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.

中文翻译：

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内生细菌是提高植物对重金属耐受性的一种选择吗？效应量的元分析

近几十年来，植物内生细菌因其改善植物对多种胁迫反应的能力而受到特别关注。许多研究已经证明了内生菌对植物应对干旱、盐分、缺氮和病原体的能力的积极影响，最近这一证据在已发表数据的荟萃分析中得到了巩固。内生细菌也与增加植物对重金属的抗性有关。尽管这种效应在重金属生物修复和农业中具有重要的生物技术应用，但系统分析该领域研究的努力仍然有限。在这项研究中，我们处理这项任务的目的是确定对其他类型压力的研究结果是否扩展到对重金属的反应。具体来说，我们试图确定植物接种促进植物生长的内生细菌是否会影响它们对重金属胁迫的耐受性？我们对接种内生细菌对宿主植物生物量响应重金属胁迫（铝、砷、镉、铜、铬、锰、镍、铅和锌）的影响大小进行了荟萃分析，其中包括27 项（来自过去 10 年发表的 76 项）在受控条件下进行的研究，评估了 19 种寄主植物和 20 种细菌属。我们的研究结果表明，内生细菌增加了受到不同重金属影响的宿主植物的生物量产生，表明它们在保护植物免受各种金属毒性方面的有效性。除了从共生关联中受益最多的非积累植物外，细菌对不同植物群的胁迫缓解作用相似。宿主身份和重金属浓度似乎会影响细菌的作用。我们的分析表明，尽管最常见的生物技术应用并不针对它们，但细菌聚生体提供了最大的好处，并且支持内生细菌作为减轻各种宿主重金属压力的替代品的价值。