Nitrate signaling pathway via the transporter MtNPF 6.8 involves abscisic acid for the regulation of primary root elongation in Medicago truncatula

Medicago truncatula seedling establishment is a multigenic trait responsive to exogenous nitrate. A quantitative genetic dissection of this trait allowed us to hypothesize that nitrate-signaling pathway for the control of primary root elongation involves the double affinity nitrate transporter MtNPF6.8. This hypothesis was challenged by phenotyping wild type (M. truncatula, R108) and an RNAi mutant npf6.8 in the presence (5 mM NO3 −) and absence of exogenous nitrate. The phenotyping consisted in both of measuring the length of the primary roots of seedlings grown on vertical plates and measuring the length of the cortical cells. The results of this experiment supported the above-mentioned hypothesis by showing that the inhibitory effect of nitrate on primary root and on cortical cell elongation was abolished in the mutant lines. This result supports the assumption that in M. truncatula, nitrate regulates primary root growth by controlling the elongation of cortical cells. As several works proposed that abscisic acid (ABA) may act downstream of nitrate for its perception as a signal in its role in the modulation of root architecture, we were interested to check for a potential involvement of ABA in MtNPF6.8-mediated nitrate signaling. Treating the npf6.8 knockdown lines by exogenous ABA (10 μM) restored the inhibitory effect of nitrate on primary root growth. Accordingly, we proposed that ABA might intervene downstream of the nitrate transporter MtNPF6.8 in nitrate signaling pathway for the control of cortical cells elongation in M. truncatula. ABA action on root elongation was associated to another nitrate transporter of the NPF family, MtNPF1.7 (MtLATD/NIP) in M. truncatula. Thorough studies of the mutation affected in MtNPF1.7, showed that unlike the interplay between ABA and MtNPF6.8, ABA in concert with MtNPF1.7, exerted a positive effect on root elongation. Exogenous ABA, in the absence of an environmental nitrate signal rescued the primary and lateral root meristem arrest and root growth defect of npf1.7 mutants. Taking together these two facets of the interplay between two transporters of the NPF family and ABA for the control of primary root elongation we suggest that exogenous nitrate intervenes as a decisive partner in the determination of the ABA effect.