Deep-Sea Actinobacteria Mitigate Salinity Stress in Tomato Seedlings and Their Biosafety Testing

Abstract

Soil salinity is an enormous problem affecting global agricultural productivity. Deep-sea actinobacteria are interesting due to their salt tolerance mechanisms. In the present study, we aim to determine the ability of deep-sea Dermacoccus (D. barathri MT2.1^T and D. profundi MT2.2^T) to promote tomato seedlings under 150 mM NaCl compared with the terrestrial strain D. nishinomiyaensis DSM20448^T. All strains exhibit in vitro plant growth-promoting traits of indole-3-acetic acid production, phosphate solubilization, and siderophore production. Tomato seedlings inoculated with D. barathri MT2.1^T showed higher growth parameters (shoot and root length, dry weight, and chlorophyll content) than non-inoculated tomato and the terrestrial strain under 150 mM NaCl. In addition, hydrogen peroxide (H₂O₂) in leaves of tomatoes inoculated with deep-sea Dermacoccus was lower than the control seedlings. This observation suggested that deep-sea Dermacoccus mitigated salt stress by reducing oxidative stress caused by hydrogen peroxide. D. barathri MT2.1^T showed no harmful effects on Caenorhabditis elegans, Daphnia magna, Eisenia foetida, and Escherichia coli MC4100 in biosafety tests. This evidence suggests that D. barathri MT2.1^T would be safe for use in the environment. Our results highlight the potential of deep-sea Dermacoccus as a plant growth promoter for tomatoes under salinity stress.

Keywords: Dermacoccus; biosafety; marine actinobacteria; plant growth promotion; salt stress; sustainable agriculture.

Figure 1

Deep-sea actinobacteria promote tomato seedlings’ growth without salt stress. (a) shoot length; (b) root length; (c) fresh weight; (d) dry weight. Data represent the mean values of three replicates. Different letters (a,b,c,e,de,cd,bc,bcd) indicate a significant difference according to Duncan at p < 0.05.

Figure 2

Effect of deep-sea actinobacteria on physiological properties of tomato seedlings under salinity stress (150 mM NaCl). (a) shoot length; (b) root length; (c) fresh weight; (d) dry weight; (e) percentage of relative water content (% RWC); (f) membrane stability index (MSI). Data represent the mean values of three replicates ± SD. Different letters (a,b,c,d) indicate a significant difference according to Duncan at p < 0.05. −NaCl means non-stressed condition; +150 mM NaCl means salinity stress condition.

Figure 3

Effect of deep-sea actinobacteria on biochemical properties of tomato seedlings under salinity stress (150 mM NaCl). (a) total soluble sugar; (b) proline content; (c) total chlorophyll; (d) content of hydrogen peroxide. Data represent the mean values of three replicates ± SD. Different letters (a,b,c,d) indicate a significant difference according to Duncan at p < 0.05. −NaCl means non-stressed condition; +150 mM NaCl means salinity stress condition.

Figure 4

Pathogenicity bioassay based on Caenorhabditis elegans. Time course of changes in the number of (a) adults; (b) juveniles; (c) eggs; (d) death rate. Data represent the mean values of three replicates ± SD. Different letters indicate a significant difference according to Duncan at p < 0.05.

Figure 5

Escherichia coli MC4100 sensitivity. Data represent the mean values of three replicates ± SD. Different letters (a,b,c) indicate a significant difference according to Duncan at p < 0.05.

Figure 6

Ecotoxicity tests in earthworms (Eisenia foetida). Time course of changes in (a) weight; (b) length of Eisenia foetida after incubation with B. cepacia CC-A174 or P. putida KT2440 or D. nishinomiyaensis DSM20448^T or D. barathri MT2.1^T. Data represent the mean values of three replicates ± SD. Different letters (a,b) indicate a significant difference in weight and length of earthworms at different cultivation days according to Duncan at p < 0.05.