Metal Resistant Endophytic Bacteria Reduces Cadmium, Nickel Toxicity, and Enhances Expression of Metal Stress Related Genes with Improved Growth of Oryza Sativa, via Regulating Its Antioxidant Machinery and Endogenous Hormones.

In-Jung Lee,Kyung Min Kim,Lubna Lubna,Sajjad Asaf,Muhammad Aaqil Khan,Rahmatullah Jan

doi:10.3390/plants8100363

Abstract

The tolerance of plant growth-promoting endophytes (PGPEs) against various concentrations of cadmium (Cd) and nickel (Ni) was investigated. Two glutathione-producing bacterial strains (Enterobacter ludwigii SAK5 and Exiguobacterium indicum SA22) were screened for Cd and Ni accumulation and tolerance in contaminated media, which showed resistance up to 1.0 mM. Both strains were further evaluated by inoculating specific plants with the bacteria for five days prior to heavy metal treatment (0.5 and 1.0 mM). The enhancement of biomass and growth attributes such as the root length, shoot length, root fresh weight, shoot fresh weight, and chlorophyll content were compared between treated inoculated plants and treated non-inoculated plants. Both strains significantly increased the accumulation of Cd and Ni in inoculated plants. The accumulation of both heavy metals was higher in the roots than in the shoots, however; Ni accumulation was greater than Cd. Heavy metal stress-responsive genes such as OsGST, OsMTP1, and OsPCS1 were significantly upregulated in treated non-inoculated plants compared with treated inoculated plants, suggesting that both strains reduced heavy metal stress. Similarly, abscisic acid (ABA) was increased with increased heavy metal concentration; however, it was reduced in inoculated plants compared with non-inoculated plants. Salicylic acid (SA) was found to exert synergistic effects with ABA. The application of suitable endophytic bacteria can protect against heavy metal hyperaccumulation by enhancing detoxification mechanisms.

Highlights

Heavy metals are high-density metallic elements that can cause toxicity at very low levels of exposure [1]
All isolates were screened for indole acetic acid (IAA) production using the method of Patten and Glick [52], siderophore production using the method of Schwyn and Neilands [53], and phosphate solubilization using the method of Katznelson and Bose [54]
It is possible that during Cd and Ni stress, ORS was produced, which induced OsGST in treated non-inoculated plants; on the other hand, the expression level was decreased during interaction with either endophyte (Figure 4A). These findings suggest that E. ludwigii SAK5 and E. indicum SA22 efficiently reduced the stress caused by heavy metals; as a result, the expression level of OsGST was reduced in treated inoculated plants compared with treated non-inoculated plants

Summary

Introduction

Heavy metals are high-density metallic elements that can cause toxicity at very low levels of exposure [1]. Some heavy metals are highly toxic even in small amounts, such as Cd, Hg, and Pb. The main sources of heavy metal contamination in the environment include industrial, agricultural, pharmaceutical, and domestic sources [2]. Wastewater irrigation of agricultural lands can result in considerable heavy metal. Plants 2019, 8, 363 accumulation and accelerate the uptake of heavy metals by plants, which would in turn reduce food quality and safety [3]. Due to the high accumulation of heavy metals in soil and plants, the risk on human health has increased in the past few decades; heavy metals are potentially hazardous environmental pollutants [4]. Anthropogenic activities continuously increase heavy metal pollution in both soil and aquatic environments. The resulting heavy metal toxicity in plants could negatively affect plant growth [5]

Objectives

Methods

Results

Discussion

Conclusion