Effects of Nitrate on Accumulation of Trehalose and other Carbohydrates and on Trehalase Activity in Soybean Root Nodules

Abstract

Soybean ( Glycine max cv. Maple Arrow) plants were infected with Bradyrhizobium japonicum (strain 61-A-101), grown in sterilized Leonard jars, exposed to various amounts of nitrate either from the beginning or after completion of nodulation. The presence of 5 mM and more nitrate during nodulation caused a considerable reduction of the number and biomass of nodules per plant, of nitrogenase activity per nodule fresh weight. The carbohydrate content of nodules was determined on a dry weight basis. The level of the disaccharide trehalose, produced by the microsymbiont, was 50% lower in nodules formed in the presence of 20 mM nitrate than in control nodules formed in its absence. With regard to the non-structural carbohydrates produced by the plant, nodules formed in the presence of high amounts of nitrate contained about 75 % less starch but three- to fourfold higher levels of sucrose and pinitol than control nodules. Sucrose was the most abundant non-structural carbohydrate in nodules formed in the presence of 20 mM nitrate, accounting for 4–5 % of the dry weight. When plants with fully established nodules, grown in the absence of nitrate, were shifted to 20 mM nitrate, the levels of trehalose and starch decreased over a period of 3 weeks while the level of sucrose increased, until the carbohydrate levels attained similar values as found in nodules established in the presence of nitrate. The activity of trehalase, an enzyme known to be induced in nodules, was about 75% lower in nodules formed in the presence of nitrate than in control nodules. However, trehalase activity did not change in established nodules during a 3-week exposure to 20 mM nitrate. Similarly, the number of colony- forming bacteria recovered from the nodules and the activities of endochitinase and endoglucanase, two plant defense hydrolases, were not affected during a 3-week exposure to nitrate.