| Summary: | 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 <i>Pseudomonas hormoni</i> G20-18<sup>T</sup> (previously named <i>P. fluorescens</i> G20-18) harbors the genes <i>acdR</i> and <i>acdS</i> that encode regulation and synthesis of ACC deaminase, respectively. Regulation of the <i>acdS</i> 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 <i>acdS</i> and <i>acdR</i> genes in <i>P. hormoni</i> G20-18<sup>T</sup>. <i>acdS</i> and <i>acdR</i> promoters were fused to the fluorescence reporter gene mCherry enabling the study of <i>acdS</i> and <i>acdR</i> promoter activity. In planta studies in wheat seedlings indicated an induced expression of <i>acdS</i> in association with the roots. Exudate molecules such as aspartate, alanine, arginine, and fumarate as well as glucose, fructose, and mannitol actively induced the <i>acdS</i> 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 <i>acdS</i> promoter activity in <i>P. hormoni</i> G20-18<sup>T</sup>.
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