The Innovation Team of Agricultural Microbial Resources of the Institute of Agricultural Resources and Regional Planning (IARRP) of the Chinese Academy of Agricultural Sciences (CAAS) has identified and dissected a novel peptide-like plant immune elicitor produced by beneficial Pseudomonas and its mechanism of regulating plant immunity. The related findings, titled "Pseudomonas Cyclic Lipopeptide Medpeptin: Biosynthesis and Modulation of Plant Immunity," were published in the prestigious academic journal "Engineering".  

Pseudomonas, as an important resource of beneficial microorganisms, is a major source of biopesticides, biofertilizers, and biological feed. The team collected over 200 plant and soil samples from 19 regions nationwide and established a microbial resource bank containing more than 12,000 Pseudomonas through functional-oriented isolation and screening. Through precise identification and systematic evaluation, strain S58 was found to possess multiple functional characteristics, such as antibacterial, growth promotion, phosphate solubilization, enzyme production, and induced plant resistance. Several mutants associated with antimicrobial activity and induced resistance were obtained through genome mining and Tn5 transposon random mutagenesis.

Further analysis revealed that the deletion of a non-ribosomal peptide synthetase gene cluster resulted in the loss of induced resistance in S58 strain, while not affecting its antimicrobial activity, indicating that the inducer of resistance and antimicrobial substances were different in strain S58. Interestingly, mutating the quorum sensing system upstream of non-ribosomal peptide synthetase gene cluster, the induced resistance and antimicrobial activity of S58 were significantly reduced or lost, suggesting that the synthesis of elicitor and antibacterial substances shared the same upstream regulatory pathway. The high-performance liquid chromatography (HPLC), matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, and nuclear magnetic resonance (NMR) were used to isolate and identify the  medpeptin, encoded by the non-ribosomal peptide synthetase gene cluster.

Medpeptin is a novel cyclic lipopeptide composed of 22 amino acid residues, including a 5-amino acid moiety forming a lactone ring, and a fatty acid chain, with a mass-to-charge ratio of 2107.24. The purified medpeptin demonstrated significant induced plant resistance but no direct antimicrobial activity. Signal pathways responsible for medpeptin-induced resistance were identified through comparative transcriptome analysis and virus-induced gene silencing (VIGS) technique. Silencing of a cell wall leucine-rich repeat extension protein (NbLRX3) or receptor-like protein kinase (NbRLK25) compromises S58- and medpeptin-triggered plant immunity. This cell wall-mediated immune pathway is distinct from the canonical two-layered immune system depended on BAK1 and SGT1.

This study provides the first evidence that non-ribosomal cyclic lipopeptides activating cell surface immunity via cell wall perception and cytoplastic signaling, which broadens the array of Pseudomonas metabolites used as biological control agents and highlights a new immune network and mechanism involved in CLP-induced plant resistance. Three national invention patents have been granted for related achievements.

Associate professor Gu Yilin and postdoctoral Fellow Li Junzhou of the IARRP are the co-first authors of the paper, while professor Wei Hailei is the corresponding author. Professor Li Yan from the Institute of Vegetables and Flowers, Chinese Academy of Agricultural Sciences, participated in the work. The research was supported by the National Key Research and Development Program, the National Natural Science Foundation of China, the Chinese Academy of Agricultural Sciences Science and Technology Innovation Program, and the Modern Agricultural Industry Technology System Beijing Municipality Characteristic Crop Innovation Team, among others.

Figure 1: Structural identification and functional detection of Medpeptin

Figure 2: Medpeptin perception and immune pathway

Paper Link: https://doi.org/10.1016/j.eng.2023.05.016