Phosphorus Stress-Induced Proteoid Roots Show Altered Metabolism in Lupinus albus

Abstract

Proteoid roots develop in Lupinus albus 1. in response to nutrient stress, especially P. Proteoid roots excrete citrate and thus increase the availability of P, Fe, and Mn in the rhizosphere. In an effort to understand citrate synthesis and organic acid metabolism in proteoid roots of lupin, we have evaluated i n vitro enzyme activities of citrate synthase (CS), malate dehydrogenase (MDH), and phosphoenolpyruvate carboxylase (PEPC) i n proteoid and normal roots of plants grown with or without P. Organic acid concentrations, respiration rates, and dark ''COz-labeling patterns were also determined. The in vitro specific activities of CS, MDH, and PEPC and in vivo dark "COZ fixation were higher i n proteoid roots compared to normal roots, particularly under P stress. Western blot analysis showed that PEPC enzyme protein was more highly expressed in -P proteoid roots compared to other tissues. The majority of the fixed 14C was found in organic acids, predominantly malate and citrate. A larger fraction of citrate was labeled in Pstressed proteoid roots compared to other root tissue. Respiration rates of proteoid roots were 31% less than those of normal roots. The data provide evidence for increased synthesis of citrate in proteoid roots compared to normal roots, particularly under P stress. A portion of the carbon for citrate synthesis is derived from nonautotrophic COz fixation via PEPC in proteoid roots.

The nutritional control of root development

“…Some species (e.g., white lupin, L. albus) are less sensitive to the presence of P in the soil and will produce some cluster roots even at P levels found in fertile agricultural soils (Keerthisinghe et al, 1998). When the ability of other nutrient stresses to stimulate cluster root formation in white lupin was tested, it was found that a -Mn treatment had a small stimulatory effect, while -N, -K and -Fe had none (Johnson et al, 1994).…”

Mycorrhizal Fungi Increase Chromium Uptake by Sunflower Plants: Influence on Tissue Mineral Concentration, Growth, and Gas Exchange

“…[24,40] Iron-and P-deficiency induce accumulation of organic acids (i.e. citric acid), [41] which may play an important role in Cr translocation. However, while Cr(III) Non-AM sunflower plants had marginal P concentrations, there was no difference in tissue Cr concentration and total Cr accumulation (roots, leaves, total plant) was less than in AM plants.…”

Developmental Physiology of Cluster-Root Carboxylate Synthesis and Exudation in Harsh Hakea. Expression of Phosphoenolpyruvate Carboxylase and the Alternative Oxidase

“…citrate, malate; Johnson et al, 1996aJohnson et al, , 1996bWatt and Evans, 1999a;Ryan et al, 2001). The role of PEPC is to replenish TCA intermediates by catalyzing the carboxylation of phosphoenolpyruvate to form oxaloacetate (Johnson et al, 1994). Johnson et al (1994) found that PEPC-mediated nonphotosynthetic CO 2 fixation provided a quarter of the carbon exuded as citrate from cluster roots of L. albus.…”

“…The evidence to support metabolic specialization comes predominately from studies with L. albus (Johnson et al, 1994(Johnson et al, , 1996a(Johnson et al, , 1996bKeerthisinghe et al, 1998;Neumann and Rö mheld, 1999;Neumann et al, 1999;Evans, 1999a, 1999b), and is based on three sets of observations: (1) genes involved in citrate synthesis are up-regulated when citrate exudation occurs from cluster roots (e.g. phosphoenolpyruvate carboxylase [PEPC]; Neumann et al, 1999;Kania et al, 2003;Uhde-Stone et al, 2003); (2) genes involved in citrate catabolism are down-regulated when citrate exudation occurs from cluster roots (e.g.…”

“…These short-lived structures have been termed proteoid roots because they were first described for Proteaceae (Purnell, 1960) but have since been found in a wide range of other species and families and are now often referred to as cluster roots . Most of our advances in cluster-root biology have been derived from studies of the crop species Lupinus albus (Fabaceae family) (Gardner et al, 1983;Johnson et al, 1994Johnson et al, , 1996aJohnson et al, , 1996bKeerthisinghe et al, 1998;Neumann et al, 1999;Evans, 1999a, 1999b). The importance of developing short-lived cluster roots, in terms of nutrient acquisition from the rhizosphere, is illustrated by the tremendous increase in fine-root surface area following cluster-root initiation (Lamont, 1972;Dell and Kuo, 1980).…”