Microbial functional traits in the hyperaccumulating Noccaea praecox rhizobiome are metal-dependent and host-driven

Abstract Background Noccaea praecox is a zinc (Zn), cadmium (Cd), and lead (Pb) hyperaccumulating plant native to the Italian peninsula and Western Balkans, where it occurs naturally in both metalliferous and non-metalliferous soils. In the present study, we investigated the effects of soil metal concentrations and the plant host on microbial functional traits, specifically the resistome (i.e., microbial functions associated with metal tolerance and resistance) in two soil compartments: the roots and rhizosphere of N. praecox . For this, we collected four plants from each metalliferous and non-metalliferous site and used a metagenomic sequencing approach to characterise microbial functions from paired root and rhizosphere samples, with three root samples per site obtained due to limited biomass, and four rhizosphere samples. Results The compartment was the primary driver of the general microbial functional structure. By contrast, the soil metal concentrations and root compartment significantly shaped the microbial resistome. Functions associated with the cobalt-zinc-cadmium efflux system and copper-transporting P-type ATPase V were significantly enriched at the metalliferous compared to the non-metalliferous site, with log 2 fold change being 2.62 and 1.72, respectively. Transporters associated with manganese/iron and cobalt/nickel were shaped by the host, regardless of soil metal levels, consistent with host-mediated filtering of microbial functions. Notably, several Zn transporter-related microbial functions associated with the ZIP family were more abundant in the rhizosphere, potentially supporting the plant’s high Zn demand. Conclusion Overall, our results demonstrate that both environmental conditions and plant host play interactive roles in shaping the microbial functional potential, with the host sometimes exerting a stronger influence than soil metal content. The enrichment of Zn transporters (Zrt-/Irt-like proteins) in the rhizosphere of the Zn-hyperaccumulating N. praecox suggests a specific microbial adaptation that may facilitate Zn uptake. These findings provide new insight into the functional dynamics of plant-microbe interactions that support the N. praecox lifestyle.