Biomedical subjects

Michael W Shane

Publications and source records attributed to Michael W Shane.

6 recordsLinked to original sources

PubMed2006

Root structure and functioning for efficient acquisition of phosphorus: Matching morphological and physiological traits.

BACKGROUND: Global phosphorus (P) reserves are being depleted, with half-depletion predicted to occur between 2040 and 2060. Most of the P applied in fertilizers may be sorbed by soil, and not be available for plants lacking specific adaptations. On the severely P-impoverished soils of south-western Australia and the Cape region in South Africa, non-mycorrhizal species exhibit highly effective adaptations to acquire P. A wide range of these non-mycorrhizal species, belonging to two monocotyledonous and eight dicotyledonous families, produce root clusters. Non-mycorrhizal species with root clusters appear to be particularly effective at accessing P when its availability is extremely low. SCOPE: There is a need to develop crops that are highly effective at acquiring inorganic P (Pi) from P-sorbing soils. Traits such as those found in non-mycorrhizal root-cluster-bearing species in Australia, South Africa and other P-impoverished environments are highly desirable for future crops. Root clusters combine a specialized structure with a specialized metabolism. Native species with such traits could be domesticated or crossed with existing crop species. An alternative approach would be to develop future crops with root clusters based on knowledge of the genes involved in development and functioning of root clusters. CONCLUSIONS: Root clusters offer enormous potential for future research of both a fundamental and a strategic nature. New discoveries of the development and functioning of root clusters in both monocotyledonous and dicotyledonous families are essential to produce new crops with superior P-acquisition traits.

Phosphorus↗

PubMed2006

Specialized 'dauciform' roots of Cyperaceae are structurally distinct, but functionally analogous with 'cluster' roots.

When grown in nutrient solutions of extremely low [P] (<or=1.0 microm), the sedge Schoenus unispiculatus Benth. (Cyperaceae) develops dauciform roots, which are short and carrot shaped, and produce dense numbers of long root hairs. It has been suggested that dauciform roots of monocotyledonous sedges function to acquire P from nutrient-poor, P-fixing soils in a manner similar to that of cluster (proteoid) roots developed by some dicotyledonous species, but without evidence to substantiate this claim. To elucidate the ecophysiological role of dauciform roots, we assessed carboxylate exudation, internal carboxylate and P concentrations and O(2) uptake rates during dauciform root development. We showed that O(2) consumption was fastest [9 nmol O(2) g(-1) fresh mass (FM) s(-1)] and root [P] greatest (0.4 mg P g(-1) FM) when dauciform roots were young and rapidly developing. Citrate was the most abundant carboxylate in root tissues at all developmental stages, and was most concentrated (22.2 micromol citrate g(-1) FM) in young dauciform roots, decreasing by more than half in mature dauciform roots. Peak citrate-exudation rates (1.7 nmol citrate g(-1) FM s(-1)) occurred from mature dauciform roots, and were approximately an order of magnitude faster than those from roots of species without root clusters, and similar to those of mature proteoid (cluster) roots of Proteaceae. Both developing and mature dauciform roots had the capacity to acidify (but not alkalinize) the rhizosphere. Anatomical studies showed that epidermal cells in dauciform roots were greatly elongated in the transverse plane; epidermal cells of parent roots were unmodified. Although structurally distinct, the physiology of dauciform roots in sedges appears to be analogous to that of proteoid roots of Proteaceae and Fabaceae, and hence, dauciform roots would facilitate access to sorbed P and micronutrients from soils of low fertility.

Carboxylic Acids↗

PubMed2005

Systemic suppression of cluster-root formation and net P-uptake rates in Grevillea crithmifolia at elevated P supply: a proteacean with resistance for developing symptoms of 'P toxicity'.

Grevillea crithmifolia R. Br. is a species of Proteaceae that is resistant to developing P-toxicity symptoms at phosphorus supplies in the root environment that induce P-toxicity symptoms in the closely related Hakea prostrata (Proteaceae). It was discovered previously that development of P-toxicity symptoms in H. prostrata is related to its low capacity to down-regulate net P-uptake rates (i.e. its low plasticity). The plasticity of net P-uptake rates and whole-plant growth responses in G. crithmifolia has now been assessed in two separate experiments: (i) a range of P, from 0 to 200 micromol P d-1, was supplied to whole root systems; (ii) using a split-root design, one root half was supplied with 0, 3, 75, or 225 micromol P d-1, while the other root half invariably received 3 micromol P d-1. Fresh mass was significantly greater in G. crithmifolia plants that had received a greater daily P supply during the pretreatments, but symptoms of P toxicity were never observed. Cluster-root growth decreased from about half the total root fresh mass when the leaf [P] was lowest (c. 0.1 mg P g-1 DM) to complete suppression of cluster-root growth when leaf [P] was 1-2 mg P g-1 DM. Split-root studies revealed that cluster-root initiation and growth, and net P-uptake rates by roots were regulated systemically, possibly by shoot P concentration. It is concluded that, in response to higher P supply, G. crithmifolia does not develop symptoms of P toxicity because of (i) greater plasticity of its net P-uptake capacity, and (ii) its greater plasticity for allocating P to growth and P storage in roots. This ecologically important difference in plasticity is most probably related to a slightly higher nutrient availability in the natural habitat of G. crithmifolia when compared with that of H. prostrata.

Acclimatization↗

PubMed2005

The occurrence of dauciform roots amongst Western Australian reeds, rushes and sedges, and the impact of phosphorus supply on dauciform-root development in Schoenus unispiculatus (Cyperaceae).

* The incidence of species that develop specialised 'dauciform' lateral roots, which are hypothesised to be important for phosphorus (P) acquisition, is uncertain. We investigated their occurrence in Australian reed, rush and sedge species, grown at low P concentration in nutrient solution, and studied the response of Schoenus unispiculatus (Cyperaceae) to a range of P concentrations. * We assessed the fraction of root biomass invested in dauciform roots, their respiration and net P-uptake rate, and the P status of roots and leaves. * Dauciform-root development occurred only in particular genera of Cyperaceae when grown at low P supply. Increased P supply was associated with increased growth of S. unispiculatus and increased leaf [P]. Dauciform-root growth was reduced by increased P supply, and reduced P uptake co-occurred with the complete suppression of dauciform roots. * The P-induced suppression of dauciform roots in Cyperaceae is similar to that observed for proteoid roots in members of Proteaceae and Lupinus albus. The response of dauciform roots to altered P supply and their absence from root systems of some sedge species are discussed in terms of managed and natural systems.

Cyperaceae↗

PubMed2004

Developmental physiology of cluster-root carboxylate synthesis and exudation in harsh hakea. Expression of phosphoenolpyruvate carboxylase and the alternative oxidase.

Harsh hakea (Hakea prostrata R.Br.) is a member of the Proteaceae family, which is highly represented on the extremely nutrient-impoverished soils in southwest Australia. When phosphorus is limiting, harsh hakea develops proteoid or cluster roots that release carboxylates that mobilize sparingly soluble phosphate in the rhizosphere. To investigate the physiology underlying the synthesis and exudation of carboxylates from cluster roots in Proteaceae, we measured O2 consumption, CO2 release, internal carboxylate concentrations and carboxylate exudation, and the abundance of the enzymes phosphoenolpyruvate carboxylase and alternative oxidase (AOX) over a 3-week time course of cluster-root development. Peak rates of citrate and malate exudation were observed from 12- to 13-d-old cluster roots, preceded by a reduction in cluster-root total protein levels and a reduced rate of O2 consumption. In harsh hakea, phosphoenolpyruvate carboxylase expression was relatively constant in cluster roots, regardless of developmental stage. During cluster-root maturation, however, the expression of AOX protein increased prior to the time when citrate and malate exudation peaked. This increase in AOX protein levels is presumably needed to allow a greater flow of electrons through the mitochondrial electron transport chain in the absence of rapid ATP turnover. Citrate and isocitrate synthesis and accumulation contributed in a major way to the subsequent burst of citrate and malate exudation. Phosphorus accumulated by harsh hakea cluster roots was remobilized during senescence as part of their efficient P cycling strategy for growth on nutrient impoverished soils.

Carbon Dioxide↗

PubMed2004

Tissue and cellular phosphorus storage during development of phosphorus toxicity in Hakea prostrata (Proteaceae).

Storage of phosphorus (P) in stem tissue is important in Mediterranean Proteaceae, because proteoid root growth and P uptake is greatest during winter, whereas shoot growth occurs mostly in summer. This has prompted the present investigation of the P distribution amongst roots, stems, and leaves of Hakea prostrata R.Br. (Proteaceae) when grown in nutrient solutions at ten P-supply rates. Glasshouse experiments were carried out during both winter and summer months. For plants grown in the low-P range (0, 0.3, 1.2, 3.0, or 6.0 micromol d(-1)) the root [P] was > stem and leaf [P]. In contrast, leaf [P] > stem and root [P] for plants grown in the high-P range (6.0, 30, 60, 150, or 300 micromol P d(-1)). At the highest P-supply rates, the capacity for P storage in stems and roots appears to have been exceeded, and leaf [P] thereafter increased dramatically to approximately 10 mg P g(-1) dry mass. This high leaf [P] was coincident with foliar symptoms of P toxicity which were similar to those described for many other species, including non-Proteaceae. The published values (tissue [P]) at which P toxicity occurs in a range of species are summarized. X-ray microanalysis of frozen, full-hydrated leaves revealed that the [P] in vacuoles of epidermal, palisade and bundle-sheath cells were in the mM range when plants were grown at low P-supply, even though very low leaf [P] was measured in bulk leaf samples. At higher P-supply rates, P accumulated in vacuoles of palisade cells which were associated with decreased photosynthetic rates.

Cells, Cultured↗