Plants exist in a dynamic relationship with various microorganisms within their natural environment. While certain microbes can provoke plant harm and prompt defensive responses, others contribute beneficially to overall plant performance. The microbial communities closely associated with plant roots are pivotal in conferring distinct functions to their host plants. It intricately influences aspects such as plant growth, health, and overall productivity [53]. The rhizosphere, in particular, has emerged as a focal point of investigation, revealing compelling insights into the complex interactions between plants and microorganisms. A mounting body of evidence underscores the phenomenon wherein plants actively recruit protective bacteria and bolster microbial activities to counteract the proliferation of pathogens. This intricate network of interactions underscores the multifaceted strategies plants employ to ensure their well-being in the presence of various microorganisms [54, 55].
Our research findings present compelling evidence regarding the substantial impact of Bacillus introduction on the intricate facets of the composition, structural arrangement, and functional dynamics inherent within the root microbial community of Cc roots. A vital outcome of this interaction is the reduction in the complexity of the microbial network. Remarkably, the presence of Bacillus leads to the pronounced enrichment of specific genera, such as Arthrobacter, Sphingobium, Pseudomonas, Bradyrhizobium, Polaromonas, Agromyces, Ensifer, Massilia, Rivibacter, Paenisporosarcina, and Frigoribacterium. Simultaneously, this enrichment is correlated with a decrease in the relative abundance of Flavobacterium and Gemmatimonas. Arthrobacter plays a significant role as a plant growth-promoting and biocontrol agent, enhancing plant resistance against pests and diseases while promoting growth [56, 57]. Their presence in the soil maintains a healthy ecological balance by suppressing harmful microorganisms, increasing soil microbial diversity, and creating a healthier plant growth environment [57]. Sphingobium positively influences plants and soil environments through various pathways, such as stimulating plant growth [58], regulating metabolism [59], maintaining soil health [60], and degrading pollutants [61]. Ensifer establishes symbiotic relationships in non-leguminous plant roots, enhancing nutrient absorption [62], secreting growth-promoting molecules, and providing nitrogen, improving overall plant growth and physiological state [63]. Pseudomonas degrades harmful substances, enhances soil nutrient efficiency, modulates plant metabolism, and boosts stress resistance [64] while producing antibiotics to inhibit pathogenic growth and maintain plant health [65]. Bradyrhizobium engages in nitrogen-fixing symbiosis, providing nitrogen nutrition, enhancing growth [66], and stabilizing the rhizosphere microbial community [67]. Polaromonas and Agromyces bacteria influence plant growth by augmenting soil nutrients [68, 69], impacting microbial interactions [70, 71] within the root ecosystem. Ensifer affects plant roots and overall ecology through nitrogen fixation, growth promotion, rhizosphere interaction [72], and alleviation of heavy metal stress [73]. Paenisporosarcina participates in organic matter decomposition and nitrogen fixation [74], contributing to soil health [75]. Massilia consumes fungal secretions, controlling pathogenic fungi and inhibiting their growth [76, 77]. Frigoribacterium may act as antagonists to plant pathogens or promoters of plant growth [78]. Flavobacteria provide nutrients in plant roots to promote growth and combat pathogenic microorganisms, helping plant health and disease resistance [79]. Gemmatimonas bacteria engage in plant symbiosis, organic matter decomposition [80], enhancement of plant nutrient uptake [81], maintaining soil health, and promoting plant growth [82]. In summary, introducing Bacillus triggers a significant reconfiguration in the root microbial community, fostering the enrichment of beneficial genera and their pivotal roles in enhancing plant growth, combatting diseases, and fortifying soil health.
Furthermore, the introduction of Bacillus prompts a notable shift in the root microorganisms' behaviour, particularly in their synthesis of antibiotics. This transformative effect translates into the biosynthesis of various antibiotics, including ansamycin, macrolides, polyketones, streptomycin, neomycin, kanamycin, gentamicin, acarose, and vatamycin. This repertoire of synthesized antibiotics can play a pivotal role in fortifying the plant's defence mechanisms against potential pathogens. The orchestrated interplay between microbial communities and the induction of antibiotic synthesis represents a noteworthy aspect of Bacillus-mediated modulation in the root ecosystem of Cc.
The findings from our study reveal significant correlations between Bacillus and the transcriptome and the metabolome of Cc root. Through Mantel-test analysis, we established robust statistical links between these bacterial species and the genetic and chemical profiles of the root. Further analysis using WGCNA substantiated the noteworthy influence of Bacillus. The outcomes demonstrated a clear and significant correlation between Bacillus and the alterations observed in both the transcriptome and metabolome of the Cc root. This underlines the pivotal role of Bacillus in shaping the plant's molecular and metabolic responses.
Subsequent KEGG enrichment analysis, focusing on critical genes and metabolites within significantly correlated modules, uncovered the profound impact of Bacillus on critical pathways governing primary and secondary plant metabolism within the Cc root system. Essential genes KEEG enrichment pathways encompass a spectrum of vital biochemical processes, encompassing secondary metabolite biosynthesis, starch and sucrose metabolism, pyruvate metabolism, Phenylpropanoid biosynthesis, Purine metabolism, Glycolysis/Gluconeogenesis, Fatty acid biosynthesis/elongation and metabolism, Glycosaminoglycan degradation, Brassinosteroid biosynthesis, Biotin metabolism, Amino sugar and nucleotide sugar metabolism, Diterpenoid biosynthesis, Glucosinolate biosynthesis, Oxidative phosphorylation, Steroid biosynthesis, Tryptophan metabolism, Glycerolipid metabolism, Plant-pathogen interaction, and Plant hormone signal transduction, among others. Moreover, Key metabolites KEEG enrichment analysis highlighted Bacillus' involvement in pathways associated with unsaturated fatty acid biosynthesis, cysteine and methionine metabolism, arachidonic acid metabolism, tyrosine metabolism, cytochrome P450 metabolism of exogenous substances, and steroid hormone biosynthesis.
The shikimate pathway synthesises the aromatic amino acids Tyr, Phe, and Trp in plastids and cytosol [83]. Essential genes KEEG enrichment shows that Bacillus significantly affects tryptophan metabolism in the root system of Cc. At least three critical shikimic acid metabolic pathway compounds were found in crucial metabolites, such as 5-O-p-Coumaroyl shikimic Acid, 5-O-Caffeoyl shikimic Acid, and Arabidophyl shikimic Acid (supplementary table S3). A considerable share of carbon flow (≥ 30%) is directed through the shikimate pathway to produce pigments, defence compounds, and the cell wall component lignin [84]. At least nine flavonoids (Syringetin-3-O-glucoside, Kaempferol, Irigenin, Kaempferide, Naringerin, Rhapontigenin 3'-O-glucoside, Hesperetin, Isoliquiritin apioside, Eupatilin) were found in key metabolites. Plant-specialized metabolites can act as nutrient sources, signalling molecules, or toxins for individual microbial strains, thereby shaping the overall composition of the microbiome [85, 86]. Many studies have shown that plant root flavonoid exudates can enrich plant rhizosphere microorganisms [87] and act as plant antimicrobial toxins to inhibit the growth of pathogens [88].
Fatty acids are an essential class of metabolites in plant cells and play important roles in regulating the biological functions of plant roots. The release of fatty acids can alter plant rhizosphere microorganisms' community structure and diversity, affecting their functions and ecosystem processes [89]. Venugopal et al. [90] reported the roles of plant hormones (such as jasmonic Acid) derived from fatty acids in defending against plant physiological stress. Furthermore, several studies have revealed that fatty acids and their derivatives directly inhibit the growth of plant pathogens within the rhizosphere and improve the surrounding environment of plant rhizosphere to reduce the occurrence of crop diseases and promote crop growth [91, 92]. At least 16 unsaturated fatty acids (such as 9-HpOTrE, FAHFA (18:2/18:1), Methyl linoleate, FAHFA (18:2/16:0), Elaidic Acid, Arachidonic Acid, Adrenic Acid, Methyl oleate, 11Z,14Z-Eicosadienoic Acid, FAHFA ༈18:1/20:)3, trans-Petroselinic Acid, 8Z,11Z,14Z-Eicosatrienoic Acid, FAHFA (16:0/22:5), Hexadecanamide, 11Z,14Z,17Z-Eicosatrienoic Acid, 16-Hydroxyhexadecanoic Acid༉ were found in critical metabolites (supplementary table S3). Cc recruits beneficial microorganisms in the rhizosphere by synthesizing these unsaturated fatty acids and flavonoids to inhibit the outbreak of pathogens.
In summary, this study delves into the dynamic relationship between plants and microorganisms in their natural environment, mainly focusing on the role of microbial communities in the rhizosphere. It highlights the intricate strategies plants employ to interact with microorganisms, including the recruitment of protective bacteria and the modulation of microbial activities. The introduction of Bacillus significantly impacts the composition and behaviour of the root microbial community, reducing complexity and the enrichment or inactive of specific genera. It is worth noting that Bacillus can induce Cc root microorganisms to synthesize various antibiotics and induce Cc root to synthesize a variety of unsaturated fatty acids and flavonoids to enhance the defence ability of plants (Fig. 13). The study establishes robust correlations between Bacillus and the transcriptome and metabolome of Cc roots, emphasizing Bacillus's pivotal role in shaping molecular and metabolic responses. KEGG enrichment analysis unveils the profound influence of Bacillus on essential plant metabolic pathways. This research suggests the eco-friendly potential of beneficial microbes in enhancing plant disease resistance as an alternative to chemical pesticides.