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

Valeria Franco-Franklin,Thaura Ghneim-Herrera,Sandra Moreno-Riascos

doi:10.3389/fenvs.2020.603668

Valeria Franco-Franklin, Thaura Ghneim-Herrera + Show 1 more

Open Access

https://doi.org/10.3389/fenvs.2020.603668

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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.

Highlights

Plants are constantly affected by different types of stress, both biotic and abiotic, which induce an alteration in their metabolism and functioning, and reduce their productivity, generating large losses in crop yields (Rejeb et al, 2014)
Of the 76 articles found in the databases, only 27 were used to carry out the meta-analysis (Supplementary Table S3) because the rest did not meet the requirements established in the methodology
Some of the articles did not contain information on the standard deviation or did not have results related to the plant biomass, since they focused on the evaluation of the activity of several genes and enzymes involved in tolerance to heavy metals (Supplementary Table S4)

Summary

Introduction

Plants are constantly affected by different types of stress, both biotic and abiotic, which induce an alteration in their metabolism and functioning, and reduce their productivity, generating large losses in crop yields (Rejeb et al, 2014). Heavy metals such as zinc, copper, molybdenum, manganese, cobalt, and nickel are naturally found in soils and are essential for the functioning of important biological processes These elements in combination with more toxic heavy metals (arsenic, lead, cadmium, mercury, chromium, aluminum, and beryllium) can reduce crop productivity when their concentrations exceed optimal values for plant functioning, causing morphological abnormalities and metabolic disorders that increase the production of reactive oxygen species (Tiwari and Lata, 2018). Both essential and non-essential metals, when present at phytotoxic concentrations, generate common adverse effects such as chlorosis, growth inhibition, reduced photosynthesis, low biomass accumulation, altered water balance, senescence and plant death (Singh et al, 2016). Human activities and industrialization in recent decades have led to an excessive release of these elements into the environment, so rapidly that plants face pressure to develop mechanisms to cope with their phytotoxicity

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