Highlights

•Distinct functional rhizobacteria clusters assist sedum in responding to Pb stress.

•Diverse clusters assist AE to better adapt Pb stress and enhance Pb absorption.

•Clusters driven growth promotion and Pb activation enhance plant Pb accumulation.

•Highly specialized clusters reduce Pb stress for NAE by phosphorus-dissolving.

•Stronger selection effect may lead to vulnerable community under extreme Pb stress.

Abstract

Lead (Pb) contamination presents a widespread environmental plague. Sedum alfredii is widely used for soil phytoremediation owing to its capacity to extract heavy metals, such as Pb. Although efficient Pb extraction is mediated by complex interactions between the roots and rhizospheric bacteria, the mechanism by which S. alfredii recruits microorganisms under Pb stress remains unclear. The Pb-accumulating ecotype (AE) and non-accumulating ecotype (NAE) of S. alfredii recruited different rhizobacterial communities. Under Pb stress, AE rhizosphere-enriched bacteria assembled into stable-connected clusters with higher phylogenetic and functional diversity. These microbes, e.g., Flavobacterium, could release indoleacetic acid to promote plant growth and siderophores, thereby increasing Pb availability. The NAE rhizosphere-enriched functional bacteria “desperately” assembled into highly specialized functional clusters with extremely low phylogenetic diversity. These bacteria, e.g., Pseudomonas, could enhance phosphorus solubilization and Pb precipitation, thereby reducing Pb stress and plant Pb accumulation. High niche overlap level of the rhizo-enriched species raised challenges in soil resource utilization, whereas the NAE community assembly was markedly constrained by environmental “selection effect” than that of AE rhizobacterial community. These results indicate that different ecotypes of S. alfredii recruit distinct bacterial functional assemblies to drive specific plant-soil feedbacks for different survival in Pb-contaminated soils. To cope with heavy metal stress, NAE formed a highly functional and specialized but vulnerable community and efficiently blocked heavy metal absorption by plants. However, the AE community adopted a more stable and elegant strategy to promote plant growth and the accumulation of dry matter via multiple evolutionary strategies that ensured a high yield of heavy metal phytoextraction. This for the first time provides new insights into the roles of rhizosphere microbes in plant adaptations to abiotic stresses.

    铅（Pb）污染是一种普遍存在的环境污染。东南景天由于具有提取Pb等重金属的能力，被广泛应用于土壤植物修复。尽管有效Pb的提取是通过根系和根际细菌之间复杂的相互作用来实现的，但东南景天在Pb胁迫下对根际微生物的选择机制尚不清楚。东南景天的积累pb生态型（AE）和不积累pb生态型（NAE）具有不同的根际细菌群落。在Pb胁迫下，AE根际富集的细菌聚集成稳定连接的集群，具有较高的系统发育和功能多样性。这些微生物（如黄杆菌属）可以释放吲哚乙酸促进植物生长和铁载体形成，从而提高Pb的生物可利用性。而NAE根际的功能细菌则“急切地”组装成高度专门化的功能集群，系统发育多样性极低。这些细菌（如假单胞菌属）可以加强P的增溶和Pb的沉淀，从而减少Pb胁迫和植物Pb积累。根际物种丰富导致生态位重叠程度较高，从而对土壤资源利用提出了挑战。与AE根际细菌群落相比，NAE群落的组合明显受到环境“选择效应”的制约。这些结果表明，不同生态型的东南景天会选择不同的根际细菌功能组成来驱动特定的植物-土壤反馈，从而在Pb污染的土壤中实现不同的生存策略。为了应对重金属胁迫，NAE在根际形成了一个功能高度专一但脆弱的群落，有效阻断了植物对重金属的吸收。而AE则通过多种进化策略促进植物生长和干物质积累，保证了植物提取较高水平的重金属。本研究为植物适应非生物胁迫过程中根际微生物的作用提供了新的见解。

    论文网页链接：https://www.sciencedirect.com/science/article/pii/S0160412020318675