One for All and All for One! Increased Plant Heavy Metal Tolerance by Growth-Promoting Microbes: A Metabolomics Standpoint

Increasing urbanization and industrialization cause the unavoidable mass release of pollutants in the environment. Nanoparticles and heavy metals are among the most threatening agents affecting ecosystems. Agriculture is severely affected by these eco-toxicants: crop productivity dramatically drops, since plant growth and development are negatively impacted. Arable lands are sinks accumulating pollutants which establish physic-chemical associations with soil particles: this leads to changes in the properties of soils, namely texture and, ultimately, alters the availability of nutrients. Green technologies in agriculture rely on the use of environmental-friendly alternatives to boost crop productivity under exogenous constraints. In this context, the use of beneficial plant growth-promoting microbes (PGPMs) is seen as a promising strategy to protect plants against the stress triggered by eco-toxicants. A strong body of evidence in the literature has shown that some PGPMs (e.g. local strains) are natural heavy metal accumulators and may also influence the metabolome of the plants they associate with. Therefore, their action is both direct and indirect. An example of the former is the secretion of extracellular polysaccharides (EPS) which function as a mechanical barrier and entrap heavy metals; an indirect effect is the priming of defence responses leading to the synthesis of specific classes of plant secondary metabolites. In this chapter, the accent will be put on the analytical power of metabolomics in conjunction with meta-metabolomics (i.e. the analysis of the metabolome of an entire community of microbes associating with different plant organs). The ultimate goal is to unravel the mechanisms responsible for the increased heavy metal tolerance in crops establishing an interaction with PGPMs. We will end our survey with some perspectives on innovative strategies in agrobiotechnology valorising PGPMs in conjunction with fertilization using beneficial elements like silicon (Si) and capable of providing sustainable solutions facing the ever-increasing release of pollutants in the environment