The Endophytic Microbiome as a Hotspot of Synergistic Interactions, with Prospects of Plant Growth Promotion

Abstract

The plant root is the primary site of interaction between plants and associated microorganisms and constitutes the main components of plant microbiomes that impact crop production. The endophytic bacteria in the root zone have an important role in plant growth promotion. Diverse microbial communities inhabit plant root tissues, and they directly or indirectly promote plant growth by inhibiting the growth of plant pathogens, producing various secondary metabolites. Mechanisms of plant growth promotion and response of root endophytic microorganisms for their survival and colonization in the host plants are the result of complex plant-microbe interactions. Endophytic microorganisms also assist the host to sustain different biotic and abiotic stresses. Better insights are emerging for the endophyte, such as host plant interactions due to advancements in 'omic' technologies, which facilitate the exploration of genes that are responsible for plant tissue colonization. Consequently, this is informative to envisage putative functions and metabolic processes crucial for endophytic adaptations. Detection of cell signaling molecules between host plants and identification of compounds synthesized by root endophytes are effective means for their utilization in the agriculture sector as biofertilizers. In addition, it is interesting that the endophytic microorganism colonization impacts the relative abundance of indigenous microbial communities and suppresses the deleterious microorganisms in plant tissues. Natural products released by endophytes act as biocontrol agents and inhibit pathogen growth. The symbiosis of endophytic bacteria and arbuscular mycorrhizal fungi (AMF) affects plant symbiotic signaling pathways and root colonization patterns and phytohormone synthesis. In this review, the potential of the root endophytic community, colonization, and role in the improvement of plant growth has been explained in the light of intricate plant-microbe interactions.

Keywords: PGPR; plant growth promotion; plant-bacteria interaction; rhizo-microbiome; rhizosphere; root endophyte.

Figure 1

Microbial interactions at the root zone, and cellular responses of the host plant. Abbreviations: JA/ET—Jasmonate/Ethylene, SA—Salicylic acid, AB—Antibiotic, ABS—Antibiosis, RE—Root exudates, VOCs—Volatile organic compounds, VFS—Virulence factor secretion, CDE—Cell wall degrading enzymes, BFF—Biofilm formation, PS—Phosphate solubilization, SP—Siderophore production, NF—Nitrogen Fixation, QS—Quorum sensing, BTH—Benzothiadiazol, LPS—Lipopolysaccharide, IAA—Indole acetic acid, S—Suppression, PTIR—Pathogen Triggered Immune Response, NB—Nucleotide binding, CC—Coiled coil, LRR—Leucine rich repeats.