Abstract
Endophytes have the potential to enhance the ability of plants to resist salt stress, improving crop development and yield. Therefore, in this study, we isolated an endophyte that produced large amounts of exopolysaccharides (EPSs) from the roots of sea rice and examined its effects on the physiological responses of rice (Oryza sativa L. ssp. japonica “Nipponbare”) seedlings to salt stress using hydroponic experiments. The endophyte was named Pantoea alhagi NX-11 based on its morphological characteristics and 16S ribosomal DNA (rDNA) sequence alignment. Rice seedlings that had been inoculated with P. alhagi NX-11 exhibited a 30.3% increase in fresh weight, a 28.6% increase in root length, a 51.6% increase in shoot length, and a 26.3% increase in chlorophyll content compared with control seedlings under normal conditions. In addition, inoculated rice seedlings had a 37.5% lower malondialdehyde content, a 133% higher K+/Na+ ratio, and a 52.8% higher proline content after 7 days under salt stress, as well as up-regulated expression of proline synthase, down-regulated expression of proline dehydrogenase, and enhanced antioxidant enzyme activities. Interestingly, rice seedlings that were inoculated with an EPS-deficient strain named NX-11eps– that was obtained by atmospheric and room temperature plasma (ARTP) mutagenesis were damaged by salt stress and had similar physiological and biochemical indicators to the control group. Therefore, we speculate that the ability of P. alhagi NX-11 to enhance the salt tolerance of rice seedlings is related to the EPSs it produces.
Highlights
In modern agricultural production, soil salinization is a common abiotic stress that reduces crop growth and yield
The endophyte NX-11, which was isolated from sea rice growing on a beach in Lianyungang, Jiangsu Province, China, was found to be a gram-negative, rod-shaped bacterium that had a moist, smooth surface and secreted a large amount of a viscous polymer when grown in fermentation medium (Supplementary Figures S1A,B)
We found that rice seedlings that had been inoculated with P. alhagi NX-11 exhibited significantly higher activities of the antioxidant enzymes SOD, POD, and chloramphenicol acetyltransferase (CAT) than uninoculated seedlings on the 7th day after salt stress treatment, even though the antioxidant enzyme activities of seedlings in the NaCl group were much higher than those of seedlings growing under normal conditions (Figures 4B–D)
Summary
Soil salinization is a common abiotic stress that reduces crop growth and yield. Plant growth-promoting bacteria (PGPBs) attach to the roots of plants or enter plant tissues and affect their physiological state, under abiotic or biotic stress conditions, and so can enhance the tolerance of plants to salt stress and reduce salt-induced damage in plant cells. The use of PGPBs is considered an environmentally friendly and cost-effective strategy for promoting the growth of plants under salt stress (Nadeem et al, 2014; Li et al, 2017). Endophytes can promote plant growth through their interactions with the host plant and are considered more suitable than soil microorganisms because they can interact with the host more directly and exhibit better responses to environmental change (Rosenblueth and Martínez-Romero, 2006). The mechanism by which they do so is not yet fully understood because it is not easy to monitor the physiological state of endophytes in situ and their metabolites have complex effects on the host
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