Root amino acid exudation: measurement of high efflux rates of glycine and serine from six different plant species

Abstract

Amino acid concentration in the rhizosphere results from fluxes between plant roots, soil and microorganisms. In this context, root amino acid exudation process, composed of both efflux and influx, remains unclear. One main issue is to understand the selectivity of amino acid exudation resulting mainly in high proportions of glycine and serine in exudates compared to low proportions inside the root. To reach this point, a quantitative analysis of exudation with dissociated measurements of efflux from influx is needed. We measured efflux and influx by supplying 15 N-labelled glycine or serine for a short time of exposure at ecologically relevant concentrations to plants of white clover (Trifolium repens L.), perennial ryegrass (Lolium perenne L.), maize (Zea mays L.), oilseed rape (Brassica napus L.), tomato (Lycopersicon esculentum Mill.) and alfalfa (Medicago sativa L.). Efflux was estimated by the increase of 14 N content of amino acids in root exudates and influx was estimated by the increase of 15 N content in plant tissue. Glycine efflux exceeded influx for all six species and was much higher in Fabaceae than in Poaceae. Serine efflux exceeded influx in alfalfa, white clover and rape. We conclude that presence of glycine and serine in root bathing solutions results from high glycine and serine efflux rates, observed in all six species studied here. The physiological and ecological significances of these high efflux rates are discussed in the context of N metabolism and plantsoil-microorganisms interactions.

Wheat roots efflux a diverse array of organic N compounds and are highly proficient at their recapture

“…Concentrations of amino acids at steady-state and rates of efflux were, in fact, broadly similar to those reported in previous studies focusing solely on amino acids (Jones and Darrah 1993;Phillips et al 2006;Lesuffleur et al 2007;Jämtgård et al 2008; Cliquet Fig. 6 The multivariate relationships among H 2 O extracts of roots, the aqueous phase of a methanol:chloroform:water extract of roots and the concentration of exudates measured at 4 h (a) and also the gross rate of efflux calculated between 1 min and 1 h (b).…”

“…Indeed, many studies on organic N compounds have focussed solely on amino acids (Jones and Darrah 1993;Phillips et al 2006;Lesuffleur et al 2007;Jämtgård et al 2008;Lesuffleur and Cliquet 2010), while reviews and summaries often imply that amino acids are the main organic N compound in root exudates. For example, the chemical composition of root exudates has been summarised as: sugars (50-70 % of exudate), carboxylic acids (20-30 % of exudate) and amino acids (10-20 % of exudate) (Kraffczyk et al 1984;Jones 1998;Hutsch et al 2002;Farrar et al 2003).…”

“…In Medicago sativa, Medicago truncatula, Zea mays, Lolium multiflorum and Triticum aestivum the rates of uptake of 16 amino acids supplied at 2.5 μM exceeded efflux rates by 5-545 % (Phillips et al 2004(Phillips et al , 2006. In contrast Lesuffleur et al (2007) working with amino acids supplied at 1, 10 and 20 μM found that efflux of glycine was faster than uptake in six crop species, while efflux of serine was faster than uptake in Medicago sativa, Trifolium repens, and Brassica napus. These contrasting results might reflect that different methods were used to disentangle efflux from uptake.…”

“…amino acids: Schobert and Komor 1987;Jämtgård et al 2008). In this context it is worth noting that past studies showing simultaneous efflux and re-capture of amino acids supplied amino acids at arbitrary concentrations of 1 10 & 20 μM (Lesuffleur et al 2007; Lesuffleur and Cliquet 2010) or 2.5 μM (Phillips et al 2004(Phillips et al , 2006 or 100 μM (Jones and Darrah 1994). In general these concentrations are larger and bear little semblance to concentrations that occur in exudates.…”

“…1) to obtain direct estimates of efflux and uptake of a broad suite of organic N compounds at field-relevant concentrations. Previous studies using 15 N-labeling to determine efflux and uptake have supplied plants with a modest number of amino acids at arbitrarily chosen concentrations (Lesuffleur et al 2007;Lesuffleur and Cliquet 2010). The approach used in the present study was instead to harvest the natural complement of exudate compounds from plants grown with N at natural abundance.…”