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Type: Thesis
Title: Diverse functions of the HvPHT1 phosphate transporters and genetic variation in the phosphate use efficiency of Triticum aestivum.
Author: Preuss, Christian Paul
Issue Date: 2011
School/Discipline: School of Agriculture, Food and Wine
Abstract: Wheat and barley are major crops both within Australia and around the world, providing a large proportion of the protein and calories required in both human and animal diets. Phosphorus(P) is an essential nutrient for all living organisms, without adequate supply both wheat and barley will produce no yield. Thechemical reactions of P with metal ions in the soil are complex and often result in precipitation of this essential nutrient out of the soil solution, resulting in inadequate supply for plant growth. To ameliorate this P fertilisers are added (mostly in the developed world). The issue is that P fertilisers are a non-renewable resource and could be depleted in the next 50-100 years, after which what will we do to maintain production? Many developing countries, however, don’t have access to P fertilisers; as such they suffer poor cereal yields. To tackle these problems, both those in the developed world and those in the developing world ultimately require wheat and barley crops that can produce higher yield with lower P fertiliser inputs; this is termed agronomic P use efficiency (PUE). Plant modifications are one way to increase PUE; this could be done via an increase in uptake efficiency or an increase in remobilisation of inorganic phosphate (Pi) within the plant. Remobilisation of Pi within a plant is critical for sustaining growth and seed production under external Pi fluctuation. The barley phosphate transporter, HvPHT1;6 has been implicated in Pi remobilisation. Expression of HvPHT1;6 in Xenopus laevis oocytes allowed a detailed characterisation of voltage-dependent fluxes and currents induced by HvPHT1;6 (see Chapter 2). HvPHT1;6 increased efflux of Pi near oocyte resting membrane potentials, dependent on external Pi concentration. Time-dependent inward currents were observed when membrane potentials were more negative than -130 mV. This is consistent with nH[superscript]+:HPO₄²⁻(n>2) co-transport, based on simultaneous radiotracer and oocyte voltage clamping, calculations based on SO₄²⁻ nonselective uptake, and differences found with change in Pi concentration gradient and pH. Time- and voltage-dependent inward currents through HvPHT1;6 were also observed for SO₄²⁻, and to a lesser degree for NO³⁻, and C¹⁻, but not for malate. Inward and outward currents showed linear dependence on the concentration of external HPO₄²⁻, similar to low affinity Pi transport in plant studies. The electrophysiological properties of HvPHT1;6, which locates to the plasma membrane when expressed in onion (Allium cepa L.) epidermal cells, are consistent with its suggested role in the remobilisation of Pi in barley plants. Pi remobilisation is just one strategy that plants have to cope with external P fluctuation. An understanding then manipulation of Pi remobilisation processes could lead to improvements in plant PUE. Unlike their low affinity counterparts, high-affinity phosphate transporters mediate uptake of Pi from soil solution under low Pi environments and could also be an important target for increasing plant PUE. The electrophysiological properties of any plant high-affinity Pi transporter has not been described yet. HvPHT1;1 was characterised in Xenopus laevis oocytes. A very low K[subscript]m (1.9 μM) for Pi transport was observed in HvPHT1;1 (see Chapter 3); this Km value is similar to that found for high affinity Pi uptake into barley roots. Inward currents at negative membrane potentials were identified as nH⁺:Pi⁻ (n>1) co-transport based on simultaneous Pi radiotracer uptake, oocyte voltage clamping, and pH dependence. HvPHT1;1 showed preferential selectivity for Pi and As, but no transport of the other oxyanions SO₄²⁻ and NO³⁻ in contrast to HvPHT1;6. HvPHT1;1 also locates to the plasma membrane when expressed in onion (A. cepa) epidermal cells, and is highly expressed in root segments with dense hairs. The electrophysiological properties, plasma membrane localisation and cell-specific expression pattern of HvPHt1;1 support its role in the uptake of Pi under low external Pi. Early plant growth, after seed P depletion, is likely to be more dependent on Pi uptake rather than remobilisation. In many regions of the developing world, wheat productivity is constrained by both low moisture availability and inherently low soil P availability; interestingly these two factors interact and therefore influence plant P nutrition (see Chapter 5). Breeding bread wheat genotypes that perform better under low P availability and dry conditions seems the most viable option for the resource poor farmers in much of the developing world. Early vigour is an important trait for improving bread wheat productivity, but little is known about the effect of P fertility on the expression of early vigour. A large genetic variation for percent early vegetative cover (%EVC) under varying P stress was found in elite bread wheats for the dry areas; where the ICARDA genotype, Hamam-4 showed only a 2 % reduction in %EVC whereas the Australian genotype, Yitpi lost 55% of its ground cover under P stress (see Chapter 4). Phosphate fertiliser application increased %EVC by 25 % on average; and the presence of surface limestone significantly reduced %EVC. A large genetic variation in %EVC was found. %EVC was captured digitally and processed using new (free-licence) software developed by Douglas Johnson, Oregon State University. It had the strongest correlation with shoot biomass. %EVC capture is cheap, accurate and efficient, and could easily be implemented in wheat breeding programs to improve not only PUE, but also water use efficiency, weed competitiveness, and ultimately productivity. Wheat yield production is the optimal determinate of PUE, basically because yield feeds the worlds’ population. Importantly we describe the significant variation in PUE of elite Australian bread wheat germplasm. The cultivar Gladius showed the highest and most consistent PUE among the tested genotypes (see Chapter 5). This suggests that initially farmers can select varieties with higher PUE to manage the risk of rising P prices, and subsequently wheat breeders could improve the PUE of wheat by incorporating genes that confer higher PUE into their breeding programs. The next step in this project would be to define mechanisms are involved in the high PUE of Gladius, be it differences high or low affinity phosphate transporters, root traits, or root rhizosphere modifications.
Advisor: Huang, Chunyuan
Tyerman, Stephen Donald
Dissertation Note: Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 2011
Keywords: HvPHT1;6; HvPHT1;1; bread wheat; early vigour; yield analysis
Provenance: Copyright material removed from digital thesis. See print copy in University of Adelaide Library for full text.
Appears in Collections:Research Theses

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