Transition from a maternal to external nitrogen source in maize seedlings.

Maximizing NO uptake during seedling development is important as it has a major influence on plant growth and yield. However, little is known about the processes leading to, and involved in, the initiation of root NO uptake capacity in developing seedlings. This study examines the physiological processes involved in root NO uptake and metabolism, to gain an understanding of how the NO uptake system responds to meet demand as maize seedlings transition from seed N use to external N capture. The concentrations of seed-derived free amino acids within root and shoot tissues are initially high, but decrease rapidly until stabilizing eight days after imbibition (DAI). Similarly, shoot N% decreases, but does not stabilize until 12-13 DAI. Following the decrease in free amino acid concentrations, root NO uptake capacity increases until shoot N% stabilizes. The increase in root NO uptake capacity corresponds with a rapid rise in transcript levels of putative NO transporters, ZmNRT2.1 and ZmNRT2.2. The processes underlying the increase in root NO uptake capacity to meet N demand provide an insight into the processes controlling N uptake.