BOR1 is an efflux transporter of boron (B), responsible for loading B into the xylem. It has been reported that nitrate (NO3 - ) concentrations significantly influence B concentrations in leaves and BOR1 mRNA accumulation in roots. Here, to unravel the interactive effects of B and NO3 - on plant growth and the function of BOR1 under the combination of B and NO3 - , seedling growth was analyzed in Col-0 and bor1 mutants. The growth of bor1 mutants was negatively affected by high NO3 - but neither by potassium chloride (KCl) nor ammonium (NH4 + ) under low B conditions, suggesting the involvement of BOR1 in growth under high NO3 - . Mutants of bor2 and bor4 did not exhibit such growth responses, suggesting that this effect was specific to BOR1 among the BORs tested. Under low B conditions, loss of the BOR1 function led to a more significant decrease in B concentrations in the presence of high NO3 - compared to normal NO3 - . Additionally, grafting experiments demonstrated that these effects of NO3 - occurred when BOR1 is absent in roots. High NO3 - treatment elevated BOR1 mRNA accumulation while the BOR1 protein accumulation was downregulated. These apparent opposite responses indicated that the transcriptional and (post-)translational regulations follow different patterns. Our work provides evidence of a novel regulation of BOR1 and another B transport system by both B and NO3 - in an interactive manner.
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