A Study On Identifying Signals Controlling Nitrate Uptake By Wheat Roots Using A New Non-invasive Technique

Nitrogen is needed in large quantity by plants and nitrate, as one major form of N, was absorbed by plant roots. Therefore, nitrate uptake and its controlling signals have attracted more and more attention so far. Wheat (Triticum aestivum) is one of the most important food crops in the world, improving its efficiency of nitrate uptake is important to increase yield, decrease the waste of fertilizer and protect the environment. Identifying signals, including feed-forward and feedback signals controlling nitrate uptake by wheat roots, have been investigated. A new non-invasive technique has been used, by which intact plants grow at the normal rate during the experiments.To identify feed-forward signal(s) controlling nitrate uptake, the effects of light and carbohydrates on time course of net nitrate influx have been studied. Illuminating the shoot stimulated nitrate uptake by the roots in many species, including wheat, as we report here. The mechanism is thought to be via a signal transported in the phloem, probably as a kind of sugar. Sugars supplied directly to the root system also stimulate nitrate uptake, which is generally viewed as an expression of the same signalling process. Our aim is to determine whether sucrose is a specific signalling molecule, and whether the light effect occurs via sucrose. To identify a signalling molecule we have compared the response of net nitrate influx to the supply of several sugars to the root system. A signal sugar should cause a rapid, large stimulus of net nitrate influx; other sugars acting indirectly should have a smaller effect after a lag. A method of measuring net nitrate influx with a time resolution of minutes has enabled us to make this comparison. Amongst nine different sugars and mannitol, sucrose alone caused an immediate, large stimulation of net nitrate influx. Glucose, fructose and raffinose caused smaller responses with a lag. Other sugars and mannitol had no effect (except mannose which has an effect related to its biochemistry). When the plant was in the light, the stimulation by external sucrose was less than in the dark, indicating that light affects nitrate uptake co-operatively with externally supplied sucrose. Sucrose showed the features of a specific molecule signalling to nitrate uptake by the root system. This has long been supposed but not previously supported experimentally. The interaction between light and externally supplied sucrose showed that the mechanism of the light effect on nitrate uptake involved sucrose. Sucrose acts as a positive feed forward signal, which must interact with other types of signal to overcome environmental perturbations.To seek the feedback signal(s) controlling nitrate uptake, the effects of thirteen amino acids on time course of net nitrate influx have been studied. The results showed that glutamate caused rapid a rapid and large decrease of net nitrate influx. Aspartic acid, asparagine, glutamine, arginine, serine and lysine caused a relatively slow decrease of net nitrate influx with a lag of 0.5-1.5 h. Other amino acids had a very small effect or even no effect. It is speculated that glutamate is most possible to be a negative feedback signal because the change of net nitrate influx is bigger and faster. Glutamate as a feedback signal, which has a smaller effect than sucrose, collaborates with sucrose (as a feed-forward signal) and controls nitrate uptake by wheat roots.