Plant-Root Exudate Analogues Influence Activity of the 1-Aminocyclopropane-1-Carboxylate (ACC) Deaminase Gene in Pseudomonas hormoni G20-18(T)

Plants exposed to abiotic stress such as drought and salinity produce 1-aminocyclopropane-1-carboxylic acid (ACC) that is converted into the stress hormone ethylene. However, plant growth-promoting bacteria (PGPB), which synthesize the enzyme ACC deaminase, may lower the ACC concentration thereby reducing the concentration of ethylene and alleviating the abiotic stress. The PGPB Pseudomonas hormoni G20-18(T) (previously named P. fluorescens G20-18) harbors the genes acdR and acdS that encode regulation and synthesis of ACC deaminase, respectively. Regulation of the acdS gene has been investigated in several studies, but so far, it has been an open question whether plants can regulate microbial synthesis of ACC deaminase. In this study, small molecules in wheat root exudates were identified using untargeted metabolomics, and compounds belonging to amino acids, organic acids, and sugars were selected for evaluation of their influence on the expression of the acdS and acdR genes in P. hormoni G20-18(T). acdS and acdR promoters were fused to the fluorescence reporter gene mCherry enabling the study of acdS and acdR promoter activity. In planta studies in wheat seedlings indicated an induced expression of acdS in association with the roots. Exudate molecules such as aspartate, alanine, arginine, and fumarate as well as glucose, fructose, and mannitol actively induced the acdS promoter, whereas the plant hormone indole-3-acetic acid (IAA) inhibited expression. Here, we present a model for how stimulatory and inhibitory root exudate molecules influence acdS promoter activity in P. hormoni G20-18(T).