Nitrate inhibits primary root growth by reducing accumulation of reactive oxygen species in the root tip in Medicago truncatula.

In Medicago truncatula, nitrate, acting as a signal perceived by NITRATE TRANSPORTER1/PEPTIDE TRANSPORTER FAMILY 6.8 (MtNPF6.8), inhibits primary root growth through a reduction of root cell elongation. Since reactive oxygen species (ROS) produced and converted in root tip (O2•- → H2O2 → •OH) have been reported to control cell elongation, the impact of nitrate on the distribution of these ROS in the primary root of M. truncatula was analyzed. We found that nitrate reduced the content of O2•-, H2O2 and •OH in the root tip of three wild type genotypes sensitive to nitrate (R108, DZA, A17), inhibition of root growth and O2•- accumulation being highly correlated. Nitrate also modified the capacity of R108 root tip to produce or remove ROS. The ROS content decrease observed in R108 in response to nitrate is linked to changes in peroxidase activity (EC1.11.1.7) with an increase in peroxidative activity that scavenge H2O2 and a decrease in hydroxylic activity that converts H2O2 into •OH. These changes impair the accumulation of H2O2 and then the accumulation of •OH, the species responsible for cell wall loosening and cell elongation. Accordingly, nitrate inhibitory effect was abolished by externally added H2O2 or mimicked by KI, an H2O2 scavenger. In contrast, nitrate has no effect on ROS production or removal capacities in npf6.8-2, a knockdown line insensitive to nitrate, affected in the nitrate transporter MtNPF6.8 (in R108 background) by RNAi. Altogether, our data show that ROS are mediators acting downstream of MtNPF6.8 in the nitrate signaling pathway.