Dual pathways for regulation of root branching by nitrate

Root development is extremely sensitive to variations in nutrient supply, b ut the mechanisms are poorly understood. We have investigated the processes by which nitrate (NO3-), depending on its availability and distribution, c an have both positive and negative effects on the development and growth of lateral roots. When Arabidopsis roots were exposed to a locally concentrat ed supply of NO3- there was no increase in lateral root numbers within the NO3--rich zone, but there was a localized 2-fold increase in the mean rate of lateral root elongation, which was attributable to a corresponding incre ase in the rate of cell production in the lateral root meristem. Localized applications of other N sources did not stimulate lateral root elongation, consistent with previous evidence that the NO3- ion is acting as a signal r ather than a nutrient. The axr4 auxin-resistant mutant was insensitive to t he stimulatory effect of NO3-, suggesting an overlap between the NO3- and a uxin response pathways. High rates of NO3- supply to the roots had a system ic inhibitory effect on lateral root development that acted specifically at the stage when the laterals had just emerged from the primary root, appare ntly delaying final activation of the lateral root meristem, A nitrate redu ctase-deficient mutant showed increased sensitivity to this systemic inhibi tory effect, suggesting that tissue NO3- levels may play a role in generati ng the inhibitory signal, We present a model in which root branching is mod ulated by opposing signals from the plant's internal N status and the exter nal supply of NO3-.