobia as nutrients. Although the symbiotic relationship of rhizobia is less common than that of mycorrhizal fungi, the reciprocal interaction between leguminous plants and rhizobia shares a common symbiotic signaling pathway with AM fungi. Interestingly, the formation of the AM symbiotic relationship can also promote the symbiosis between leguminous plants and rhizobia, indicating a potential promoting effect between these two reciprocal interactions.
In recent years, significant progress has been made in revealing the diversity and chemical structures of root exudates and microbial metabolites. Evidence shows that plant genetics drives the assembly and function of the rhizosphere microbiome through metabolic control. Therefore, uncovering the chemical and genetic clues in plant-microbe communication will contribute to the development of sustainable agriculture. However, due to the complex plant-microbe interactions involving metabolites, the application of metabolite-triggered microbial engineering in improving plant adaptability and the sustainability of agricultural ecosystems has had mixed results. The latest advancements in technologies such as mass spectrometry, multi-omics, and synthetic community research will help us gain a more detailed understanding of the mechanisms of plant-microbe interactions mediated by root exudates and microbial metabolites. In addition, the targeted manipulation of SynComs designed through machine learning may pave the way for precision agriculture microbiome engineering to enhance plant stress resistance and nutrient absorption capacity.