Citramalic acid and salicylic acid in sugar beet root exudates solubilize soil phosphorus

Root exudates were collected from plants grown in hydroponics under low- and high-P availability. Exudate components were separated by HPLC, ionized by electrospray, and detected by mass spectrometry in the range of mass-to-charge ratio (m/z) from 100 to 1000. Eight mass spectrometric signals were enhanced at least 5-fold by low P availability at all harvest times. Among these signals, negative ions with an m/z of 137 and 147 were shown to originate from salicylic acid and citramalic acid. The ability of both compounds to mobilize soil P was demonstrated by incubation of pure substances with Oxisol soil fertilized with calcium phosphate.Root exudates of sugar beet contain salicylic acid and citramalic acid, the latter of which has rarely been detected in plants so far. Both metabolites solubilize soil P and their exudation by roots is stimulated by P deficiency. These results provide the first assignment of a biological function to citramalic acid of plant origin.Sugar beet and wheat are similar in their phosphorus (P) efficiency with regard to shoot production [1] but they appear to use different mechanisms to overcome low availability of soil P. Wheat has a large root system that compensates for low P influx when P availability is low, whereas sugar beet is able to achieve high P influx despite low P availability in soil [1]. The higher P influx of sugar beet compared to other plant species cannot solely be due to a more efficient uptake physiology. At low P availability, soil P transport is the limiting factor in P uptake [2]. Hence, the high P influx of sugar beet is attributed to the ability of the plant to mobilize, i.e. solubilize, P in the soil. This mobilization is most likely due to chemical modification of the rhizosphere by root exudates.Plants exude up to 30% of assimilated carbon into the rhizosphere [3-5]. The composition of root exudates is complex and includes high molecular weight (HMW) and low molecular weight (LMW) molecules. HMW exudates include