A Novel fry1 Allele Reveals the Existence of a Mutant Phenotype Unrelated to 5′->3′ Exoribonuclease (XRN) Activities in Arabidopsis thaliana Roots

International audienceBackgroundMutations 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 FindingsA 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/SignificanceOur 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.

Transcriptomic Events Involved in Melon Mature-Fruit Abscission Comprise the Sequential Induction of Cell-Wall Degrading Genes Coupled to a Stimulation of Endo and Exocytosis

Background: Mature-fruit abscission (MFA) in fleshy-fruit is a genetically controlled process with mechanisms that, contrary to immature-fruit abscission, has not been fully characterized. Here, we use pyrosequencing to characterize the transcriptomes of melon abscission zone (AZ) at three stages during AZ-cell separation in order to understand MFA control at an early stage of AZ-activation. Principal Findings: The results show that by early induction of MFA, the melon AZ exhibits major gene induction, while by late induction of MFA, melon AZ shows major gene repression. Although some genes displayed similar regulation in both early and late induction of abscission, such as EXT1-EXT4, EGase1, IAA2, ERF1, AP2D15, FLC, MADS2, ERAF17, SAP5 and SCL13 genes, the majority had different expression patterns. This implies that time-specific events occur during MFA, and emphasizes the value of characterizing multiple time-specific abscission transcriptomes. Analysis of gene-expression from these AZs reveal that a sequential induction of cell-wall-degrading genes is associated with the upregulation of genes involved in endo and exocytosis, and a shift in plant-hormone metabolism and signaling genes during MFA. This is accompanied by transcriptional activity of small-GTPases and synthaxins together with tubulins, dynamins, V-type ATPases and kinesin-like proteins potentially involved in MFA signaling. Early events are potentially controlled by down-regulation of MADS-box, AP2/ERF and Aux/IAA transcription-factors, and up-regulation of homeobox, zinc finger, bZIP, and WRKY transcription-factors, while late events may be controlled by up-regulation of MYB transcription-factors. Significance: Overall, the data provide a comprehensive view on MFA in fleshy-fruit, identifying candidate genes and pathways associated with early induction of MFA. Our comprehensive gene-expression profile will be very useful for elucidating gene regulatory networks of the MFA in fleshy-fruit

X-ray structure of yeast Hal2p, a major target of lithium and sodium toxicity, and identification of framework interactions determining cation sensitivity

12 pags, 4 figs, 2 tabsThe product of the yeast HAL2 gene (Hal2p) is an in vivo target of sodium and lithium toxicity and its overexpression improves salt tolerance in yeast and plants. Hal2p is a metabolic phosphatase which catalyses the hydrolysis of 3'-phosphoadenosine-5'-phosphate (PAP) to AMP. It is, the prototype of an evolutionarily conserved family of PAP phosphatases and the engineering of sodium insensitive enzymes of this group may contribute to the generation of salt-tolerant crops. We have solved the crystal structure of Hal2p in complex with magnesium, lithium and the two products of PAP hydrolysis, AMP and Pi, at 1.6 Å resolution. A functional screening of random mutations of the HAL2 gene in growing yeast generated forms of the enzyme with reduced cation sensitivity. Analysis of these mutants defined a salt bridge (Glu238 ... Arg152) and a hydrophobic bond (Va170 ... Trp293) as important framework interactions determining cation sensitivity. Hal2p belongs to a larger superfamily of lithium-sensitive phosphatases which includes inositol monophosphatase. The hydrophobic interaction mutated in Hal2p is conserved in this superfamily and its disruption in human inositol monophosphatase also resulted in reduced cation sensitivity. (C) 2000 Academic Press.A.A. is a holder of a contract of the Spanish Ministerio de Educacion y Cultura and he was funded in part by a ROPA grant from the BBSRC and by a grant of the Spanish ``Plan Nacional de Biotecnologia'' (CICYT, Madrid). S.P. is supported by a BBSRC earmarked studentship. T.L.B. thanks the Wellcome Trust for provision of X-ray and computing equipment. Work in R.S. laboratory was funded by grants of the Spanish ``Plan Nacional de Biotecnologia'' (CICYT, Madrid) and the European Programme on Biotechnology (European Commission, Brussels). L.Y. holds an International Research Fellow Award of the National Science Foundation (USA) and P.L.R. is a holder of a contract of the Spanish Ministerio de Eduacion y Cultura

GmSAL1 Hydrolyzes Inositol-1,4,5-Trisphosphate and Regulates Stomatal Closure in Detached Leaves and Ion Compartmentalization in Plant Cells

<div><p>Inositol polyphosphatases are important regulators since they control the catabolism of phosphoinositol derivatives, which are often signaling molecules for cellular processes. Here we report on the characterization of one of their members in soybean, GmSAL1. In contrast to the substrate specificity of its <i>Arabidopsis</i> homologues (AtSAL1 and AtSAL2), GmSAL1 only hydrolyzes inositol-1,4,5-trisphosphate (IP₃) but not inositol-1,3,4-trisphosphate or inositol-1,4-bisphosphate.The ectopic expression of <i>GmSAL1</i> in transgenic <i>Arabidopsis thaliana</i> led to a reduction in IP₃ signals, which was inferred from the reduction in the cytoplasmic signals of the <i>in vivo</i> biomarker pleckstrin homology domain–green florescent protein fusion protein and the suppression of abscisic acid-induced stomatal closure. At the cellular level, the ectopic expression of <i>GmSAL1</i> in transgenic BY-2 cells enhanced vacuolar Na⁺ compartmentalization and therefore could partially alleviate salinity stress.</p> </div