Roles of endophytic bacteria in Suaeda salsa grown in coastal wetlands: Plant growth characteristics and salt tolerance mechanisms

Highlights

• Endophytic bacteria regulated osmotic metabolism of Suaeda salsa under salt stress.

• Sphingomonas prati regulated soluble sugar and CAT, as salt tolerance factor in plant.

• Sphingomonas prati increased plants carbohydrate, Sphingomonas zeicaulis decreased.

• For plants, Sphingomonas prati and Sphingomonas zeicaulis have antagonistic effect.

Abstract

Salinity is a limiting factor in the growth of plants in coastal wetlands. The interaction of halophytes with salt-tolerant endophytes has been one of the major concerns in this area. However, the mechanism by which endophytes promote halophyte growth remains unclear. The growth and physiological responses of Suaeda salsa inoculated with endophytic bacteria (Sphingomonas prati and Sphingomonas zeicaulis) at 0 ‰ and 20 ‰ NaCl were studied. The results showed that Sphingomonas zeicaulis had stronger positive effects on the growth of Suaeda salsa under 0 ‰ NaCl, and Sphingomonas prati performed better under 20 ‰ NaCl. Sphingomonas prati inoculation increased the mean height, root length, fresh weight and dry weight by 45.43%, 9.91%, 82.00% and 102.25%, respectively, compared with the uninoculated treatment at 20 ‰ NaCl. Sphingomonas prati inoculation decreased MDA content by 23.78%, while the soluble sugar and soluble protein contents increased by 15.08% and 12.57%, respectively, compared to the control, at 20 ‰ NaCl. Increases in SOD and CAT in the Sphingomonas prati inoculation were 1.03 and 1.47-fold greater, respectively, than in the Sphingomonas zeicaulis inoculation, under 20 ‰ NaCl. Moreover, Sphingomonas prati and Sphingomonas zeicaulis had antagonistic interactions in Suaeda salsa according to the results of the “interaction equation” (most G values were negative). PCA, clustering analysis and the PLS model revealed two mechanisms for regulating plant salt tolerance by which Sphingomonas prati enhanced Suaeda salsa growth: (1) Sphingomonas prati improved intracellular osmotic metabolism and (2) Sphingomonas prati promoted the production of CAT in the antioxidant enzyme system and retained permeability. This study provides new insight into the comprehensive understanding and evaluation of endophytic bacteria as biological inoculants in plants under salt stress.

Introduction

Coastal wetlands are essential for mitigating climate change, sequestering carbon and supporting biodiversity (Oyuela Leguizamo et al., 2017; Villa and Bernal, 2018; Xi et al., 2021). Globally, over one-third of coastal wetlands have lost ecological function in the past several decades (Sapkota and White, 2020). Plants play important roles in the ecological function of coastal wetlands, especially in enhancing biodiversity. Nonetheless, extreme fluctuations in salinity in coastal wetlands may directly impair plant functions, leading to restrained growth or even death (Herbert et al., 2015).

High salt (NaCl) concentrations are believed to affect osmotic metabolism in plants. Excess accumulation of Na⁺ may damage the normal physiological and biochemical processes of the cell (Kaur et al., 2018). Apart from osmotic stress, another damaging consequence of salinity is oxidative damage, resulting from excess generation of reactive oxygen species (ROS) (Van Zelm et al., 2020). Toxic ROS include superoxide (O₂·⁻), hydrogen peroxide (H₂O₂), and hydroxyl radicals (OH⁻), and their excess accumulation causes membrane deterioration and lipid peroxidation, leading to irreparable metabolic, structural dysfunction and ending in cell death (Alam et al., 2019). To maintain normal physiological functions and improve salt tolerance, plants have the following mechanisms: (1) accumulating compatible osmolytes, such as soluble sugars, that affect water uptake and the associated physiological pathways; (2) partitioning and compartmentalizing salt ions in vacuoles; and (3) upregulating the antioxidant defence system for the rapid removal of toxic ROS (Ahanger et al., 2018; Chrysargyris et al., 2019).

Inoculation with rhizosphere and endophytic microorganisms may enhance the salt tolerance of plants (Egamberdieva et al., 2017; Kearl et al., 2019; Moreira et al., 2020). Compared to epiphytic bacteria, endophytic bacteria colonize internal tissues of plants without causing disease symptoms, and form extensive associations with plants (Alcántara-Martínez et al., 2018; Zhao et al., 2019). Apart from stimulating plant growth, some endophytic bacteria may protect plants under stressful conditions (Papik et al., 2020). Mechanisms of bacterial regulation include direct regulation and indirect regulation. Direct regulation is achieved by enhancing diverse chemical entities and producing phytohormone signals, such as ACC deaminase, whereas indirect regulation occurs by altering plant defense and metabolic pathways, that accumulate osmolytes and antioxidant compounds (Qin et al., 2016; Xiong et al., 2019). Studies have shown that various endophytic bacteria, such as Sphingomonas, Pantoea, Bacillus, and Arthrobacter endophyticus, may enhance the resistance of glycophytes under salt stress (Li et al., 2016; Dong et al., 2019). However, it is not clear whether endophytic bacteria will affect halophytes, such as Suaeda salsa, during salt stress. We hypothesize that salt-tolerant endophytes will balance osmotic metabolism and relieve oxidative stress in halophytes.

Suaeda salsa, as a representative wetland plant, was inoculated with two endophytic bacterial strains (Sphingomonas zeicaulis and Sphingomonas prati) under 0 ‰ NaCl and 20 ‰ NaCl conditions in this study. The objectives were as follows: (1) to estimate whether endophytic bacterial strains could enhance the growth of Suaeda salsa under salt stress; (2) to analyse the effects of endophytic bacterial inoculation on metabolite production related to osmotic stress and antioxidant enzyme synthesis; and (3) to illustrate the mechanism by which endophytic bacteria enhance the growth of Suaeda salsa under salt stress.