Metal movement within the plant: contribution of nicotianamine and yellow stripe 1-like transporters.

Background Since the identification of the genes controlling the root acquisition of iron (Fe), the control of inter- and intracellular distribution has become an important challenge in understanding metal homeostasis. The identification of the yellow stripe-like (YSL) transporter family has paved the way to decipher the mechanisms of long-distance transport of Fe. Scope Once in the plant, Fe will systematically react with organic ligands whose identity is poorly known so far. Among potential ligands, nicotianamine has been identified as an important molecule for the circulation and delivery of metals since it participates in the loading of copper (Cu) and nickel in xylem and prevents Fe precipitation in leaves. Nicotianamine is a precursor of phytosiderophores, which are high-affinity Fe ligands exclusively synthesized by Poaceae species and excreted by roots for the chelation and acquisition of Fe. Maize YS1 is the founding member of a family of membrane transporters called YS1-like (YSL), which functions in root Fe-phytosiderophore uptake from the soil. Next to this well-known Fe acquisition role, most of the other YSL family members are likely to function in plant-wide distribution of metals since (a) they are produced in vascular tissues throughout the plant and (b) they are found in non-Poaceae species that do not synthesize phytosiderophores. The hypothesized activity as Fe-nicotianamine transporters of several YSL members has been demonstrated experimentally by heterologous expression in yeast or by electrophysiology in Xenopus oocytes but, despite numerous attempts, proof of the arabidopsis YSL substrate specificity is still lacking. Reverse genetics, however, has revealed a role for AtYSL members in the remobilization of Cu and zinc from senescing leaves, in the formation of pollen and in the Fe, zinc and Cu loading of seeds. Conclusions Preliminary data on the YSL family of transporters clearly argues in favour of its role in the long-distance transport of metals through and between vascular tissues to eventually support gametogenesis and embryo development.

A novel fry1 allele reveals the existence of a mutant phenotype unrelated to 5'->3' exoribonuclease (XRN) activities in Arabidopsis thaliana roots.

BACKGROUND: Mutations in the FRY1/SAL1 Arabidopsis locus are highly pleiotropic, affecting drought tolerance, leaf shape and root growth. FRY1 encodes a nucleotide phosphatase that in vitro has inositol polyphosphate 1-phosphatase and 3',(2'),5'-bisphosphate nucleotide phosphatase activities. It is not clear which activity mediates each of the diverse biological functions of FRY1 in planta. PRINCIPAL FINDINGS: A fry1 mutant was identified in a genetic screen for Arabidopsis mutants deregulated in the expression of Pi High affinity Transporter 1;4 (PHT1;4). Histological analysis revealed that, in roots, FRY1 expression was restricted to the stele and meristems. The fry1 mutant displayed an altered root architecture phenotype and an increased drought tolerance. All of the phenotypes analyzed were complemented with the AHL gene encoding a protein that converts 3'-polyadenosine 5'-phosphate (PAP) into AMP and Pi. PAP is known to inhibit exoribonucleases (XRN) in vitro. Accordingly, an xrn triple mutant with mutations in all three XRNs shared the fry1 drought tolerance and root architecture phenotypes. Interestingly these two traits were also complemented by grafting, revealing that drought tolerance was primarily conferred by the rosette and that the root architecture can be complemented by long-distance regulation derived from leaves. By contrast, PHT1 expression was not altered in xrn mutants or in grafting experiments. Thus, PHT1 up-regulation probably resulted from a local depletion of Pi in the fry1 stele. This hypothesis is supported by the identification of other genes modulated by Pi deficiency in the stele, which are found induced in a fry1 background. CONCLUSIONS/SIGNIFICANCE: Our results indicate that the 3',(2'),5'-bisphosphate nucleotide phosphatase activity of FRY1 is involved in long-distance as well as local regulatory activities in roots. The local up-regulation of PHT1 genes transcription in roots likely results from local depletion of Pi and is independent of the XRNs.

A genome-wide transcriptional analysis using Arabidopsis thaliana Affymetrix gene chips determined plant responses to phosphate deprivation.

Phosphorus, one of the essential elements for plants, is often a limiting nutrient because of its low availability and mobility in soils. Significant changes in plant morphology and biochemical processes are associated with phosphate (Pi) deficiency. However, the molecular bases of these responses to Pi deficiency are not thoroughly elucidated. Therefore, a comprehensive survey of global gene expression in response to Pi deprivation was done by using Arabidopsis thaliana whole genome Affymetrix gene chip (ATH1) to quantify the spatio-temporal variations in transcript abundance of 22,810 genes. The analysis revealed a coordinated induction and suppression of 612 and 254 Pi-responsive genes, respectively. The functional classification of some of these genes indicated their involvement in various metabolic pathways, ion transport, signal transduction, transcriptional regulation, and other processes related to growth and development. This study is a detailed analysis of Pi starvation-induced changes in gene expression of the entire genome of Arabidopsis correlated with biochemical processes. The results not only enhance our knowledge about molecular processes associated with Pi deficiency, but also facilitate the identification of key molecular determinants for improving Pi use by crop species.

Transcriptional regulation and functional properties of Arabidopsis Pht1;4, a high affinity transporter contributing greatly to phosphate uptake in phosphate deprived plants.

Phosphate mobilization into the plant is a complex process requiring numerous transporters for absorption and translocation of this major nutrient. In the genome of Arabidopsis thaliana, nine closely related high affinity phosphate transporters have been identified but their specific roles remain unclear. Here we report the molecular, histological and physiological characterization of Arabidopsis pht1;4 high affinity phosphate transporter mutants. Using GUS-gene trap and in situ hybridization, Pht1;4 was found mainly expressed in inorganic phosphate (Pi) limiting medium in roots, primarily in the epidermis, the cortex and the root cap. In addition to this, expression was also observed at the lateral root branch points on the primary root and in the stele of lateral roots, suggesting a role of Pht1;4 in phosphate absorption and translocation from the growth medium to the different parts of the plant. Pi-starved pht1;4 plantlets exhibited a strong reduction of phosphate uptake capacity (40). This phenotype appears only related to the pht1;4 mutation as there were no obvious changes in the expression of other Pht1 family members in the mutants background. However, after 10 days of growth on phosphate deficient or sufficient medium, the Pi content in the mutants was not significantly different from that of the corresponding wild type controls. Furthermore, the mutants did not display any obvious growth defects or visible phenotypes when grown on a low phosphate containing medium. The work described here offers a first step in the complex genetic dissection of the phosphate transport system in planta.