Rhizosphere Associated Denitrifying Bacteria RWX31 Induce Exudation of Unsaturated Fatty Acid, Fatty Acid Amides and Sterols in Root Exudates of Duckweed

The aquatic plant duckweed could stimulate bacterial denitrification by secretion of specific root exudates. However, the effect of denitrifying bacteria on duckweed growth and metabolites remains unclear. Here, we tested the plant growth-promoting properties of two denitrifying bacteria RWX31 and RHZ31 (Pseudomonas. sp) previously isolated from the rhizosphere of duckweed (Spirodela polyr­rhiza HZ1 and Lemna minor WX3). After 5 day, we found that RWX31 could stimulate the growth of two duckweed species by increased root length and leaf area whereas RHZ31 showed no significant activity. After RWX31 colonization, the composition of duckweed root exudate was assessed by gas chromatography/mass spectrometry (GC/MS). The unsaturated fatty acids (Linolenic acid, Oleic acid, cis-9-Hexadecenoic acid, cis-10-Hexadecenoic acid) levels were increased 1-3 fold while the saturated fatty acids (Hexadecanoic acid and Octadecanoic acid) levels were decreased compared to the controls. The secretion of two fatty acid amides (Oleamide and Erucamide) which could act as denitrification stimulators was also increased. Stigmasterol was only identified at high concentration in RWX31- colonized duckweed root exudates, while no free sterols were detected in controls. These findings provide the evidence for function of unsaturated fatty acids, fatty acid amides and sterols in the “crosstalk” between aquatic plants and denitrifying bacteria in the rhizosphere.