Mechanisms of nitrogen-form preference in shoots and roots of invasive and native Xanthium species

BACKGROUND: Nitrogen (N) form preference influences responses of invasive plants to soil N form changes caused by global change and the evolution of their invasiveness, and the differences in root uptake and shoot assimilation for different forms of N affect N form utilization in plants. However, the relative roles of shoots and roots in N-form preference are still unclear for invasive plants. METHODS: We grew the homo- and heterografted seedlings of the nitrate-preferred invasive plant Xanthium strumarium and the ammonium-preferred native plant X. sibiricum under two levels of nitrate and ammonium. KEY RESULTS: Under high N levels, the grafted seedlings with X. strumarium as scion, regardless of the source of the rootstock always preferred nitrate, whereas those with X. sibiricum as scion always preferred ammonium. Higher expression of nitrate reductase genes in leaves of X. strumarium relative to X. sibiricum was associated with its nitrate preference. Regardless of scion origin, the grafted seedlings with X. strumarium as rootstock always accumulated more total biomass and N than those with X. sibiricum as rootstock under both low and high N levels, which was associated with higher root surface area, volume, and NPF6.3 expression in the invasive relative to the native species. CONCLUSIONS: Our results indicate that N-form preference was determined primarily by shoot identity, whereas roots contributed to the magnitude of the response, providing a novel perspective for understanding the mechanisms underlying exotic plant invasion from the viewpoint of N form utilization.