The development of phosphate stress and its functional consequences in the model legume plant Medicago truncatula

[摘要] ENGLISH ABSTRACT: Phosphate is an abundant nutrient in the soil; however it is mostly bound to otherelements that make phosphate unavailable for plant uptake. This bound state makesphosphate the second most limiting nutrient for plant growth. Phosphate is also a nonrenewablemined resource that forms a major constituent of fertiliser given to crops grownin nutrient poor soils. The second most important crop family in agriculture isLeguminosae. In an attempt to to reduse possible nitrogen stress, legumes can form asymbiosis with nitrogen-fixing soil bacteria. This symbiosis, found in the nodules,exchanges fixed nitrogen with host photosynthate and phosphate. The nodules are thus aphosphate sink that place stress on the rest of the plant. Legumes have adapted differentways to optimise the limited available phosphate to continue their own growth whilemaintaining the adenosine-triphosphate expensive nitrogen-fixing reaction.In this study, we looked at how the genetic model legume, Medicago truncatulaGaertner, has adapted to phosphate stressed conditions as it relied solely on biologicalnitrogen fixation as a source of nitrogen.In the first treatment, Medicago truncatula seedlings were infected withSinorhizobium meliloti and received a low concentration of phosphate throughout thegrowth period. This was done to simulate Medicago truncatula growing in alreadyphosphate deprived soils. The comparisons of biomass and growth, internal freephosphate concentrations, and organic acid and acid phosphatases enzyme activitieswere done on the above versus below ground tissues. Photosynthesis parameters werealso recorded. Above ground tissues responded to phosphate stress with increasedactivity of bypass enzymes at the steps that required adenosine-triphosphate. While thebelow ground tissues focused on using acid phosphatases to recycle phosphate. Althoughthe rate of photosynthesis had decreased in the phosphate stressed plants, the efficiencyof photosynthesis with the phosphate that was available in the leaves had increased.The second treatment involved the growth of nodulated Medicago truncatula with anoptimal phosphate concentration, followed by an induced phosphate stress period. In thismanner, soil that had been depleted of phosphate during plant growth was simulated. With the addition of determining differences in activities of nitrogen assimilating enzymes, theabove-mentioned comparisons were made on the nodules and roots of the sample plants.Under the induced stress condition, available phosphate was concentrated to the nodules.A possible cause for this was the increase in activity of the organic acid synthesisingenzymes present in the nodule. The nitrogen assimilating enzyme activities indicated thatstressed nodules may export glutamine rather than asparagine to the roots. Root nitrogenassimilating enzyme activities remained relatively constant during phosphate stress.Reduced nitrogen and carbon content of stressed plants indicated that phosphate had adirect impact on nitrogen fixation.From this study, we deduced that above ground tissues adapted metabolically forimproved photosynthesis phosphate use efficiency; while below ground tissues recycle theavailable phosphate to be used for nitrogen-fixation. After the induction of phosphatestress it was found that the nodules relied on saving available phosphate for nitrogenfixation,while the roots recyled assimilated glutamine to maintain function.

[发布机构] Stellenbosch University