Marcel Bucher

Molecular mechanisms of phosphate transport in plants.

Abstract. Membrane-spanning transport proteins are responsible for the selective passage of most mineral nutrients and metabolites across cellular and intracellular membranes. This reviews focus is on summarising the current state of research covering the molecular regulation and biochemical mechanisms involved in the transport of phosphorus, an often growth-limiting nutrient, in vascular plants. Physiological data illustrating the tight control of Pi homeostasis on the cellular as well as on the organisms level are discussed together with the recent results on molecular transport mechanisms.

Potential for increasing the content and bioavailability of Fe, Zn and Ca in plants for human nutrition

This paper reviews the possibility and limits for increasing the content and bioavailability of iron (Fe), zinc (Zn) and calcium (Ca) in edible parts of staple crops, such as cereals, pulses, roots and tubers as a way to combat mineral deficiencies in human populations. Theoretically, this could be achieved by increasing the total level of Fe, Zn and Ca in the plant foods, while at the same time increasing the concentration of compounds which promote their uptake (ascorbic acid), and/or by decreasing the concentration of compounds which inhibit their absorption (phytic acid or phenolic compounds). The content of Zn and Ca in grains and fruits can in some cases be increased through soil and/or foliar applications of Zn and Ca fertilisers. Plant breeding and genetic engineering techniques, however, have the greatest potential to increase Fe and Zn content in grains, roots and tubers. The possibility of enhancing Ca and ascorbic acid content in plant foods by plant breeding and genetic engineering remained to be explored. The critical factor is to ensure that the extra minerals have an adequate bioavailability for man. Given the important role of phytic acid and polyphenols in plant physiology, reducing the levels of these compounds in the edible parts of plants does not appear to be wise although introduction of phytases which are active during digestion is an exciting possibility.

A phosphate transporter expressed in arbuscule-containing cells in potato

Arbuscular mycorrhizas are the most common non-pathogenic symbioses in the roots of plants. It is generally assumed that this symbiosis facilitated the colonization of land by plants. In arbuscular mycorrhizas, fungal hyphae often extend between the root cells and tuft-like branched structures (arbuscules) form within the cell lumina that act as the functional interface for nutrient exchange. In the mutualistic arbuscular-mycorrhizal symbiosis the host plant derives mainly phosphorus from the fungus, which in turn benefits from plant-based glucose. The molecular basis of the establishment and functioning of the arbuscular-mycorrhizal symbiosis is largely not understood. Here we identify the phosphate transporter gene StPT3 in potato (Solanum tuberosum). Functionality of the encoded protein was confirmed by yeast complementation. RNA localization and reporter gene expression indicated expression of StPT3 in root sectors where mycorrhizal structures are formed. A sequence motif in the StPT3 promoter is similar to transposon-like elements, suggesting that the mutualistic symbiosis evolved by genetic rearrangements in the StPT3 promoter.

Functional analysis and cell-specific expression of a phosphate transporter from tomato.

Abstract. For a better understanding of the molecular and biochemical processes involved in orthophosphate (Pi) uptake at the root/soil interface, we cloned a Pi-transporter cDNA (LePT1) from a root air-specific cDNA library of tomato (Lycopersicon esculentum Mill.). The corresponding protein belongs to the growing family of ion transporters with twelve putative transmembrane domains. It is highly homologous to recently isolated Pi transporters from higher plants, yeast and fungi. When expressed in a Pi-uptake-deficient yeast mutant, the L. esculentum phosphate transporter 1 (LePT1) protein exhibits an apparent Km of 31 μM. The transporter is still active at submicromolar Pi concentrations and mediates highest Pi uptake at pH 5. The activity of LePT1 is dependent on the electrochemical membrane potential mediated by the yeast P-type H+-ATPase. Transcript levels of LePT1 in tomato seedlings are detectable in all vegetative organs under Pi-sufficient conditions, with highest concentrations in root hairs. In␣situ hybridization studies demonstrate cell-specific expression of LePT1 in the tomato root. The LePT1 mRNA is detectable in peripheral cell layers such as rhizodermal and root cap cells. Under Pi-deprivation conditions, mRNA levels are also detectable in young stelar tissue. This work presents molecular and biochemical evidence for distinct root cells playing an important role in Pi acquisition at the root/soil interface.

Phosphate transport and homeostasis in Arabidopsis

Phosphorus (P) is an essential macronutrient for all living organisms. It serves various basic biological functions as a structural element in nucleic acids and phospholipids, in energy metabolism, in the activation of metabolic intermediates, as a component in signal transduction cascades, and the regulation of enzymes. Of the major nutrients, P is the most dilute and the least mobile in soil. High sorbing capacity for P in the soil (e.g. sorbtion to metal oxides), P mineralization (e.g. calcium phosphates such as apatite), and/or fixation of P in organic soil matter (by converting soluble P into organic molecules) result in low availability of this macronutrient for uptake into plants (Marschner, 1995). P is absorbed by plants as orthophosphate (Pi, inorganic phosphate). Pi concentration in the soil solution hardly reaches 10 µM and may even drop to submicromolar levels at the root/soil interface, where Pi uptake by plants and root surface-colonizing microorganisms leads to the generation of a zone of Pi depletion around the root cylinder that is maintained due to slow diffusion of Pi from regions distant to the root surface (Figure 1). Fig. 1. A transverse section through the tip of a primary root. The dotted line indicates the outer border of the P depletion zone. The arrow indicates the direction of growth. In industrialized countries, low P availability in agricultural soils is compensated by a high input of P fertilizer to guarantee high crop productivity and yield. Water run-off, soil erosion and leakage in highly fertilized agricultural soils may cause environmental problems such as eutrophication of lakes and rivers. As forecasted by Tilman et al. (2001), during the next 50 years, which is likely to be the final period of rapid agricultural expansion, demand for food by global population will be a major driver of global environmental change. Conversion of natural ecosystems to agriculture by 2050 will be accompanied by an approximate 2.5-fold increase in nitrogen- and P- driven eutrophication of terrestrial, freshwater, and near-shore marine ecosystems. Modern agricultural soils are almost universally maintained at high fertilization. Selection of new cultivars is usually made under such conditions and will not normally distinguish between plants varying in nutrient efficiency (Stevens and Rick, 1986). To alleviate the forecasted adverse negative effects of agricultural expansion, scientists have started to use classical breeding strategies and biotechnology to improve crop plants, based on the current knowledge and aiming at an improved crop yield with a lower input of fertilizer, thus protecting the environment. In contrast, in many developing tropical countries, subsistence farmers can not buy enough fertilizer due to limited financial capacities or poor infrastructure (Sanchez et al., 1997). As a consequence, P deprivation dramatically limits crop yield and is one of the reasons for poverty and malnutrition. In the future, agriculture from both developed as well as developing countries could thus benefit from modern crop varieties with enhanced P efficiency, thus leading to improved fertilizer management and increased crop yield on low-P soils. Thorough knowledge of the plants response to P deprivation stress will contribute to the rational and targeted breeding of P efficient crop plants. Therefore, the authors of this chapter focus on summarizing the current state of research covering physiology, biochemistry, and molecular genetics of P acquisition and allocation, and P homeostasis within the plant. Although this review will mainly focus on knowledge acquired on Arabidopsis thaliana, some specific results obtained with other plant species will also be included in this work. For example, formation of mycorrhizae, which is observed in most vascular plants and strongly contributes to plant P nutrition, does not occur in Brassicaceae and therefore Arabidopsis is not a suitable model for mycorrhizae studies.

Phosphate systemically inhibits development of arbuscular mycorrhiza in Petunia hybrida and represses genes involved in mycorrhizal functioning

Most terrestrial plants form arbuscular mycorrhiza (AM), mutualistic associations with soil fungi of the order Glomeromycota. The obligate biotrophic fungi trade mineral nutrients, mainly phosphate (P(i) ), for carbohydrates from the plants. Under conditions of high exogenous phosphate supply, when the plant can meet its own P requirements without the fungus, AM are suppressed, an effect which could be interpreted as an active strategy of the plant to limit carbohydrate consumption of the fungus by inhibiting its proliferation in the roots. However, the mechanisms involved in fungal inhibition are poorly understood. Here, we employ a transcriptomic approach to get insight into potential shifts in metabolic activity and symbiotic signalling, and in the defence status of plants exposed to high P(i) levels. We show that in mycorrhizal roots of petunia, a similar set of symbiosis-related genes is expressed as in mycorrhizal roots of Medicago, Lotus and rice. P(i) acts systemically to repress symbiotic gene expression and AM colonization in the root. In established mycorrhizal roots, P(i) repressed symbiotic gene expression rapidly, whereas the inhibition of colonization followed with a lag of more than a week. Taken together, these results suggest that P(i) acts by repressing essential symbiotic genes, in particular genes encoding enzymes of carotenoid and strigolactone biosynthesis, and symbiosis-associated phosphate transporters. The role of these effects in the suppression of symbiosis under high P(i) conditions is discussed.

Pht2;1 Encodes a Low-Affinity Phosphate Transporter from Arabidopsis

An Arabidopsis genomic sequence was recently shown to share similarity with bacterial and eukaryotic phosphate (Pi) transporters. We have cloned the corresponding cDNA, which we named Pht2;1, and subsequently performed gene expression studies and functional analysis of the protein product. The cDNA encodes a 61-kD protein with a putative topology of 12 transmembrane (TM) domains interrupted by a large hydrophilic loop between TM8 and TM9. Two boxes of eight and nine amino acids, located in the N- and C-terminal domains, respectively, are highly conserved among species across all kingdoms (eubacteria, archea, fungi, plants, and animals). The Pht2;1 gene is predominantly expressed in green tissue, the amount of transcript staying constant in leaves irrespective of the Pi status of the shoot; in roots, however, there is a marginal increase in mRNA amounts in response to Pi deprivation. Although the protein is highly similar to eukaryotic sodium-dependent Pi transporters, functional analysis of the Pht2;1 protein in mutant yeast cells indicates that it is a proton/Pi symporter dependent on the electrochemical gradient across the plasma membrane. Its fairly high apparent Km for Pi (0.4 mM) and high mRNA content in the shoot, especially in leaves, suggest a role for shoot organs in Pi loading. Pht2;1 thus differs from members of the recently described plant Pi transporter family in primary structure, affinity for Pi, and presumed function.

Mycorrhizal phosphate uptake pathway in tomato is phosphorus‐repressible and transcriptionally regulated

Plants colonized by arbuscular mycorrhizal (AM) fungi take up phosphate (Pi)via the mycorrhizal and the direct Pi uptake pathway. Our understanding of the molecular mechanisms involved in the regulation of these pathways is just emerging.Here, we have analyzed the molecular physiology of mycorrhizal Pi uptake in the tomato (Solanum lycopersicum) variety Micro-Tom and integrated the data obtained with studies on chemical signaling in mycorrhiza-inducible Pi transporter gene regulation.At high plant phosphorus (P) status, the mycorrhizal Pi uptake pathway was almost completely repressed and the mycorrhiza-inducible Pi transporter genes were down-regulated. A high plant P status also suppressed the activation of the mycorrhiza-specific StPT3 promoter fragment by phospholipid extracts containing the mycorrhiza signal lysophosphatidylcholine.Our results suggest that the mycorrhizal Pi uptake pathway is controlled at least partially by the plant host. This control involves components in common

AEROBIC FERMENTATION IN TOBACCO POLLEN

We characterized the genes coding for the two dedicated enzymes of ethanolic fermentation, alcohol dehydrogenase (ADH) and pyruvate decarboxylase (PDC), and show that they are functional in pollen. Two PDC-encoding genes were isolated, which displayed reciprocal regulation: PDC1 was anaerobically induced in leaves, whereas PDC2 mRNA was absent in leaves, but constitutively present in pollen. A flux through the ethanolic fermentation pathway could be measured in pollen under all tested environmental and developmental conditions. Surprisingly, the major factor influencing the rate of ethanol production was not oxygen availability, but the composition of the incubation medium. Under optimal conditions for pollen tube growth, approximately two-thirds of the carbon consumed was fermented, and ethanol accumulated into the surrounding medium to a concentration exceeding 100 mM.

Root Endophyte Colletotrichum tofieldiae Confers Plant Fitness Benefits that Are Phosphate Status Dependent.

Summary A staggering diversity of endophytic fungi associate with healthy plants in nature, but it is usually unclear whether these represent stochastic encounters or provide host fitness benefits. Although most characterized species of the fungal genus Colletotrichum are destructive pathogens, we show here that C. tofieldiae (Ct) is an endemic endophyte in natural Arabidopsis thaliana populations in central Spain. Colonization by Ct initiates in roots but can also spread systemically into shoots. Ct transfers the macronutrient phosphorus to shoots, promotes plant growth, and increases fertility only under phosphorus-deficient conditions, a nutrient status that might have facilitated the transition from pathogenic to beneficial lifestyles. The host’s phosphate starvation response (PSR) system controls Ct root colonization and is needed for plant growth promotion (PGP). PGP also requires PEN2-dependent indole glucosinolate metabolism, a component of innate immune responses, indicating a functional link between innate immunity and the PSR system during beneficial interactions with Ct.