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The Arabidopsis mutant pho1 is deficient in the transfer of Pi from root epidermal and cortical cells to the xylem. The PHO1 gene was identified by a map-based cloning strategy. The N-terminal half of PHO1 is mainly hydrophilic, whereas the C-terminal half has six potential membrane-spanning domains. PHO1 shows no homology with any characterized solute transporter, including the family of H ؉ -Pi cotransporters identified in plants and fungi. PHO1 shows highest homology with the Rcm1 mammalian receptor for xenotropic murine leukemia retroviruses and with the Saccharomyces cerevisiae Syg1 protein involved in the mating pheromone signal transduction pathway. PHO1 is expressed predominantly in the roots and is upregulated weakly under Pi stress. Studies with PHO1 promoter-␤ -glucuronidase constructs reveal predominant expression of the PHO1 promoter in the stelar cells of the root and the lower part of the hypocotyl. There also is ␤ -glucuronidase staining of endodermal cells that are adjacent to the protoxylem vessels. The Arabidopsis genome contains 10 additional genes showing homology with PHO1 . Thus, PHO1 defines a novel class of proteins involved in ion transport in plants. INTRODUCTIONThe radial movement of ions from root epidermal and cortical cells to the xylem can be mediated by two major pathways. In the apoplastic pathway, ions move radially toward the stele through the extracellular space, whereas in the symplastic pathway, ions move intracellularly from cell to cell via plasmodesmata (Bowling, 1981;Clarkson, 1993). Although a third pathway is possible, namely, one in which ions move from cell to cell through a successive uptake and release of ions from and into the extracellular space, the high energy requirement of this pathway makes it unlikely to play a major role in ion transport to the xylem.The movement of ions and water through the apoplast of the root is blocked at the level of the endoderm by the Casparian strip, a zone in which the cell wall is impregnated with hydrophobic compounds such as suberin and lignin. Thus, passage of ions beyond the Casparian strip and toward the stele must proceed via the symplasm. Once in the cells of the stele, the release of ions into the xylem requires their efflux out of the stelar cells. Thus, radial transport of ions from the external solution to the xylem requires a minimum of two passages across the plasma membrane, once for the uptake of ions into the epidermal, cortical, or outer surface of the endodermal cells, and then again for the efflux of ions out of the stelar cells before entering the xylem vessel (Bowling, 1981;Clarkson, 1993).The uptake of anions such as Pi into a cell is an energyrequiring process. The negatively charged phosphate ion (HPO 4 Ϫ 2 or H 2 PO 4 Ϫ ) must move against an electrical gradient, the interior of the cell being negatively charged ( ‫ف‬ Ϫ 100 mV), as well as against a concentration gradient, the intracellular concentration of Pi being 1000 to 10,000 times higher than the extracellular concentration (the concentration of P...

A mutant of Arabidopsis thaliana deficient in the accumulation of inorganic phosphate has been isolated by screening directly for plants with altered quantities of total leaf phosphate. The mutant plants accumulate approximately 5% as much inorganic phosphate, and 24 to 44% as much total phosphate, as wild-type plants in aerial portions of the plant. Growth of the mutant is reduced, relative to wild type, and it exhibits other symptoms normally associated with phosphate deficiency. The phosphate deficiency is caused by a single nuclear recessive mutation at a locus designated phol. The rate of phosphate uptake into the roots was similar between mutant and wild-type plants over a wide range of external phosphate concentrations. In contrast, when plants were grown in media containing 200 micromolar phosphate or less, phosphate transfer to the shoots of the mutant was reduced to 3 to 10% of the wild-type levels. The defect in phosphate transfer to the shoots could be overcome by providing higher levels of phosphate. Transfer of sulfate to the shoots was essentially normal in the mutant, indicating that the phol lesion was not a general defect in anion transport. Movement of phosphate through the xylem of the shoots was not impaired. The results suggest that the mutant is deficient in activity of a protein required to load phosphate into the xylem.in uptake kinetics (3,7,11). Under conditions of limited phosphate availability, the capacity to transfer the absorbed phosphate to the shoots is also increased, suggesting an increase in phosphate release into the xylem (3, 7, 11). Compartmentalization of Pi appears to be important at the cellular level, with the vacuole acting as a storage site for excess Pi that can be released under conditions ofcytosolic Pi deficiency (3, 12). Thus, understanding the mechanisms that regulate Pi acquisition is complicated by both the existence of several potential sites of control and the presence of adaptive responses.As one approach to understanding how Pi levels are regulated, a genetic study was initiated to define loci that play a role in this process. The goal was to identify plants from a mutagenized population of Arabidopsis thaliana that display quantitative differences in leaf total phosphate content when compared with wild-type plants. We describe here the properties of a novel mutant that displayed a leaf Pi content that is only approximately 5% of the wild-type level. Measurements of the rate of root Pi uptake and transfer to the shoots, under various external phosphate concentrations and physiological conditions, indicate that the mutant is most likely deficient in activity of a protein required to load phosphate into the xylem.The mechanisms that regulate fluxes of phosphate between various tissues of higher plants are not well characterized. Potential points of control are the level of root Pi uptake, transfer of the absorbed Pi to the shoot, and compartmentalization into different subcellular organelles. There appear to be two major points of regulation for ion...

SUMMARYPhosphate is a crucial and often limiting nutrient for plant growth. To obtain inorganic phosphate (P i ), which is very insoluble, and is heterogeneously distributed in the soil, plants have evolved a complex network of morphological and biochemical processes. These processes are controlled by a regulatory system triggered by P i concentration, not only present in the medium (external P i ), but also inside plant cells (internal P i ). A 'splitroot' assay was performed to mimic a heterogeneous environment, after which a transcriptomic analysis identified groups of genes either locally or systemically regulated by P i starvation at the transcriptional level. These groups revealed coordinated regulations for various functions associated with P i starvation (including P i uptake, P i recovery, lipid metabolism, and metal uptake), and distinct roles for members in gene families. Genetic tools and physiological analyses revealed that genes that are locally regulated appear to be modulated mostly by root development independently of the internal P i content. By contrast, internal P i was essential to promote the activation of systemic regulation. Reducing the flow of P i had no effect on the systemic response, suggesting that a secondary signal, independent of P i , could be involved in the response. Furthermore, our results display a direct role for the transcription factor PHR1, as genes systemically controlled by low P i have promoters enriched with P1BS motif (PHR1-binding sequences). These data detail various regulatory systems regarding P i starvation responses (systemic versus local, and internal versus external P i ), and provide tools to analyze and classify the effects of P i starvation on plant physiology.

Fatty acid degradation in most organisms occurs primarily via the beta-oxidation cycle. In mammals, beta-oxidation occurs in both mitochondria and peroxisomes, whereas plants and most fungi harbor the beta-oxidation cycle only in the peroxisomes. Although several of the enzymes participating in this pathway in both organelles are similar, some distinct physiological roles have been uncovered. Recent advances in the structural elucidation of numerous mammalian and yeast enzymes involved in beta-oxidation have shed light on the basis of the substrate specificity for several of them. Of particular interest is the structural organization and function of the type 1 and 2 multifunctional enzyme (MFE-1 and MFE-2), two enzymes evolutionarily distant yet catalyzing the same overall enzymatic reactions but via opposite stereochemistry. New data on the physiological roles of the various enzymes participating in beta-oxidation have been gathered through the analysis of knockout mutants in plants, yeast and animals, as well as by the use of polyhydroxyalkanoate synthesis from beta-oxidation intermediates as a tool to study carbon flux through the pathway. In plants, both forward and reverse genetics performed on the model plant Arabidopsis thaliana have revealed novel roles for beta-oxidation in the germination process that is independent of the generation of carbohydrates for growth, as well as in embryo and flower development, and the generation of the phytohormone indole-3-acetic acid and the signal molecule jasmonic acid.