Disruption of OsSULTR3;3 reduces phytate and phosphorus concentrations and alters the metabolite profile in rice grains.

Two low phytic acid (lpa) mutants have been developed previously with the aim to improve the nutritional value of rice (Oryza sativa) grains. In the present study, the impacts of lpa mutations on grain composition and underlying molecular mechanisms were investigated. Comparative compositional analyses and metabolite profiling demonstrated that concentrations of both phytic acid (PA) and total phosphorus (P) were significantly reduced in lpa brown rice, accompanied by changes in other metabolites and increased concentrations of nutritionally relevant compounds. The lpa mutations modified the expression of a number of genes involved in PA metabolism, as well as in sulfate and phosphate homeostasis and metabolism. Map-based cloning and complementation identified the underlying lpa gene to be OsSULTR3;3. The promoter of OsSULTR3;3 is highly active in the vascular bundles of leaves, stems and seeds, and its protein is localized in the endoplasmic reticulum. No activity of OsSULTR3;3 was revealed for the transport of phosphate, sulfate, inositol or inositol 1,4,5 triphosphate by heterologous expression in either yeast or Xenopus oocytes. The findings reveal that OsSULTR3;3 plays an important role in grain metabolism, pointing to a new route to generate value-added grains in rice and other cereal crops.

Functional expression of PHO1 to the Golgi and trans-Golgi network and its role in export of inorganic phosphate.

Arabidopsis thaliana PHO1 is primarily expressed in the root vascular cylinder and is involved in the transfer of inorganic phosphate (Pi) from roots to shoots. To analyze the role of PHO1 in transport of Pi, we have generated transgenic plants expressing PHO1 in ectopic A. thaliana tissues using an estradiol-inducible promoter. Leaves treated with estradiol showed strong PHO1 expression, leading to detectable accumulation of PHO1 protein. Estradiol-mediated induction of PHO1 in leaves from soil-grown plants, in leaves and roots of plants grown in liquid culture, or in leaf mesophyll protoplasts, was all accompanied by the specific release of Pi to the extracellular medium as early as 2-3 h after addition of estradiol. Net Pi export triggered by PHO1 induction was enhanced by high extracellular Pi and weakly inhibited by the proton-ionophore carbonyl cyanide m-chlorophenylhydrazone. Expression of a PHO1-GFP construct complementing the pho1 mutant revealed GFP expression in punctate structures in the pericycle cells but no fluorescence at the plasma membrane. When expressed in onion epidermal cells or in tobacco mesophyll cells, PHO1-GFP was associated with similar punctate structures that co-localized with the Golgi/trans-Golgi network and uncharacterized vesicles. However, PHO1-GFP could be partially relocated to the plasma membrane in leaves infiltrated with a high-phosphate solution. Together, these results show that PHO1 can trigger Pi export in ectopic plant cells, strongly indicating that PHO1 is itself a Pi exporter. Interestingly, PHO1-mediated Pi export was associated with its localization to the Golgi and trans-Golgi networks, revealing a role for these organelles in Pi transport.

Over-expression of PHO1 in Arabidopsis leaves reveals its role in mediating phosphate efflux.

Inorganic phosphate (Pi) homeostasis in multi-cellular eukaryotes depends not only on Pi influx into cells, but also on Pi efflux. Examples in plants for which Pi efflux is crucial are transfer of Pi into the xylem of roots and release of Pi at the peri-arbuscular interface of mycorrhizal roots. Despite its importance, no protein has been identified that specifically mediates phosphate efflux either in animals or plants. The Arabidopsis thaliana PHO1 gene is expressed in roots, and was previously shown to be involved in long-distance transfer of Pi from the root to the shoot. Here we show that PHO1 over-expression in the shoot of A. thaliana led to a two- to threefold increase in shoot Pi content and a severe reduction in shoot growth. (31) P-NMR in vivo showed a normal initial distribution of intracellular Pi between the cytoplasm and the vacuole in leaves over-expressing PHO1, followed by a large efflux of Pi into the infiltration medium, leading to a rapid reduction of the vacuolar Pi pool. Furthermore, the Pi concentration in leaf xylem exudates from intact plants was more than 100-fold higher in PHO1 over-expressing plants compared to wild-type. Together, these results show that PHO1 over-expression in leaves leads to a dramatic efflux of Pi out of cells and into the xylem vessel, revealing a crucial role for PHO1 in Pi efflux.

Regulation of phosphate starvation responses in plants: signaling players and cross-talks.

Phosphate (Pi) availability is a major factor limiting growth, development, and productivity of plants. In both ecological and agricultural contexts, plants often grow in soils with low soluble phosphate content. Plants respond to this situation by a series of developmental and metabolic adaptations that are aimed at increasing the acquisition of this vital nutrient from the soil, as well as to sustain plant growth and survival. The development of a comprehensive understanding of how plants sense phosphate deficiency and coordinate the responses via signaling pathways has become of major interest, and a number of signaling players and networks have begun to surface for the regulation of the phosphate-deficiency response. In practice, application of such knowledge to improve plant Pi nutrition is hindered by complex cross-talks, which are emerging in the face of new data, such as the coordination of the phosphate-deficiency signaling networks with those involved with hormones, photo-assimilates (sugar), as well as with the homeostasis of other ions, such as iron. In this review, we focus on these cross-talks and on recent progress in discovering new signaling players involved in the Pi-starvation responses, such as proteins having SPX domains.

Getting the most sulfate from soil: Regulation of sulfate uptake transporters in Arabidopsis.

Sulfur (S) is an essential macronutrient for all living organisms. Plants require large amounts of sulfate for growth and development, and this serves as a major entry point of sulfate into the food web. Plants acquire S in its ionic form from the soil; they have evolved tightly controlled mechanisms for the regulation of sulfate uptake in response to its external and internal availability. In the model plant Arabidopsis thaliana, the first key step in sulfate uptake is presumed to be carried out exclusively by only two high-affinity sulfate transporters: SULTR1;1 and SULTR1;2. A better understanding of the mode of regulation for these two transporters is crucial because they constitute the first determinative step in balancing sulfate in respect to its supply and demand. Here, we review the recent progress achieved in our comprehension of (i) mechanisms that regulate these two high-affinity sulfate transporters at the transcriptional and post-transcriptional levels, and (ii) their structure-function relationship. Such progress is important to enable biotechnological and agronomic strategies aimed at enhancing sulfate uptake and improving crop yield in S-deficient soils.

Differential regulation of the expression of two high-affinity sulfate transporters, SULTR1.1 and SULTR1.2, in Arabidopsis.

The molecular mechanisms regulating the initial uptake of inorganic sulfate in plants are still largely unknown. The current model for the regulation of sulfate uptake and assimilation attributes positive and negative regulatory roles to O-acetyl-serine (O-acetyl-Ser) and glutathione, respectively. This model seems to suffer from exceptions and it has not yet been clearly validated whether intracellular O-acetyl-Ser and glutathione levels have impacts on regulation. The transcript level of the two high-affinity sulfate transporters SULTR1.1 and SULTR1.2 responsible for sulfate uptake from the soil solution was compared to the intracellular contents of O-acetyl-Ser, glutathione, and sulfate in roots of plants submitted to a wide diversity of experimental conditions. SULTR1.1 and SULTR1.2 were differentially expressed and neither of the genes was regulated in accordance with the current model. The SULTR1.1 transcript level was mainly altered in response to the sulfur-related treatments. Split-root experiments show that the expression of SULTR1.1 is locally regulated in response to sulfate starvation. In contrast, accumulation of SULTR1.2 transcripts appeared to be mainly related to metabolic demand and is controlled by photoperiod. On the basis of the new molecular insights provided in this study, we suggest that the expression of the two transporters depends on different regulatory networks. We hypothesize that interplay between SULTR1.1 and SULTR1.2 transporters could be an important mechanism to regulate sulfate content in the roots.

Functional genomics of cell elongation in developing cotton fibers.

Cotton fibers are single-celled seed trichomes of major economic importance. Factors that regulate the rate and duration of cell expansion control fiber morphology and important agronomic traits. For genetic characterization of rapid cell elongation in cotton fibers, approximately 14,000 unique genes were assembled from 46,603 expressed sequence tags (ESTs) from developmentally staged fiber cDNAs of a cultivated diploid species ( Gossypium arboreum L.). Conservatively, the fiber transcriptome represents 35-40% of the genes in the cotton genome. In silico expression analysis revealed that rapidly elongating fiber cells exhibit significant metabolic activity, with the bulk of gene transcripts, represented by three major functional groups - cell wall structure and biogenesis, the cytoskeleton and energy/carbohydrate metabolism. Oligonucleotide microarrays revealed dynamic changes in gene expression between primary and secondary cell wall biogenesis showing that fiber genes in the dbEST are highly stage-specific for cell expansion - a conclusion supported by the absence of known secondary cell wall-specific genes from our fiber dbEST. During the developmental switch from primary to secondary cell wall syntheses, 2553 "expansion-associated" fiber genes are significantly down regulated. Genes (81) significantly up-regulated during secondary cell wall synthesis are involved in cell wall biogenesis and energy/carbohydrate metabolism, which is consistent with the stage of cellulose synthesis during secondary cell wall modification in developing fibers. This work provides the first in-depth view of the genetic complexity of the transcriptome of an expanding cell, and lays the groundwork for studying fundamental biological processes in plant biology with applications in agricultural biotechnology.

DNA microarray experiments: biological and technological aspects.

DNA microarray technologies, such as cDNA and oligonucleotide microarrays, promise to revolutionize biological research and further our understanding of biological processes. Due to the complex nature and sheer amount of data produced from microarray experiments, biologists have sought the collaboration of experts in the analytical sciences, including statisticians, among others. However, the biological and technical intricacies of microarray experiments are not easily accessible to analytical experts. One aim for this review is to provide a bridge to some of the relevant biological and technical aspects involved in microarray experiments. While there is already a large literature on the broad applications of the technology, basic research on the technology itself and studies to understand process variation remain in their infancy. We emphasize the importance of basic research in DNA array technologies to improve the reliability of future experiments.