White lupin has developed a complex strategy to limit microbial degradation of secreted citrate required for phosphate acquisition

White lupins (Lupinus albus L.) respond to phosphate deficiency by producing special root structures called cluster roots. These cluster roots secrete large amounts of carboxylates into the rhizosphere, mostly citrate and malate, which act as phosphate solubilizers and enable the plant to grow in soils with sparingly available phosphate. The success and efficiency of such a P-acquisition strategy strongly depends on the persistence and stability of the carboxylates in the soil, a parameter that is influenced to a large extent by biodegradation through rhizosphere bacteria and fungi. In this study, we show that white lupin roots use several mechanisms to reduce microbial growth. The abundance of bacteria associated with cluster roots was decreased at the mature state of the cluster roots, where a burst of organic acid excretion and a drastic pH decrease is observed. Excretion of phenolic compounds, mainly isoflavonoids, induced fungal sporulation, indicating that vegetative growth, and thus potential citrate consumption, is reduced. In addition, the activity of two antifungal cell wall-degrading enzymes, chitinase and glucanase, were highest at the stage preceding the citrate excretion. Therefore, our results suggest that white lupin has developed a complex strategy to reduce microbial degradation of the phosphate-solubilizing agents.     

Routes of pyruvate synthesis in phosphorus-deficient lupin roots and nodules

Here, nodulated lupins (Lupinus angustifolius (cv Wonga)) were hydroponically grown at low phosphate (LP) or adequate phosphate (HP). Routes of pyruvate synthesis were assessed in phosphorus (P)-starved roots and nodules, because P-starvation can enhance metabolism of phosphoenolpyruvate (PEP) via the nonadenylate-requiring PEP carboxylase (PEPc) route. Since nodules and roots may not experience the same degree of P stress, it was postulated that decreases in metabolic inorganic phosphorus (Pi) of either organ, should favour more pyruvate being synthesized from PEPc-derived malate. Compared with HP roots, the LP roots had a 50% decline in Pi concentrations and 55% higher ADP : ATP ratios. However, LP nodules maintained constant Pi levels and unchanged ADP : ATP ratios, relative to HP nodules. The LP roots had greater PEP metabolism via PEPc and synthesized more pyruvate from PEPc-derived malate. In nodules, P supply did not influence PEPc activities or levels of malate-derived pyruvate. These results indicate that nodules were more efficient than roots in maintaining optimal metabolic Pi and adenylate levels during LP supply. This caused an increase in PEPc-derived pyruvate synthesis in LP roots, but not in LP nodules.     

Yellow lupin cytoplasmic tRNAGlu is not a cofactor in chlorophyll biosynthesis

Yellow lupin seeds (Lupinus luteus) cytoplasmic tRNA^Glu was isolated and the primary structure was determined to be: pUCCGUUGUAGUCAGDDGGDCAGGAUAUUCGGCUCUCACCCGAAAGACm⁵CCGGGTCAm¹ AGU CCCGGCGACGGAACCA_OH. It is 76 nucleotides long and contains 8 modified nucleosides: 2 residues of pseudouridine, ribothymidine, 3 dihydrouridines, 5-methylcytosine and 1-methyladenosine. This tRNA^Glu assayed in -aminolevulinic acid synthesis was shown to be inactive. Its structural festures are discussed.     

An analysis of the repair processes in ultraviolet-irradiated Micrococcus luteus using purified ultraviolet-endonuclease

The measurement of the frequency of endonucleolytic incisions in ultraviolet-irradiated DNA serves as the test for the presence of pyrimidine dimers. In accordance with this approach, the lysates of three Micrococcus luteus strains containing radioactively labeled chromosomes were treated with purified M. luteus ultraviolet-endonuclease to trace segregation of dimers amongst parental and newly synthesized DNA and their removal during postreplication and excision DNA repair. A considerable proportion of the dimers in all strains tested proved to be insensitive to the action of exogenous incising enzyme. The use of chloramphenicol as an inhibitor of postirradiation protein synthesis in combination with ultraviolet-endonuclease treatment of DNA allowed to reveal at least two alternative pathways of postreplication repair: constitutively active recombinational pathway and inducible nonrecombinational one.     

Peroxidase and not laccase is the enzyme responsible for cell wall lignification in the secondary thickening of xylem vessels inLupinus

Summary The post-exponential growth phase of lupin (Lupinus albus cv. Multolupa) hypocotyls is characterized by a strong deposition of lignins in the primary and secondary walls of the xylem vessels. Coinciding with this phenomenon, there is a clearly peroxidatic activity in both the primary cell walls and the outer-most layers of the secondary thickening of the xylem vessels, as demonstrated by 3,3-diaminobenzidine cytochemistry. This activity was completely inhibited by KCN and the removal of H₂O₂ and was not due to laccase since this enzyme shows an almost total inability to oxidize 3,3-diaminobenzidine both in the presence and in the absence of H₂O₂. The absence of laccase-like activities in cell walls of vascular cells was supported by the fact that cell wall proteins from vascular cells were only capable of oxidizing 3,3-diaminobenzidine and coniferyl alcohol in the presence of H₂O₂. These results support the idea of an exclusive role of peroxidase (and exclude any role for laccase) in lignin formation in the secondary thickening of xylem vessels inLupinus.     

pH above 6.0 reduces nodulation in Lupinus species

Lupinus angustifolius and Lupinus pilosus differ substantially in root growth in response to high solution pH with L. angustifolius showing much greater sensitivity to pH above 6. This study examined the effect of pH above 6 on nodulation of these two species in buffered solution. Shoot weight and root weight and length in L. pilosus was not significantly affected by pH, whereas the growth of shoots and roots of L. angustifolius was markedly impaired by increasing pH. Total root length, the number of lateral roots, and the length of individual lateral roots were greatly decreased, resulting in decreased uptake of iron and phosphorus. In addition, L. angustifolius had a higher internal requirement for iron than L. pilosus. A solution pH above 6 decreased the number of nodule initials and nodules similarly in both species but decreased nodule mass much more in L. angustifolius. The effect of high pH on nodule formation occurred prior to that on host shoot growth. High pH also decreased nitrogen concentration and content in both species but to a greater extent for L. angustifolius. The results suggested that pH above 6 has a specific effect in the impairment of nodulation in lupins.     

The use of inositol hexaphosphate as a phosphorus source by mycorrhizal and non-mycorrhizal Scots Pine (Pinus sylvestris)

1. The external mycelia of the ectomycorrhizal fungi Thelephora terrestris and Suillus luteus , associated with Pinus sylvestris roots, exhibited a substantial extracellular acid phosphatase activity. The activity was positively correlated with the ergosterol concentration in the growth substratum and decreased with an increasing P nutrition.
2. The pioneer species T. terrestris grew best at a high P_i nutrition level whereas S. luteus , a 'late-stage' mycobiont, produced more active biomass at a low P_i nutrition level.
3. The phytase activity of the external mycelia could not be detected; at the root surface a phytase activity was observed. Mycorrhizas had significantly higher activities than uninfected roots.
4. The addition of a relatively high concentration of a soluble phytate to the growth substratum resulted in an increased relative growth rate (RGR) in both mycorrhizal and non-mycorrhizal plants. The influence of the mycorrhizal fungi on the use of the phytate-P was small, despite the phytase activity of the mycorrhizal feeder roots.
5. The addition of phytate fixed on a HPLC resin did not result in an increase of the RGR and P uptake neither in the non-mycorrhizal nor in the mycorrhizal Pines. The experiment did not support the hypothesis that phytate, which has a low solubility in soils, is a useful P source for ectomycorrhizal plants.     

Selection of balancing ions for nutritional studies in nutrient culture experiments

Nutrient culture studies frequently involve the use of balancing ions to equalize concentrations of essential nutrient elements. In a pot experiment in controlled environment with Lupinus angustifolius, growth and nodulation were assessed following calcium treatment (15 mM) using the acetate, chloride and sulphate salts in various combinations. Chloride depressed nodulation at levels higher than 20 mM; nodule mass and number were highest at the maximum sulphate concentration (13 mM). At the lowest sulphate level (2 mM), nodulation and root growth were depressed by 4 mM or higher acetate. Nodulation (dry weight and numbers of nodules) was maximized at 13 mM sulphate/4 mM chloride.     

Effects of tall fescue turf on growth and nitrogen fixation potential of the woody legume Lupinus albifrons

The effect of tall fescue turf on growth, flowering, nodulation, and nitrogen fixing potential of Lupinus albifrons Benth. was examined for greenhouse and field grown plants. No allelopathic effect was observed for lupine plants treated with tall fescue leachates. The nitrogen-fixing potential measured by nodule dry weight and acetylene reduction rates was not significantly affected by tall fescue turf.Both the greenhouse and field studies showed that the growth, sexual reproductive allocation and number of inflorescences were significantly reduced when lupine plants were grown with tall fescue. The root-length densities of tall fescue turf and lupine monoculture were measured. The tall fescue turf had 20 times higher root-length density (20 cm cm^-3 soil) than the lupine plant monoculture. This suggests that intense competition at the root zone may be a dominant factor which limits the growth of the lupine plants.The flowering characters of the lupine plants were improved by phosphorus fertilization. Transplanting of older lupine plants into the turf substantially alleviated the tall fescue turf competitive effect.