Effect of Natural Polysaccharide Matrix-Based Selenium Nanocomposites on Phytophthora cactorum and Rhizospheric Microorganisms

Abstract

We studied the effects of new chemically synthesized selenium (Se) nanocomposites (NCs) based on natural polysaccharide matrices arabinogalactan (AG), starch (ST), and kappa-carrageenan (CAR) on the viability of phytopathogen Phytophthora cactorum, rhizospheric bacteria, and potato productivity in the field experiment. Using transmission electron microscopy (TEM), it was shown that the nanocomposites contained nanoparticles varying from 20 to 180 nm in size depending on the type of NC. All three investigated NCs had a fungicidal effect even at the lowest tested concentrations of 50 µg/mL for Se/AG NC (3 µg/mL Se), 35 µg/mL for Se/ST NC (0.5 µg/mL Se), and 39 µg/mL for Se/CAR NC (1.4 µg/mL Se), including concentration of 0.000625% Se (6.25 µg/mL) in the final suspension, which was used to study Se NC effects on bacterial growth of the three common rhizospheric bacteria Acinetobacter guillouiae, Rhodococcus erythropolis and Pseudomonas oryzihabitans isolated from the rhizosphere of plants growing in the Irkutsk Region, Russia. The AG-based Se NC (Se/AG NC) and CAR-based Se NC (Se/CAR NC) exhibited the greatest inhibition of fungal growth up to 60% (at 300 µg/mL) and 49% (at 234 µg/mL), respectively. The safe use of Se NCs against phytopathogens requires them to be environmentally friendly without negative effects on rhizospheric microorganisms. The same concentration of 0.000625% Se (6.25 µg/mL) in the final suspension of all three Se NCs (which corresponds to 105.57 µg/mL for Se/AG NC, 428.08 µg/mL for Se/ST NC and 170.30 µg/mL for Se/CAR NC) was used to study their effect on bacterial growth (bactericidal, bacteriostatic, and biofilm formation effects) of the three rhizospheric bacteria. Based on our earlier studies this concentration had an antibacterial effect against the phytopathogenic bacterium Clavibacter sepedonicus that causes diseases of potato ring rot, but did not negatively affect the viability of potato plants at this concentration. In this study, using this concentration no bacteriostatic and bactericidal activity of all three Se NCs were found against Rhodococcus erythropolis based on the optical density of a bacterial suspension, agar diffusion, and intensity of biofilm formation, but Se/CAR and Se/AG NCs inhibited the growth of Pseudomonas oryzihabitans. The cell growth was decrease by 15-30% during the entire observation period, but the stimulation of biofilm formation by this bacterium was observed for Se/CAR NC. Se/AG NC also had bacteriostatic and antibiofilm effects on the rhizospheric bacterium Acinetobacter guillouiae. There was a 2.5-fold decrease in bacterial growth and a 30% decrease in biofilm formation, but Se/CAR NC stimulated the growth of A. guillouiae. According to the results of the preliminary field test, an increase in potato productivity by an average of 30% was revealed after the pre-planting treatment of tubers by spraying them with Se/AG and Se/CAR NCs with the same concentration of Se of 0.000625% (6.25 µg/mL) in a final suspension. The obtained and previously published results on the positive effect of natural matrix-based Se NCs on plants open up prospects for further investigation of their effects on rhizosphere bacteria and resistance of cultivated plants to stress factors.

Keywords: Acinetobacter guillouiae; Phytophthora cactorum; Pseudomonas oryzihabitans; Rhodococcus erythropolis; antibacterial activity; arabinogalactan; fungicidal effect; nanocomposites; polysaccharides; potato productivity; selenium.

Figure 1

Fourier-transform infrared (FTIR) absorption spectra of all Se NCs and their polysaccharides. Continuous ovals depicted four intense IR absorption regions that were almost the same for all Se NCs in our study. Dotted solid red ovals depicted three additional IR absorption regions specific for CAR and Se/CAR NC.

Figure 2

UV-Vis optical density spectra of SeNPs.

Figure 3

Typical diffractograms of Se/AG and Se CAR NCs.

Figure 4

Diffractogram of the Se/ST NC and the original ST (vertical lines depict the Se/ST NC reflexes, which coincide with those for the original ST).

Figure 5

Results of energy-dispersive X-ray spectroscopy microanalysis (EDXMA) of Se/AG (A), Se/ST (B), and Se/CAR (C) NCs.

Figure 6

Photos of the Se NCs obtained using a transmission electron microscope LEO 906E. Scale bar = 200 nm.

Figure 7

Effect of the Se NCs on the growth of rhizosphere bacteria (with 105.57 µg/mL for Se/AG NC, 428.08 µg/mL for Se/ST NC, and 170.30 µg/mL for Se/CAR NC, which corresponds to the Se concentration of 6.25 µg/mL in the final solution): (A) R. erythropolis, (B) A. guillouiae, (C) P. oryzihabitans. 1—control, 2—Se/AG NC, 3—Se/ST NC, 4—Se/CAR NC.

Figure 8

Effect of the Se NC treatments (with 105.57 µg/mL for Se/AG NC, 428.08 µg/mL for Se/ST NC, and 170.30 µg/mL for Se/CAR NC, which corresponds to the Se concentration of 6.25 µg/mL in the final solution) on the growth dynamics of rhizosphere bacteria A. guillouiae (A), R. erythropolis (B), and P. oryzihabitans (C). CS—control samples; * p < 0.01, ** p < 0.05 (in comparison with CS at each respective time point) based on 9 independent observations per each Se NC, control and bacteria.

Figure 9

Effect of the Se NC treatments (with 105.57 µg/mL for Se/AG NC, 428.08 µg/mL for Se/ST NC, and 170.30 µg/mL for Se/CAR NC, which corresponds to the Se concentration of 6.25 µg/mL in the final solution) on the biofilm formation of rhizosphere bacteria A. guillouiae (A), R. erythropolis (B), and P. oryzihabitans (C); * p < 0.01, ** p < 0.05 (in comparison with CS) based on 9 independent observations per each Se NC, control and bacteria.

Figure 10

Effect of the Se NC treatments (with 105.57 µg/mL for Se/AG NC, 428.08 µg/mL for Se/ST NC, and 170.30 µg/mL for Se/CAR NC, which corresponds to the Se concentration of 6.25 µg/mL in the final solution) on the average weight of potatoes (A) and number of tubers (B) per plant, and crop structure (C). Large tubers weighed 150 g or more; commercial (marketable)—85–150 g; seed—50–80 g; small—less than 50 g; * p < 0.01, ** p < 0.05 (in comparison with CS) based on 3 plots per each Se NC and control.