The Phosphatidylserine Synthase ( pssA ) Mutant of Agrobacterium fabrum Exhibits Membrane Lipid Remodeling, Altered Physiological Traits, and Reduced Plant Infectivity

Agrobacterium fabrum is a plant pathogen that induces tumor formation through the genetic transformation of infected plant tissues. Its pathogenicity is strongly influenced by the composition and structural organization of bacterial membranes. In this study, we characterized a mutant defective in the phosphatidylserine synthase gene (pssA), which encodes a key enzyme in the phosphatidylserine/phosphatidylethanolamine biosynthetic pathway. The lipid profile of the mutant was analyzed using thin-layer chromatography and high-resolution electrospray ionization mass spectrometry. The mutant lacked phosphatidylethanolamine (PE) and PE methylation intermediates and exhibited extensive lipid remodeling, including altered levels of phosphatidylglycerol, cardiolipin, and ornithine lipids compared with the wild-type strain. We also examined the physiological traits of the mutant, including resistance to environmental stressors, motility, and plant infectivity, all of which were markedly impaired. To investigate the global consequences of membrane lipid disruption, we performed transcriptomic analysis. A total of 283 genes were differentially expressed in the mutant relative to the wild-type strain, with a predominance of upregulated genes associated with carbohydrate, amino acid, inorganic ion, and lipid transport and metabolism, particularly genes encoding ABC transporters. In contrast, genes involved in flagellar assembly, chemotaxis, signal transduction, and transcription were strongly downregulated, consistent with the observed motility defect. Genes encoding components of several two-component regulatory systems and transcription factors were also downregulated, suggesting that membrane lipid remodeling affects global regulatory networks.