A recombinant plant natriuretic peptide causes rapid and spatially differentiated K+, Na+ and H+ flux changes in Arabidopsis thaliana roots

Plant natriuretic peptides (PNPs) belong to a novel class of systemically mobile molecules that are structurally similar to the N-terminal domain of expansins and affect physiological processes such as protoplast volume regulation at nano-molar concentrations. Here we demonstrate that AtPNP-A, a recombinant Arabidopsis thaliana PNP causes rapid H(+) influx in the elongation zone of A. thaliana roots but not in the mature zone. AtPNP-A also induces significant K(+) and Na(+) efflux and this effect is seen in the mature root zone only. These observations suggest that responses to AtPNP-A are developmental stage and tissue specific and point to a complex role in plant growth and homeostasis.     

The Arabidopsis thaliana natriuretic peptide AtPNP-A is a systemic regulator of leaf dark respiration and signals via the phloem

Plant natriuretic peptides (PNPs) belong to a novel class of peptidic signaling molecules that share some structural similarity to the N-terminal domain of expansins and affect physiological processes such as water and ion homeostasis at nano-molar concentrations. Here we show that a recombinant Arabidopsis thaliana PNP (AtPNP-A) rapidly increased the rate of dark respiration in treated leaves after 5 min. In addition, we observed increases in lower leaves, and with a lag time of 10 min, the effect spread to the upper leaves and subsequently (after 15 min) to the opposite leaves. This response signature is indicative of phloem mobility of the signal, a hypothesis that was further strengthened by the fact that cold girdling, which affects phloem but not xylem or apoplastic processes, delayed the long distance AtPNP-A effect. We conclude that locally applied AtPNP-A can induce a phloem-mobile signal that rapidly modifies plant homeostasis in distal parts.     

AtPNP-A is a systemically mobile natriuretic peptide immunoanalogue with a role in Arabidopsis thaliana cell volume regulation

Cellular and physiological evidence suggests the presence of a novel class of systemically mobile plant molecules that are recognized by antibodies against vertebrate atrial natriuretic peptides (ANPs). In order to characterize the function of these immunoanalogues we have expressed the full-length recombinant (AtPNP-A[1-126]) and demonstrate that this molecule induces osmoticum-dependent H(2)O uptake into protoplasts at nanomolar concentrations and thus affects cell volume. A similar response is also seen with a recombinant that does not contain the signal peptide (AtPNP-A[26-126]) as well as a short domain (AtPNP-A[33-66]) that shows homology to the vertebrate peptide. Taken together, these findings suggest that AtPNP-A has an important and systemic role in plant growth and homeostasis.     

Co-expression and promoter content analyses assign a role in biotic and abiotic stress responses to plant natriuretic peptides

Background  

Plant natriuretic peptides (PNPs) are a class of systemically mobile molecules distantly related to expansins. While several physiological responses to PNPs have been reported, their biological role has remained elusive. Here we use a combination of expression correlation analysis, meta-analysis of gene expression profiles in response to specific stimuli and in selected mutants, and promoter content analysis to infer the biological role of the Arabidopsis thaliana PNP, AtPNP-A.     

Natriuretic peptides--a class of heterologous molecules in plants

Immunological and physiological evidence suggests the presence of biologically active natriuretic peptide hormones (NPs) in plants. Evidence includes specific binding of rat atrial NP, [rANP (99-126)] to plant membranes and the promotion of cyclic guanosine-3',5'-monophosphate (cGMP) mediated stomatal responses. Furthermore, anti-ANP affinity purifies biologically active plant immunoreactants (irPNPs) and a biologically active Arabidopsis thaliana irPNP (AtPNP-A) has been identified. AtPNP-A belongs to a novel class of molecules that share some similarity with the cell wall loosening expansins but do not contain the carbohydrate-binding wall anchor, thus suggesting that irPNPs and ANP are heterologues. We hypothesise that irPNP-like molecules have evolved from primitive glucanase-like molecules that have been recruited to become systemically mobile modulators of homeostasis acting via the plasma membrane. Such a function is compatible with localisation in the conductive tissue and the physiological and cellular modes of action of irPNPs reported to-date.     

SDIR1 is a RING finger E3 ligase that positively regulates stress-responsive abscisic acid signaling in Arabidopsis

Ubiquitination plays important roles in plant hormone signal transduction. We show that the RING finger E3 ligase, Arabidopsis thaliana SALT- AND DROUGHT-INDUCED RING FINGER1 (SDIR1), is involved in abscisic acid (ABA)-related stress signal transduction. SDIR1 is expressed in all tissues of Arabidopsis and is upregulated by drought and salt stress, but not by ABA. Plants expressing the ProSDIR1-beta-glucuronidase (GUS) reporter construct confirmed strong induction of GUS expression in stomatal guard cells and leaf mesophyll cells under drought stress. The green fluorescent protein-SDIR1 fusion protein is colocalized with intracellular membranes. We demonstrate that SDIR1 is an E3 ubiquitin ligase and that the RING finger conservation region is required for its activity. Overexpression of SDIR1 leads to ABA hypersensitivity and ABA-associated phenotypes, such as salt hypersensitivity in germination, enhanced ABA-induced stomatal closing, and enhanced drought tolerance. The expression levels of a number of key ABA and stress marker genes are altered both in SDIR1 overexpression and sdir1-1 mutant plants. Cross-complementation experiments showed that the ABA-INSENSITIVE5 (ABI5), ABRE BINDING FACTOR3 (ABF3), and ABF4 genes can rescue the ABA-insensitive phenotype of the sdir1-1 mutant, whereas SDIR1 could not rescue the abi5-1 mutant. This suggests that SDIR1 acts upstream of those basic leucine zipper family genes. Our results indicate that SDIR1 is a positive regulator of ABA signaling.     

The expression patterns of arabinogalactan-protein AtAGP30 and GLABRA2 reveal a role for abscisic acid in the early stages of root epidermal patterning

In the Arabidopsis root, patterning of the epidermal cell types is position-dependent. The epidermal cell pattern arises early during root development, and can be visualized using reporter genes driven by the GLABRA (GL)2 promoter as markers. The GL2 gene is preferentially expressed in the differentiating hairless cells (atrichoblasts) during a period in which epidermal cell identity is believed to be established. We show that AtAGP30 is also expressed in atrichoblasts. This gene encodes an arabinogalactan-protein (AGP) that is known to play a role in root regeneration and increases abscisic acid (ABA)-response rates. Although the expression level of this gene is regulated by the plant growth factors ABA and ethylene, only ABA was found to affect the tissue-specific pattern of expression. ABA also disrupts the expression pattern of the GL2::GUS (beta-glucuronidase) reporter gene. Our results indicate that ABA regulates epidermal cell-type-specific gene expression in the meristematic zone of the Arabidopsis root, while ethylene is known to act at later stages of epidermal differentiation. Despite its effects on the early stages of root epidermal patterning, ABA does not affect root hair formation on mature wild-type epidermal cells, suggesting that other developmental cues, like positional information, can progressively over-ride the ABA-mediated disruption of early epidermal patterning.     

Cloning and functional analysis of wheat V-H+-ATPase subunit genes

The root microsomal proteomes of salt-tolerant and salt-sensitive wheat lines under salt stress were analyzed by two-dimensional electrophoresis and mass spectrum. A wheat V-H(+)-ATPase E subunit protein was obtained whose expression was enhanced by salt stress. In silicon cloning identified the full-length cDNA sequences of nine subunits and partial cDNA sequences of two subunits of wheat V-H(+)-ATPase. The expression profiles of these V-H(+)-ATPase subunits in roots and leaves of both salt-tolerant and salt-sensitive wheat lines under salt and abscisic acid (ABA) stress were analyzed. The results indicate that the coordinated enhancement of the expression of V-H(+)-ATPase subunits under salt and ABA stress is an important factor determining improved salt tolerance in wheat. The expression of these subunits was tissue-specific. Overexpression of the E subunit by transgenic Arabidopsis thaliana was able to enhance seed germination, root growth and adult seedling growth under salt stress.