C:N interaction in the metabolic response of the root system of soybean to waterlogging

Abstract

Flooding of land is a common event in nature, especially with compact soils or soils with deficient drainage properties. Flooding leads to a deficiency in O2, a stress known as hypoxia, resulting in impaired root respiration. O2 deficiency is associated with a series of modifications in plant metabolism and growth, affecting the productivity of economically important species. The most well known modification of metabolism is the switch to fermentation when the product of glycolysis, pyruvate, is transformed to lactate and ethanol. Besides these fermentation products the amino acid alanine is also prominent as a product of pyruvate metabolism under hypoxia. The formation of alanine under these conditions involves aspartate and glutamate as donors of N in transamination reactions. The other products of transamination are the organic acids oxaloacetate and alpha-ketoglutarate, respectively derived from aspartate and glutamate. The formation of these organic acids may explain how alanine formation can lead to NAD+ regeneration, just as when lactate and ethanol are produced. The objective of the present study is to seek an understanding of the role of these organic acids in the metabolic response of the plant to hypoxia. This role may involve not only the regeneration of NAD+ but the transport of organic acids to the shoot and thereby function as a mechanism whereby excess electrons produced by fermentation reactions in the roots are carried to the shoot for oxidation. Once the existence of this process can be shown, it follows that organic acids would have a role in the tolerance of plants to hypoxic stress. In order to reach these objectives, it is our plan to carry out an analysis of the modifications to primary metabolism in the roots of soybean induced by hypoxia and to verify the effect of treatments with NO3- e NH4+ on this response, since these N sources have a strong influence on the synthesis of alanine under hypoxia. Measurements of organic acids and amino acids (mainly) will be made in different parts of the plant: that is, root, leaf, xylem and phloem sap. Such data should allow the determination of which organic acids accumulate under hypoxia and whether they are transported in the xylem to the shoot. Data for leaves and phloem should reveal to what extent the compounds are metabolized after their transport to the leaf and determine which are recycled back to the root. The study is expected to contribute to a better understanding of the metabolic response and adaptation of soybean to O2 deficiency, an important point in view of the present-day climatic change and worldwide increases in flooding events. (AU)