Legumes have the unique ability to fix atmospheric nitrogen (N 2 ) via symbiotic bacteria in their nodules but depend heavily on phosphorus (P), which affects nodulation, and the carbon costs and energy costs of N 2 fixation. Consequently, legumes growing in nutrient-poor ecosystems (e.g. sandstone derived soils) have to enhance P recycling and/ or acquisition in order to maintain N 2 fixation. In this study, we investigated the flexibility of P recycling and distribution within the nodules and their effect on N nutrition in Virgilia divaricata Adamson, Fabaceae, an indigenous legume in the Cape Floristic Region of South Africa. Specifically, we assessed tissue elemental localization using micro-particle induced x-ray emission 1 (PIXE), measured N fixation using nutrient concentrations derived from inductively coupled mass spectrometry (ICP-MS), calculated nutrient costs, and determined P recycling from enzyme activity assays. Morphological and physiological features characteristic of adaptation to P-deprivation were observed for V. divaricata. Decreased plant growth and nodule production with parallel increased root: shoot ratios are some of the plastic features exhibited in response to P deficiency. Plants resupplied with P resembled those supplied with optimal P levels in terms of growth and nutrient acquisition. Under low P conditions, plants maintained an increase in N 2 -fixing efficiency despite lower levels of orthophosphate (Pi) in the nodules.This can be attributed to two factors: i) an increase in Fe concentration under low P, and ii) greater APase activity in both the roots and nodules under low P. These findings suggest that V. divaricata is well-adapted to acquire N under P deficiency, owing to the plasticity of its nodule physiology.
Virgilia divaricata is an indigenous forest margin legume growing in nutrient richer soils, but it is also known to invade the N and P poorer soils of the mature fynbos.This implies that the legume has a functional tolerance for variable soil N and P levels. It is not known how the legume utilizes inorganic N from soil and atmospheric sources under variable P supply. Moreover, very little is known about how P deficiency affects root nodule metabolic functioning of V. divaricata and their associated energy costs of N assimilation. Therefore the aim of this study was to determine whether the P deficiency affects the metabolic status of root and nodules and the consequent impact on the routes of N assimilation in a Fynbos legume, V. divaricata. Our results show that V. divaricata had a reduced biomass, plant P concentration and BNF during P deficiency. Based on the adenylates data, P stressed nodules maintained their P status better than P stressed roots. Furthermore V. divaricata was able to alter C and N metabolism in different ways in roots and nodules, in response to P stress. For both roots and nodules, this was achieved via internal cycling of P, by possible replacement of membrane phospholipids with sulpholipids and galactolipids and increased reliance on the PPi-dependant metabolism of sucrose via UDPG and to Fru-6-P. P stressed roots exported mostly ureides as organic N and recycled amino acids via deamination glutamate dehydrogenase (GDH). In contrast, P stressed nodules largely exported amino acids.Compared to roots, the nodules showed a greater degree of P conservation during low P supply, this resulted in the roots and nodules of V. divaricata, metabolising N differently during P stress, this meaning that these organs may contribute differently to the success of this plant in soils ranging from forest to fynbos.
Nitrogen fixing legumes rely on phosphorus for nodule formation, nodule function and the energy costs of fixation. Phosphorus is however very limited in soils, especially in ancient sandstone-derived soils such as those in the Cape Floristic Region of South Africa. Plants growing in such areas have evolved the ability to tolerate phosphorus stress by eliciting an array of physiological and biochemical responses. In this study we investigated the effects of phosphorus limitation on N 2 fixation and phosphorus recycling in the nodules of Virgilia divaricata (Adamson), a legume native to the Cape Floristic Region. In particular, we focused on nutrient acquisition efficiencies, phosphorus fractions and the exudation and accumulation of phosphatases. Our finding indicate that during low phosphorus supply, V. divaricata internally recycles phosphorus and has a lower uptake rate of phosphorus, as well as lower levels adenylates but greater levels of phosphohydrolase exudation suggesting it engages in recycling internal nodule phosphorus pools and making use of alternate bypass routes in order to conserve phosphorus.
In phosphorus (P)-poor ecosystems, microbial communities can play a major role in the nitrogen (N) mineral nutrition during N2 fixation in legumes. This study investigated the role of P nutrition on the composition of N2-fixing bacterial community in Virgilia divaricata root nodules, grown under glasshouse conditions. V. divaricata seeds were germinated in Fynbos soil as a natural inoculum, and, thereafter, transferred into sterile quartz-sand cultures and supplied with 500 µM P and 5 µM P, respectively. The N2-fixing bacterial communities in the rhizosphere and root nodules were examined on the basis of the polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE) banding patterns of 16S rDNA and sequencing methods. The GenBank blast results showed that V. divaricata was nodulated by a wide range of root-nodule bacterial strains also found in the rhizosphere. These included Burkholderia phytofirmans, Burkholderia sp. and Bradyrhizobium sp., during both high and low P supply. The 15N natural-abundance data also confirmed that 40–50% of the N nutrition was from symbiotic N2 fixation. This is not only evidence of nodulation, but an indication of the adaptation of a range of N2-fixing bacterial strain species to the nutrient-poor, sandy, acidic soil of the Mediterranean-type ecosystems of the fynbos vegetation in the Cape Floristic Region (CFR). Legume species V. divaricata is highly adapted to the low-nutrient soils of its native range by its association with the symbiotic N2-fixing bacteria.
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