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Hyperaccumulators are plants that can extract heavy metal ions from the soil and translocate those ions to the shoots, where they are sequestered and detoxified. Hyperaccumulation depends not only on the availability of mobilized metal ions in the soil, but also on the enhanced activity of metal transporters and metal chelators which may be provided by the plant or its associated microbes. The rhizobiome is captured by plant root exudates from the complex microbial community in the soil, and may colonize the root surface or infiltrate the root cortex. This community can increase the root surface area by inducing hairy root proliferation. It may also increase the solubility of metals in the rhizosphere and promote the uptake of soluble metals by the plant. The bacterial rhizobiome, a subset of specialized microorganisms that colonize the plant rhizosphere and endosphere, makes an important contribution to the hyperaccumulator phenotype. In this review, we discuss classic and more recent tools that are used to study the interactions between hyperaccumulators and the bacterial rhizobiome, and consider future perspectives based on the use of omics analysis and microscopy to study plant metabolism in the context of metal accumulation. Recent data suggest that metal-resistant bacteria isolated from the hyperaccumulator rhizosphere and endosphere could be useful in applications such as phytoextraction and phytoremediation, although more research is required to determine whether such properties can be transferred successfully to non-accumulator species.

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Germination is related with improvements of nutritional value of seeds, since it promotes accumulation of health-promoting phytochemicals. However, only few studies have investigated on phytochemicals accumulation during sprouting under sub-optimal conditions. Thus, we investigated the effect of salinity during germination of an einkorn (TMoM), an emmer (TDiZ) and a durum wheat (TDuC) genotype on the total polyphenols (TPC), free-and bound-phenolic acids [PAs; i.e. caffeic acid, syringic acid, Pcoumaric acid, trans-ferulic acid, and salicylic acid] contents and antiradical activity (Trolox equivalent antioxidant capacity; TEAC) of sprouts and wheatgrass. The following NaCl treatments were applied: 0 (control), 25, 50 and 100 mM NaCl concentration throughout the whole experiment, or 50 and 100 mM NaCl until sprout stage and then 0 mM until wheatgrass stage (recovery treatments). TMoM showed higher total bound-PAs both in sprouts and wheatgrass with respect to the other Triticum genotypes (+25% and 24%, respectively) as well as of total bound-PAs and bound-SA in the recovery treatments. Moderate salt stress significantly increased all the investigated variables in TDiZ. Salt stress induced higher TPC and TEAC as well as total free-PAs values till 50 mM NaCl in TDuC, whilst significantly lowered total boundPAs due to the negative variation of both P-CA (-84%) and trans-FA (-81%) acids. Results indicate that salinity during germination could be efficiently modulated to improve the nutritional quality of sprouts, wheatgrass and cereal-based products.

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