Are Endophytic Bacteria an Option for Increasing Heavy Metal Tolerance of Plants? A Meta-Analysis of the Effect Size

Abstract

Plant endophytic bacteria have received special attention in recent decades for their ability to improve plant response to multiple stresses. A positive effect of endophytes on plant’s ability to cope with drought, salinity, nitrogen deficiency, and pathogens have already been demonstrated in numerous studies, and recently this evidence was consolidated in a meta-analysis of published data. Endophytic bacteria have also been implicated in increasing resistance to heavy metals in plants; despite the important biotechnological applications of such effect in heavy metal bioremediation and agriculture, efforts to systematically analyze studies in this field have been limited. In this study, we address this task with the objective of establishing whether the findings made for other types of stresses extend to the response to heavy metals. Specifically, we seek to establish if plant inoculation with plant-growth promoting endophytic bacteria have an impact on their tolerance to heavy metal stress? We carried out a meta-analysis of the effect size of inoculation with endophytic bacteria on the host plant biomass in response to heavy metal stress (aluminum, arsenic, cadmium, copper, chromium, manganese, nickel, lead, and zinc), which included 27 (from 76 published in the last 10 years) studies under controlled conditions that evaluated 19 host plants and 20 bacterial genera. Our results suggest that endophytic bacteria increase the biomass production of host plants subjected to different heavy metals, indicating their effectiveness in protecting plants from a wide range of metal toxicities. Stress mitigation by the bacteria was similar among the different plant groups with the exception of non-accumulating plants that benefit most from the symbiotic association. Host identity and heavy metal concentration seem to influence the effect of the bacteria. Our analysis revealed that bacterial consortia provide the greatest benefit although the most common biotechnological applications are not directed towards them, and support the value of endophytic bacteria as an alternative to mitigate heavy metal stress in a wide variety of hosts.

Legumes and Nodule Associated Bacteria Interaction as Key Factor for Abiotic Stresses Impact Mitigation

“…This section shed light on how the interaction between the legumes and their associated endophytes can contribute in addressing the major types of abiotic stress face the plants which include drought, salinity, acidity/alkalinity and heavy metal toxicity [40].…”

“…Soil contamination with heavy metals results in toxic effects on plants [10]. To address this environmental problem, using association of plants with various microorganisms represents a sustainable strategy [40]. However, till now very few studies evaluated the effect of bacterial consortia for heavy metal contamination mitigation [40].…”

“…To address this environmental problem, using association of plants with various microorganisms represents a sustainable strategy [40]. However, till now very few studies evaluated the effect of bacterial consortia for heavy metal contamination mitigation [40]. Some studies reported that some bacteria have adapted well to environments polluted with heavy metals and exhibit resistance mechanisms like enhancing the expression of stress related gene, metal bioaccumulation, anti-oxidant activities and alteration of the levels of 1-aminocyclopropane-1-carboxylate (ACC) [10,70,71].…”

Characterization of Arsenic-Resistant Endophytic Bacteria From Alfalfa and Chickpea Plants

“…Excessive release of heavy metals into the environment results in plants to develop different mechanisms to cope with their negative effects (Franco-Franklin et al, 2021). A study of plant responses and tolerance revealed that several genes are induced under heavy metal stress.…”

“…Endophytic bacteria can promote plant growth by several mechanisms that include indole acetic acid (IAA) synthesis, phosphate solubilization activity, production of siderophore under Fe-limiting conditions, improving mineral nutrient uptake by plants, and nitrogen fixation activity (Ryan et al, 2008). Endophytic bacteria can promote the growth of the plant hosts, even under abiotic stress conditions (Franco-Franklin et al, 2021). Several reports also indicate the induction of genes in plants that are central to heavy metal stress signaling in the presence of beneficial bacteria, such as Pseudomonas and Bacillus (Tiwari and Lata, 2018).…”