Antioxidative Response of Duckweed (<i>Lemna minor</i> L.) to Rhizosphere-Associated <i>Pseudomonas</i> Strains and Exogenous Indole-3-Acetic Acid

Aquatic plants, just like terrestrial plants, are exposed to oxidative stress. However, their responses are still under-researched. In this study, we examined the physiological and antioxidative responses of an aquatic plant, duckweed (<i>Lemna minor</i> L.), to four indole-3-acetic acid (IAA)-degrading and -producing <i>Pseudomonas</i> bacteria (<i>Pseudomonas oryzihabitans</i> D1-104/3, <i>P. putida</i> A3-104/5, <i>P. gessardii</i> C31-106/3 and <i>P. yamanorum</i> C44-104/1) and/or a supraphysiological level of IAA (10 mg L⁻¹). Growth characteristics, total photosynthetic pigment content, histochemical localization of reactive oxygen species and antioxidant enzyme activity (SOD, CAT and POX) were evaluated at two time points, after 3 and 7 days of co-cultivation. Superoxide anion and hydrogen peroxide were produced and accumulated mainly in the roots, daughter fronds and veins of duckweeds. Duckweeds’ responses depended on the strain of <i>Pseudomonas</i>, time and exogenous IAA. Co-cultivation of duckweed with bacteria has positive or neutral effects. Exogenous application of IAA had a negative or neutral effect on enzyme activity and other parameters. Co-cultivation with <i>P. gessardii</i> C31-106/3 showed plant-growth-promoting effects on duckweed: increased biomass production, modulation of duckweeds’ antioxidant enzymatic activity and reduction in hydrogen peroxide content. This study widens our knowledge of aquatic plants and their response to oxidative stress, supports the hypothesis that plant growth-promoting bacteria (PGPB) induce tolerable levels of oxidative stress in plants and introduces a new PGPB strain, <i>P. gessardii</i> C31-106/3.