Silicon isotope fractionation with a low-silica rice mutant reflects plant uptake strategy in response to different silicon supply levels

Plants may modulate uptake pathways when the nutrient concentration varies. The identified transporters Lsi1 and Lsi2 belong to low-affinity systems; however, it is unknown whether a high-affinity system responsible for silicon (Si) transport exists or how plants modulate Si uptake pathways. We aimed at developing isotope fractionation as a novel tool for nutrient uptake and cycling in plants. We grew the wild-type rice 'Oochikara' and its mutant lsi1 under low (Si-L: 0.17 mM) and high (Si-H: 1.70 mM) Si supply levels and evaluated Si isotope fractionation between different Si pools. The isotope fractionations between Oochikara and nutrient solution were -0.15 and -0.63 parts per thousand at Si-L and Si-H, respectively; between lsi1 and nutrient solution they were -0.18 and -1.33 parts per thousand at Si-L and Si-H, respectively. The qualitative isotope fractionation caused by Lsi1 were -0.09 and -0.49 parts per thousand at Si-L and Si-H, respectively. We thus verified the hypothesis that Lsi1 preferred Si-28 transport over Si-30 and that this preference was independent of Si supply levels. Changes in Delta Si-30(plant-solution) suggested that a high-affinity system exists and that plants may regulate Si uptake strategy by changing the participation of low-affinity and high-affinity pathways. Another root-shoot translocation that favors Si-28 has been found in Si-H-lsi1 that previously occurred in banana (Musa spp.), which suggested that Si xylem loading is related to the different Si transport abilities of Lsi1 and Lsi2. Our results demonstrated that Si isotope fractionation can shed light on plants' response to variable nutrient conditions.