Strigolactone Regulates Anthocyanin Accumulation,Acid Phosphatases Production and Plant Growth under Low Phosphate Condition in Arabidopsis

Phosphate is an essential macronutrient in plant growth and development; however, the concentration of inorganic phosphate (Pi) in soil is often suboptimal for crop performance. Accordingly, plants have developed physiological strategies to adapt to low Pi availability. Here, we report that typical Pi starvation responses in Arabidopsis are partially dependent on the strigolactone (SL) signaling pathway. SL treatment induced root hair elongation, anthocyanin accumulation, activation of acid phosphatase, and reduced plant weight, which are characteristic responses to phosphate starvation. Furthermore, the expression profile of SL-response genes correlated with the expression of genes induced by Pi starvation. These results suggest a potential overlap between SL signaling and Pi starvation signaling pathways in plants.     

Differential responses of oat cultivars to phosphate deprivation: plant growth and acid phosphatase activities

The effect of phosphate starvation on growth and acid phosphatases (APases) localization and activity in oat tissues was investigated. Oat cultivars (Avena sativa L.??Arab, Polar, Szakal) were grown for 1?C3?weeks in complete nutrient medium (+P) and without phosphate (?P). Pi concentration in plant tissues decreased strongly after culturing on ?P medium. Pi deficit reduced shoot growth, stimulated root elongation and increased ratio of root/shoot in all oat cultivars. Pi deficit had a greater impact on growth of oat cv. Polar than other varieties. A decrease in the internal Pi status led to an increase of acid phosphatase activities in extracts from shoots and roots, and in root exudates. The highest activity of secreted APases was observed for oat cv. Arab, during the third week of growth under Pi-deficient conditions. The activity of extracellular APase was high in young, growing zones of roots of ?P plants. Histochemical visualization indicated high activity of APases in the epidermis and vascular tissues of ?P plants. Pi deficiency increased intracellular APase activity in shoot mainly in oat cv. Polar, whereas APase activity in roots was the highest in oat cv. Szakal. Protein extracts from roots and shoots were run on native discontinuous PAGE to determine which isoform(s) may be affected by Pi deficiency. Three major APase isoforms were detected in all oat plants; one was strongly induced by Pi deficit. The studied oat cultivars differed in terms of acclimation to deficiency of phosphate??used various pools of APases to acquire Pi from external or internal sources.     

A type 5 acid phosphatase gene from Arabidopsis thaliana is induced by phosphate starvation and by some other types of phosphate mobilising/oxidative stress conditions

Low phosphorous availability, a common condition of many soils, is known to stimulate phosphatase activity in plants; however, the molecular details of this response remain mostly unknown. We purified and sequenced the N-terminal region of a phosphate starvation induced acid phosphatase (AtACP5) from Arabidopsis thaliana, and cloned its cDNA and the corresponding genomic DNA. The nucleotide sequence of the cDNA predicted that AtACP5 is synthesised as a 338 amino acid-long precursor with a signal peptide. AtACP5 was found to be related to known purple acid phosphatases, especially to mammal type 5 acid phosphatases. Other similarities with purple acid phosphatases, which contain a dinuclear metal centre, include the conservation of all residues involved in metal ligand binding and resistance to tartrate inhibition. In addition, AtACP5, like other type 5 acid phosphatases, displayed peroxidation activity. Northern hybridisation experiments, as well as in situ glucuronidase (GUS) activity assays on transgenic plants harbouring AtACP5:GUS translational fusions, showed that AtACP5 is not only responsive to phosphate starvation but also to ABA and salt stress. It is also expressed in senescent leaves and during oxidative stress induced by H2O2, but not by paraquat or salicylic acid. Given its bifunctionality, as it displays both phosphatase and peroxidation activity, we propose that AtACP5 could be involved in phosphate mobilisation and in the metabolism of reactive oxygen species in stressed or senescent parts of the plant.     

Chlamydomonas reinhardtii mutants abnormal in their responses to phosphorus deprivation.

P-starved plants scavenge inorganic phosphate (Pi) by developing elevated rates of Pi uptake, synthesizing extracellular phosphatases, and secreting organic acids. To elucidate mechanisms controlling these acclimation responses in photosynthetic organisms, we characterized the responses of the green alga Chlamydomonas reinhardtii to P starvation and developed screens for isolating mutants (designated psr [phosphorus-stress response]) abnormal in their responses to environmental levels of Pi. The psr1-1 mutant was identified in a selection for cells that survived exposure to high concentrations of radioactive Pi. psr1-2 and psr2 were isolated as strains with aberrant levels of extracellular phosphatase activity during P-deficient or nutrient-replete growth. The psr1-1 and psr1-2 mutants were phenotypically similar, and the lesions in these strains were recessive and allelic. They exhibited no increase in extracellular phosphatase activity or Pi uptake upon starvation. Furthermore, when placed in medium devoid of P, the psr1 strains lost photosynthetic O2 evolution and stopped growing more rapidly than wild-type cells; they may not be as efficient as wild-type cells at scavenging/accessing P stores. In contrast, psr2 showed elevated extracellular phosphatase activity during growth in nutrient-replete medium, and the mutation was dominant. The mutant phenotypes and the roles of Psr1 and Psr2 in P-limitation responses are discussed.     

Membrane lipid alteration during phosphate starvation is regulated by phosphate signaling and auxin/cytokinin cross-talk

During phosphate (Pi) starvation in plants, membrane phospholipid content decreases concomitantly with an increase in non-phosphorus glycolipids. Although several studies have indicated the involvement of phytohormones in various physiological changes upon Pi starvation, the regulation of Pi-starvation induced membrane lipid alteration remains unknown. Previously, we reported the response of type B monogalactosyl diacylglycerol synthase genes (atMGD2 and atMGD3) to Pi starvation, and suggested a role for these genes in galactolipid accumulation during Pi starvation. We now report our investigation of the regulatory mechanism for the response of atMGD2/3 and changes in membrane lipid composition to Pi starvation. Exogenous auxin activated atMGD2/3 expression during Pi starvation, whereas their expression was repressed by cytokinin treatment in the root. Moreover, auxin inhibitors and the axr4 aux1 double mutation in auxin signaling impaired the increase of atMGD2/3 expression during Pi starvation, showing that auxin is required for atMGD2/3 activation. The fact that hormonal effects during Pi starvation were also observed with regard to changes in membrane lipid composition demonstrates that both auxin and cytokinin are indeed involved in the dynamic changes in membrane lipids during Pi starvation. Phosphite is not metabolically available in plants; however, when we supplied phosphite to Pi-starved plants, the Pi-starvation response disappeared with respect to both atMGD2/3 expression and changes in membrane lipids. These results indicate that the observed global change in plant membranes during Pi starvation is not caused by Pi-starvation induced damage in plant cells but rather is strictly regulated by Pi signaling and auxin/cytokinin cross-talk.     

Phosphate Starvation Inducible Metabolism in Lycopersicon esculentum: III. Changes in Protein Secretion under Nutrient Stress

Phosphate starvation increased the secretion of at least six proteins by suspension cultured tomato (Lycopersicon esculentum L. and L. pennellii) cells. Cells exhibited a biphasic response to phosphate (Pi) starvation. The early phase involved enhanced secretion of three proteins in response to transfer to a Pi-depleted media, while biomass accumulation continued at the same rate as in the Pi-sufficient cells. Severe starvation, defined as inhibition of biomass accumulation, induced enhanced secretion of three additional proteins. After sodium dodecyl sulfate-polyacrylamide gel electrophoresis, media proteins were immunoblotted with antibodies reacting specifically to oligosaccharides processed by the Golgi apparatus. Binding patterns showed that the enhancement in secretion during both phases of starvation was Golgi-mediated. Cells undergoing severe starvation had a respiration rate approximately twice that of unstressed cells and secreted 4.4 times more protein into the media per unit biomass. These data suggest overlapping Pi starvation-specific and global stress responses in plant cells. Under these conditions, Golgi-mediated protein secretion is enhanced. We present evidence for phosphate starvation inducible enhancement of Pi uptake. Secreted proteins specific for N and Fe starvation are also identified.     

Functional characterization of lysophosphatidic acid phosphatase from Arabidopsis thaliana

Lysophosphatidic acid (LPA) acts as a signaling molecule that regulates diverse cellular processes and it can rapidly be metabolized by phosphatase and acyltransferase. LPA phosphatase gene has not been identified and characterized in plants so far. The BLAST search revealed that the At3g03520 is similar to phospholipase family, and distantly related to bacterial phosphatases. The conserved motif, (J)4XXXNXSFD, was identified in both At3g03520 like phospholipases and acid phosphatases. In silico expression analysis of At3g03520 revealed a high expression during phosphate starvation and abiotic stresses. This gene was overexpressed in Escherichia coli and shown to posses LPA specific phosphatase activity. These results suggest that this gene possibly plays a role in signal transduction and storage lipid synthesis.     

Negative Regulation of Phosphate Starvation-Induced Genes

Phosphate (Pi) deficiency is a major nutritional problem faced by plants in many agro-ecosystems. This deficiency results in altered gene expression leading to physiological and morphological changes in plants. Altered gene expression is presumed to be due to interaction of regulatory sequences (cis-elements) present in the promoters with DNA binding factors (trans-factors). In this study, we analyzed the expression and DNA-protein interaction of promoter regions of Pi starvation-induced genes AtPT2 and TPSI1. AtPT2 encodes the high-affinity Pi transporter in Arabidopsis, whereas TPSI1 codes for a novel gene induced in the Pi-starved tomato (Lycopersicon esculentum). Expression of AtPT2 was induced rapidly under Pi deficiency and increased with decreasing concentrations of Pi. Abiotic stresses except Pi starvation had no affect on the expression of TPSI1. DNA mobility-shift assays indicated that specific sequences of AtPT2 and TPSI1 promoter interact with nuclear protein factors. Two regions of AtPT2 and TPSI1 promoters specifically bound nuclear protein factors from Pi-sufficient plants. Interestingly, the DNA binding activity disappeared during Pi starvation, leading to the hypothesis that Pi starvation-induced genes may be under negative regulation.