Physiological Aspects of Cluster Root Function and Development in Phosphorus-deficient White Lupin (Lupinus albus L.)

Cluster root formation in white lupin (Lupinus albus L.) isinduced mainly by phosphorus (P) starvation, and seems to beregulated by the endogenous P status of the plant. Increasedformation of cluster roots, when indole acetic acid is suppliedto the growth medium of P sufficient plants, and inhibitoryeffects of kinetin application suggest the involvement of endogenousphytohormones (auxins and cytokinins), which may act in an antagonisticmanner in the P-starvation response. Phosphorus deficiency-inducedadaptations of white lupin, involved in P acquisition and mobilizationof sparingly available P sources, are predominantly confinedto the cluster roots, and moreover to distinct stages duringtheir development. Increased accumulation and exudation of citrateand a concomitant release of protons were found to be mainlyrestricted to mature root clusters after prolonged culture (3–4weeks) under P-deficient conditions. Inhibition of citrate exudationby exogenous application of anion channel antagonists such asethacrynic- and anthracene-9-carboxylic acids may indicate involvementof an anion channel. Phosphorus deficiency-induced accumulationand subsequent exudation of citric acid seems to be a consequenceof both enhanced biosynthesis and reduced turnover of citricacid in the cluster root tissue, indicated by enhanced expressionof sucrose synthase, fructokinase, phosphoglucomutase, phosphoenol-pyruvatecarboxylase, but reduced activity of aconitase and slower rootrespiration. The release of acid phosphatase and of phenoliccompounds (isoflavonoids) as well as the induction of a putativehigh-affinity P uptake system was more highly expressed in juvenile,mature and even senescent cluster regions than in apical zonesof non-proteoid roots. An AFLP-cDNA library for cluster root-specificgene expression was constructed to assist in the identificationof further genes involved in cluster root development. Copyright2000 Annals of Botany Company Acid phosphatase, auxin, citric acid, cluster roots, cytokinin, Lupinus albus L., P acquisition, P uptake, root exudates     

Spatial and temporal variation in citrate and malate exudation and tissue concentration as affected by P stress in roots of white lupin

Exudation of carboxylates represents one the most efficient strategies used by P-starved white lupin (Lupinus albus L.) to acquire phosphorus from sparingly soluble sources. This exudation occurs through proteoid root clusters, with citrate being the predominant organic acid released. The occasional detection of malate in whole root exudates suggests that this acid would also be released, but from tissues other than root clusters. To investigate the spatial and temporal pattern of exudation, citrate and malate exudation and concentration were measured in whole roots and root sections of white lupin, from seedling emergence to plant senescence due to P starvation. Both organic acids were detected in whole root exudates of P-stressed plants, and they were released at similar rates throughout the experiment. Malate was predominantly exuded from apices of both seedling taproots and proteoid roots, whereas citrate exudation was restricted to proteoid root clusters. Studies directed to address the association between carboxylate exudation and concentration in proteoid root clusters showed a non-linear response for citrate, within the range of 7 to 23 mol g^–1 fresh weight. This association was further assessed by altering citrate concentration in the whole root. Adding P to 24-day-old P-starved plants reduced citrate concentration and exudation to the level of the control P-fed plants, demonstrating that citrate exudation and concentration are associated. Malate exudation and concentration did not correlate significantly. Results indicate that citrate release by P-starved white lupin would occur whenever a certain threshold of citrate concentration is attained, and that the sites, the rates and the span of transient exudation depend on the physiological age of the tissue.     

Differences in cluster-root formation and carboxylate exudation in Lupinus albusL. under different nutrient deficiencies

In contrast with the well document role of proteoid root formation and carboxylate exudation in acclimation to P deficiency in white lupin (Lupinus albus L.), their role under other nutrient deficiencies and their ecological significance are still poorly understood. In the present work, differences in proteoid root formation, exudation of carboxylates by root clusters, non-proteoid and proteoid root tips by using a non-destructive method, and concentrations of organic acids in the tissues of plants grown in the absence of P, Fe or K were studied. Proton release from roots increased soon after withdrawing Fe from the medium; within three days the solution pH decreased from 6 to about 4, and this increased release in protons continued until the end of the experiment. Acidification appeared much later, on the 10th day and the 14th day after withdrawal of P and K, respectively; the extent of the acidification was also weaker than under –Fe (5.2 for –P and 5.7 for control on the 10th day; 6.0 for –K and 6.1 for control on the 14th day). Root clusters formed when plants were grown under –P and –Fe, but not under –K conditions. The root clusters developed sooner under –Fe conditions, but the number of clusters was far less than under –P. Under P deficiency, root clusters released mainly citrate, but also some malate; while the major organic acid released by root tips of both non-proteoid and proteoid roots was malate. However, under Fe deficiency, the majority of the organic acids exuded both by the root clusters and root tips was malate, whereas only a small amount of citrate was detected. The release rate of citrate by – P root clusters was greater than that by – Fe root clusters. Moreover, the release rate of malate was greater in –Fe root clusters than in –P root clusters, but the opposite was found in proteoid root tips, i.e. faster in –P than in –Fe proteoid root tips. The significances of proteoid root formation and release of organic acids in acclimation to different nutrient deficiencies for white lupin plants are discussed.     

Structure,ecology and physiology of root clusters – a review

Hairy rootlets, aggregated in longitudinal rows to form distinct clusters, are a major part of the root system in some species. These root clusters are almost universal (1600 species) in the family Proteaceae (proteoid roots), with fewer species in another seven families. There may be 10–1000 rootlets per cm length of parent root in 2–7 rows. Proteoid roots may increase the surface area by over 140× and soil volume explored by 300× that per length of an equivalent non-proteoid root. This greatly enhances exudation of carboxylates, phenolics and water, solubilisation of mineral and organic nutrients and uptake of inorganic nutrients, amino acids and water per unit root mass. Root cluster production peaks at soil nutrient levels (P, N, Fe) suboptimal for growth of the rest of the root system, and may cease when shoot mass peaks. As with other root types, root cluster production is controlled by the interplay between external and internal nutrient levels, and mediated by auxin and other hormones to which the process is particularly sensitive. Proteoid roots are concentrated in the humus-rich surface soil horizons, by 800× in Banksia scrub-heath. Compared with an equal mass of the B horizon, the A₁ horizon has much higher levels of N, P, K and Ca in soils where species with proteoid root clusters are prominent, and the concentration of root clusters in that region ensures that uptake is optimal where supply is maximal. Both proteoid and non-proteoid root growth are promoted wherever the humus-rich layer is located in the soil profile, with 4× more proteoid roots per root length in Hakea laurina. Proteoid root production near the soil surface is favoured among hakeas, even in uniform soil, but to a lesser extent, while addition of dilute N or P solutions in split-root system studies promotes non-proteoid, but inhibits proteoid, root production. Local or seasonal applications of water to hakeas initiate non-proteoid, then proteoid, root production, while waterlogging inhibits non-proteoid, but promotes proteoid, root production near the soil surface. A chemical stimulus, probably of bacterial origin, may be associated with root cluster initiation, but most experiments have alternative interpretations. It is possible that the bacterial component of soil pockets rich in organic matter, rather than their nutrient component, could be responsible for the proliferation of proteoid roots there, but much more research on root cluster microbiology is needed.     

Acclimation of white lupin to phosphorus deficiency involves enhanced expression of genes related to organic acid metabolism

White lupin (Lupinus albus L.) acclimates to phosphorus deficiency (–P) by the development of short, densely clustered lateral roots called proteoid (or cluster) roots. These specialized plant organs display increased exudation of citric and malic acid. The enhanced exudation of organic acids from P stressed white lupin roots is accompanied by increased in vitro phosphoenolpyruvate carboxylase (PEPC) and malate dehydrogenase (MDH) activity. Here we report the cloning of full-length white lupin PEPC and MDH cDNAs. RNA blot analysis indicates enhanced expression of these genes in –P proteoid roots, placing higher gene expression at the site of organic acid exudation. Correspondingly, macroarray analysis of about 1250 ESTs (expressed sequence tags) revealed induced expression of genes involved in organic acid metabolism in –P proteoid roots. In situ hybridization revealed that PEPC and MDH were both expressed in the cortex of emerging and mature proteoid rootlets. A C₃ PEPC protein was partially purified from proteoid roots of P deficient white lupin. Native and subunit Mr were determined to be 440 kD and 110 kD, respectively. Citrate and malate were effective inhibitors of in vitro PEPC activity at pH 7. Addition of ATP partially relieved inhibition of PEPC by malate but had little effect on citrate inhibition. Taken together, the results presented here suggest that acclimation of white lupin to low P involves modified expression of plant genes involved in carbon metabolism.     

Acid phosphatase activity in phosphorus-deficient white lupin roots

White lupin ( Lupinus albus L.) develops proteoid roots when grown in phosphorus (P)-deficient conditions. These short, lateral, densely clustered roots are adapted to increase P availability. Previous studies from our laboratory have shown proteoid roots have higher rates of non-photosynthetic carbon fixation than normal roots and altered metabolism to support organic acid exudation, which serves to solubilize P in the rhizosphere. The present work indicates that proteoid roots possess additional adaptations for increasing P availability and possibly for conserving P in the plant. Roots from P-deficient (–P) plants had significantly greater acid phosphatase activity in both root extracts and root exudates than comparable samples from P-sufficient (+P) plants beginning 10 d after emergence. The increase in activity in –P plants was most pronounced in the proteoid regions. In contrast, no induction of phytase activity was found in –P plants compared to +P plants. The number of proteoid roots present was not affected by the source of phosphorus supplied, whether organic or inorganic forms. Adding molybdate to the roots increased the number of proteoid roots in plants supplied with organic P, but not inorganic P. Increased acid phosphatase activity was detected in root exudates in the presence of organic P sources. Native-polyacrylamide gel electrophoresis demonstrated that under P-deficient conditions, a unique isoform of acid phosphatase was induced between 10 and 12 d after emergence. This isoform was found not only within the root, but it comprised the major form exuded from proteoid roots of –P plants. The fact that exudation of proteoid-root-specific acid phosphatase coincides with proteoid root development and increased exudation of organic acids indicates that white lupin has several coordinated adaptive strategies to P-deficient conditions.     

子叶磷在白羽扇豆缺磷适应性反应中的作用

实验用液体培养的方法,对比分析了在不同供磷条件下,白羽扇豆子叶中的磷对植物生长发育的影响,以及排根和根尖中有机酸积累和分泌的作用,结果表明,子叶中的磷能使白羽扇豆在完全缺磷２３d的环境中,不仅没有使干物质的积累减少,反而使干物质的积累略有增加,相反,如果没有子叶磷的供给,则使白羽扇豆在缺磷环境中产生强烈的抗胁迫反应,表现在干物质的积累明显下降,根系能产生大量的排根,排根能积累和分泌大量的柠檬酸,而根尖能积累和分泌萍果酸,在整个缺磷反应过程中,根尖中苹果酸的分泌要早于排根可柠檬酸的积累和分泌。     

Phosphorus deficiency-induced modifications in citrate catabolism and in cytosolic pH as related to citrate exudation in cluster roots of white lupin

A possible contribution of alterations in metabolic sequences involved in citrate catabolism, to intracellular accumulation and subsequent release of citrate was investigated in cluster roots of phosphorus (P)-deficient white lupin (Lupinus albus L.). Citrate accumulation during maturation of root clusters was associated with decreased levels of intracellular soluble P_i and ATP, and with reduced rates of respiration. Inhibitor studies with KCN and salicylhydroxamic acid (SHAM) suggest a reduced capacity of both the cytochrome pathway and of the alternative respiration with a concomitant decrease of immunochemically detectable protein levels of the alternative oxidase. Reduced respiration seems to be related to a general impairment of the respiratory system, rather than to limitation of respiratory substrates such as P_i and adenylates, as indicated by the absence of stimulatory effects of the uncoupler CCCP. The citrate/malate ratio in juvenile root clusters with high rates of respiration and low inherent levels of citrate accumulation was increased by short-term application (4–8 h) of azide and SHAM as respiration inhibitors. During maturation of root clusters, a shift from intracellular malic acid to citric acid accumulation was associated also with down-regulation of ATP citrate lyase (ACL), which catalyzes cleavage of citrate into acetyl-CoA and oxaloacetate with a putative function as anapleurotic source for the production of acetyl-CoA under P-deficient conditions. Inhibition of nitrate uptake and assimilation is a general response to P limitation in many plant species including white lupin. Reduced consumption of the amino acceptor 2-oxoglutaric acid as a product of citrate turnover may therefore contribute to increased citrate accumulation. Accordingly, artificial inhibition of nitrate reduction by localized application of tungstate significantly increased the citrate/malate ratio in juvenile root clusters. Lowering the cytosolic pH by external application of propionate stimulated citrate and malate exudation in non-cluster lateral roots and in developing root clusters. This effect was reverted by preincubation with phosphonate to buffer the cytosol. The results suggest that acidification of the cytosol may be an important factor, triggering the transient release of citrate and protons from mature root clusters in P-deficient white lupin.     

The relationship between silicon availability,and growth and silicon concentration of the salt marsh halophyte Spartina anglica

Incorporation of cover crops into cropping systems may contribute to a more efficient utilization of soil and fertilizer P by less P-efficient crops through exudation of P-mobilizing compounds by the roots of P-efficient plant species. The main objective of the present work was to test this hypothesis. First a method has been developed which allows the quantification of organic anion exudation from individual cluster roots formed by P-deficient white lupin (Lupinus albus L.). Lupin plants were grown in nutrient solution at 1 μM P and in a low P loess in small rhizotrons. Organic anions exuded from intact plants grown in nutrient solution were collected from individual cluster roots and root tips sealed in small compartments by an anion-exchange resin placed in nylon bags (resin-bags). Succinate was the dominant organic anion exuded followed by citrate and malate. The mean of citrate exudation-rate was 0.06 pmol mm⁻¹ s⁻¹ with exudation highly dependent on the citrate concentration and on the age of the cluster roots. Exudates from cluster roots and root tips grown at the soil surface (rhizotron-grown plants) were collected using overlayered resin–agar (resin mixed with agar). Citrate exudation from cluster roots was 10 times higher than that from root tips. Fractionation of P in the cluster root rhizosphere-soil indicates that white lupin can mobilize P not only from the available and acid-soluble P, but also from the stable residual soil P fractions. In pot experiments with an acid luvisol derived from loess low in available P, growth of wheat was significantly improved when mixed-cropped with white lupin due to improved P uptake. Both in mixed culture and in rotation wheat could benefit from the P mobilization capacity of white lupin, supporting the hypothesis above. Nine tropical leguminous cover crops and maize were grown in a pot experiment using a luvisol from Northern Nigeria low in available P. All plant species derived most of their P from the resin and bicarbonate-extractable inorganic P. Organic P (P_o) accumulated particularly in the rhizosphere of all plant species. There was a significant negative correlation between the species-specific rhizosphere acid phosphatase activity and P_o accumulation. Growth and P uptake of maize grown in rotation after legumes were enhanced indicating that improved P nutrition was a contributing factor. This revised version was published online in June 2006 with corrections to the Cover Date.     

Proteoid Root Development of Phosphorus Deficient Lupin is Mimicked by Auxin and Phosphonate

White lupin (Lupinus albus L.) develops proteoid (cluster) rootsin response to phosphorus deficiency. Proteoid roots are composedof tight clusters of rootlets that initiate from the pericycleopposite protoxylem poles and emerge from every protoxylem polewithin the proteoid root axis. Auxins are required for lateralroot development, but little is known of their role in proteoidroot formation. Proteoid root numbers were dramatically increasedin P-sufficient (+P) plants by application of the syntheticauxin, naphthalene acetic acid (NAA), to leaves, and were reducedin P-deficient (-P) plants by the presence of auxin transportinhibitors [2,3,5-triiodobenzoic acid (TIBA) and naphthylphthalamicacid (NPA)]. While ethylene concentrations in the root zonewere 1.5-fold higher in -P plants, there was no effect on proteoidroot numbers of the ethylene inhibitors aminoethoxyvinvylglycine(AVG) and silver thiosulphate. Phosphonate, which interfereswith plant perception of internal P concentration, dramaticallyincreased the number of proteoid root segments in +P plants.Activities of phosphoenolpyruvate carboxylase (PEPC), malatedehydrogenase (MDH) and exuded acid phosphatase in proteoidroot segments were not different from +P controls when NAA wasapplied to +P lupin plants, but increased to levels comparableto -P plants in the phosphonate treatment. Addition of TIBAor NPA to -P plants reduced PEPC and MDH activity of -P proteoidroots to levels found in +P or -P normal root tissues, but didnot affect acid phosphatase in root exudates. These resultssuggest that auxin transport from the shoot plays a role inthe formation of proteoid roots during P deficiency. Auxin-stimulatedproteoid root formation is necessary, but not sufficient, tosignal the up-regulation of PEPC and MDH in proteoid root segments.In contrast, phosphonate applied to P-sufficient white lupinelicits the full suite of coordinated responses to P deficiencyCopyright2000 Annals of Botany Company Lupinus albus L., white lupin, proteoid roots, auxin, ethylene, phosphonate, phosphorus deficiency     

Adaptation of H+-pumping and plasma membrane H+ ATPase activity in proteoid roots of white lupin under phosphate deficiency

White lupin (Lupinus albus) is able to adapt to phosphorus deficiency by producing proteoid roots that release a huge amount of organic acids, resulting in mobilization of sparingly soluble soil phosphate in rhizosphere. The mechanisms responsible for the release of organic acids by proteoid root cells, especially the trans-membrane transport processes, have not been elucidated. Because of high cytosolic pH, the release of undissociated organic acids is not probable. In the present study, we focused on H+ export by plasma membrane H+ ATPase in active proteoid roots. In vivo, rhizosphere acidification of active proteoid roots was vanadate sensitive. Plasma membranes were isolated from proteoid roots and lateral roots from P-deficient and -sufficient plants. In vitro, in comparison with two types of lateral roots and proteoid roots of P-sufficient plants, the following increase of the various parameters was induced in active proteoid roots of P-deficient plants: (a) hydrolytic ATPase activity, (b) Vmax and Km, (c) H+ ATPase enzyme concentration of plasma membrane, (d) H+-pumping activity, (e) pH gradient across the membrane of plasmalemma vesicles, and (f) passive H+ permeability of plasma membrane. In addition, lower vanadate sensitivity and more acidic pH optimum were determined for plasma membrane ATPase of active proteoid roots. Our data support the hypothesis that in active proteoid root cells, H+ and organic anions are exported separately, and that modification of plasma membrane H+ ATPase is essential for enhanced rhizosphere acidification by active proteoid roots.     

Root excretion of carboxylic acids and protons in phosphorus-deficient plants

Phosphorus deficiency-induced metabolic changes related to exudation of carboxylic acids and protons were compared in roots of wheat (Triticum aestivum L. cv Haro), tomato (Lycopersicon esculentum L., cv. Moneymaker), chickpea (Cicer arietinum) and white lupin (Lupinus albus L. cv. Amiga), grown in a hydroponic culture system. P deficiency strongly increased the net release of protons from roots of tomato, chickpea and white lupin, but only small effects were observed in wheat. Release of protons coincided with increased exudation of carboxylic acids in roots of chickpea and white lupin, but not in those of tomato and wheat. P deficiency-induced exudation of carboxylic acids in chickpea and white lupin was associated with a larger increase of carboxylic acid concentrations in the roots and lower accumulation of carboxylates in the shoot tissue compared to that in wheat and tomato. - Citric acid was one of the major organic acids accumulated in the roots of all investigated species in response to P deficiency, and this was associated with increased activity and enzyme protein levels of PEP carboxylase, which is required for biosynthesis of citrate. Accumulation of citric acid was most pronounced in the roots of P-deficient white lupin, chickpea and tomato. Increased PEP carboxylase activity in the roots of these plants coincided with decreased activity of aconitase, which is involved in the breakdown of citric acid in the TCA cycle. In the roots of P-deficient wheat plants, however, the activities of both PEP carboxylase and aconitase were enhanced, which was associated with little accumulation of citric acid. The results suggest that P deficiency-induced exudation of carboxylic acids depends on the ability to accumulate carboxylic acids in the root tissue, which in turn is determined by biosynthesis, degradation and partitioning of carboxylic acids or related precursors between roots and shoot. In some plant species such as white lupin, there are indications for a specific transport mechanism (anion channel), involved in root exudation of extraordinary high amounts of citric acid. This revised version was published online in June 2006 with corrections to the Cover Date.     

Interactions between atmospheric CO2 concentration and phosphorus nutrition on the formation of proteoid roots in white lupin (Lupinus albus L.)

Atmospheric [CO₂] affects photosynthesis and therefore should affect the supply of carbon to roots. To evaluate interactions between carbon supply and nutrient acquisition, the [CO₂] effects on root growth, proteoid root formation and phosphorus (P) uptake capacity were studied in white lupin (Lupinus albus L.) grown hydroponically at 200, 410 and 750 µmol mol^?1 CO₂, under sufficient (0·25 mm P) and deficient (0·69 µm P) phosphorus. Plant size increased with increasing [CO₂] only at high P. Both P deficiency and increasing [CO₂] increased the production of proteoid clusters; the increase in response to increased [CO₂] was proportionally greater from low to ambient [CO₂] than from ambient to high. The activity of phosphoenol pyruvate carboxylase in the proteoid root, the exudation of organic acids from the roots, and the specific uptake of P increased with P deficiency, but were unaffected by [CO₂]. Increasing [CO₂] from Pleistocene levels to those predicted for the next century increased plant size and allocation to proteoid roots, but did not change the specific P uptake capacity per unit root mass. Hence, rising [CO₂] should promote nutrient uptake by allowing lupins to mine greater volumes of soil.     

LaALMT1 mediates malate release from phosphorus-deficient white lupin root tips and metal root to shoot translocation

Under phosphorus (P) deficiency, Lupinus albus (white lupin) releases large amounts of organic acid anions from specialized root structures, so-called cluster or proteoid roots, to mobilize and acquire sparingly soluble phosphates from a restricted soil volume. The molecular mechanisms underlying this release and its regulation are, however, poorly understood. Here, we identified a gene belonging to the aluminium (Al)-activated malate transporter (ALMT) family that specifically contributes to malate, but not citrate release. This gene, LaALMT1, was most prominently expressed in the root apices under P deficiency, including those of cluster roots and was also detected in the root stele. Contrary to several ALMT homologs in other species, the expression was not stimulated, but moderately repressed by Al. Aluminium-independent malate currents were recorded from the plasma membrane localized LaALMT1 expressed in Xenopus oocytes. In composite lupins with transgenic roots, LaALMT1 was efficiently mutated by CRISPR-Cas9, leading to diminished malate efflux and lower xylem sap malate concentrations. When grown in an alkaline P-deficient soil, mutant shoot phosphate concentrations were similar, but iron and potassium concentrations were diminished in old leaves, suggesting a role for ALMT1 in metal root to shoot translocation, a function that was also supported by growth in hydroponics.     

A link between citrate and proton release by proteoid roots of white lupin (Lupinus albus L.) grown under phosphorus-deficient conditions?

White lupin (Lupinus albus L.) is able to acclimate to phosphorus deficiency by forming proteoid roots that release a large amount of citric acid, resulting in the mobilization of sparingly soluble soil phosphate in the rhizosphere. The mechanisms responsible for the release of organic acids have not been fully elucidated. In this study, we focused on the link between citrate and malate release and the release of H+ and other inorganic ions by proteoid roots of white lupin. The release of citrate was closely correlated with the release of H+, K+, Na+ and Mg2+, but not with that of Ca2+. The stoichiometric relationships between citrate release and the release of H+, K+, Na+ and Mg2+ were 1 : 1.3, 1 : 2.1, 1 : 1.5 and 1 : 0.47, respectively. Similar correlations were found between exudation of malate and cations. During 30 min incubation, fusicoccin addition stimulated H+ and malate release, but not citrate release. A concomitant stimulation of H+, malate and citrate release was measured after 60 min incubation. Vanadate inhibited the release of H+ and malate, but not that of citrate. Anthracene-9-carboxylic acid, an anion channel blocker, caused a concomitant decrease in release of citrate, malate and H+. We conclude that for export of citrate across the plasma membrane of proteoid root cells, H+ release is not strictly related to citrate release. Other cations such as K+ and Na+ can also serve as counterions for citrate release. In contrast, malate release shows a strong H+ release dependency.     

Growth Medium and Phosphorus Supply Affect Cluster Root Formation and Citrate Exudation by Lupinus albus Grown in a Sand/Solution Split-Root System

We examined cluster root formation and root exudation by white lupin (Lupinus albus L. cv. Kiev Mutant) in response to growth medium and phosphorus supply in a sand/solution split-root system. The split-root system consisted of a nutrient solution compartment and a siliceous sand compartment. Phosphorus was applied at 1 (low-P plants) or 50 (high-P plants) μM as KH₂PO₄ to the solution compartment and at 10, 50 or 250 mg P kg⁻¹ as hydroxyapatite (Ca-P) to the sand compartment. In contrast to the high-P plants, P concentration and P uptake in the low-P plants increased with increasing P supply to the sand compartment. The NaHCO₃-extractable P was lower in the rhizosphere of the low-P plants than the high-P ones. The proton extrusion rate by the solution-grown roots of the low-P plants was higher than that of the high-P plants at the early growth stage. For the low-P plants, the proportion of dry root biomass allocated to cluster roots was higher in the solution compartment than that in the sand compartment. The citrate exudation increased in the sand compartment and decreased in the solution compartment with time, showing a lack of synchronization in citrate exudation by two root halves grown in different media. The cluster root proportion and citrate exudation in both compartments decreased with increasing shoot P concentration. An additional experiment with no P added to either root compartment showed that the proportion of cluster roots was about 9% lower in the sand than solution compartments. The results suggest that cluster root formation and citrate exudation can be significantly affected by the root growth medium in addition to being regulated by shoot P status. More P can be exploited from sparingly available Ca-P by the low-P plants than the high-P ones due to greater citrate exudation under P deficiency.     

The Response of Lupinus albus Roots to the Signal from Phosphorus-Deficient Substrate1

The excretion of organic acids into the rhizosphere is induced by low phosphorus content in roots of white lupine (Lupinus albus L.). The aim of this study was to investigate how did the white lupine roots respond to the signals of P-deficiency in the substrate, by using the method of separating the root system into two parts, one part being placed into P-deficient solution and another part into P-sufficient solution. The results showed that the root tips (1-cm-long) accept the signal of P-deficiency in the substrate. This signal can be transmitted from one root tip to another, and can induce the accumulation and excretion of malic acid in the root tips and the development of proteoid roots. In order to investigate whether a phytohormone was involved in the response, the external hormones were used. Indole-3-butyric acid added to P-sufficient solution can induce the development of proteoid roots, but without any accumulation of citric acid in them. On the contrary, 6-benzyladenine (BA) added to the P-deficient solution can inhibit the development of proteoid roots and the accumulation of malic acid in the root tips. The inhibitory effect increases with increasing BA supply.     

Phosphorus acquisition from soil by white lupin (Lupinus albus L.) and soybean (Glycine max L.), species with contrasting root development

White lupin and soybean have contrasting root morphologies: white lupin develops proteoid or cluster roots, roots with discreet clusters of short, determinate branch roots (rootlets) while soybean develops a more fibrous root system with evenly distributed, longer branch roots. Growth and P acquisition by white lupin and soybean were compared in a soil high in bound, total P, with or without additional inorganic P applied in solution. Additional P increased biomass by 25% and doubled total P in soybean. In contrast, white lupin did not respond to additional P in biomass or total P. However added P decreased cluster development on proteoid roots indicating that white lupin sensed the added P. The reduction in cluster weight per plant was exactly countered by an increase in dry weight of other roots. Soybean root development responded to P application, proliferating branch roots with active meristems in the upper portion of the soil profile where P was applied, and reducing root weight to plant weight by 13%. White lupin did not proliferate roots in response to P application. When P was not added to soil, soybean and lupin acquired similar P per unit root dry weight. However, white lupin accumulated 4.8 times more P per unit root length, suggesting that P acquisition in these plants involved other mechanisms such as the exudation of P solubilizing compounds. Soybean accessed P by developing more root length thus colonising more soil volume than white lupin and, therefore, was better able to take advantage of the added P. Pericycle and root tip meristem activities were critical to the differences in root development between white lupin and soybean, and therefore their responses to plant and soil P.     

Higher rates of nitrogen fertilization decrease soil enzyme activities, microbial functional diversity and nitrification capacity in a Chinese polytunnel greenhouse vegetable land

White lupin (Lupinus albus L.) is considered a model system for understanding plant acclimation to nutrient deficiency. It acclimates to phosphorus (P) and iron (Fe) deficiency by the development of short, densely clustered lateral roots called proteoid (or cluster) roots; proteoid-root development is further influenced by nitrogen (N) supply. In an effort to better understand proteoid root function under various nutrient deficiencies, we used nylon filter arrays to analyze 2,102 expressed sequence tags (ESTs) from proteoid roots of P-deficient white lupin. These have been previously analyzed for up-regulation in ?P proteoid roots, and were here analyzed for up-regulation in proteoid roots of N-deprived plants. We identified a total of 19 genes that displayed up-regulation in proteoid roots under both P and N deprivation. One of these genes showed homology to putative formamidases. The corresponding open reading frame was cloned, overexpressed in E. coli, and the encoded protein was purified; functional characterization of the recombinant protein confirmed formamidase activity. Though many homologues of bacterial and fungal formamidases have been identified in plants, to our knowledge, this is the first report of a functional characterization of a plant formamidase.     

缺磷白羽扇豆排根与非排根区根尖分泌有机酸的比较

采用根系分泌有机酸原位收集方法及高效液相色谱技术分析了供磷及缺磷后不同时间白羽扇豆(LupinusalbusL .)非排根区根尖和排根分泌有机酸的种类和数量 ,以及相应的根尖、排根组织 ,茎木质部、韧皮部汁液中有机酸含量的变化。结果表明 :(1)缺磷能够诱导白羽扇豆根系产生大量排根 ,根系的有机酸分泌量也明显增加。 (2 )无论在供磷或缺磷条件下 ,排根与非排根区根尖组织中的有机酸种类相同 ,但排根主要分泌柠檬酸和苹果酸 ,而非排根区根尖主要分泌苹果酸和乙酸。 (3)缺磷后非排根区根尖分泌苹果酸的量增加 ,至第 17天达到高峰 ;排根开始分泌柠檬酸的时间相对较晚。缺磷后排根分泌柠檬酸的量随缺磷时间的延长不断增加。 (4 )在缺磷的排根与非排根区根尖组织和茎木质部伤流液中含有大量柠檬酸和苹果酸 ,但在茎韧皮部汁液中则几乎检测不到这两种有机酸。上述结果表明 ,尽管排根和非排根区根尖组织中的有机酸种类相同 ,但它们向外分泌的有机酸种类不同。缺磷后排根及非排根区根尖增加向外分泌的有机酸主要在根中合成