Effect of secondary metabolites associated with anaerobic soil conditions on ion fluxes and electrophysiology in barley roots

The effects of secondary metabolites produced by waterlogged soils on net K(+), H(+), and Ca(2+) fluxes were studied in the mature zone of roots of two barley (Hordeum vulgare) cultivars contrasting in their waterlogging (WL) tolerance using the noninvasive microelectrode ion flux measuring technique. In WL-sensitive variety 'Naso Nijo', all three lower monocarboxylic acids (formic, acetic, and propionic acids) and three phenolic acids (benzoic, 2-hydroxybenzoic, 4-hydroxybenzoic acids) caused a substantial shift toward steady K(+) efflux, accompanied by an immediate net influx of H(+). Detrimental effects of secondary metabolites on K(+) homeostasis in root cells were absent in WL-tolerant 'TX' variety. Root treatment with Mn(2+) caused only a temporary K(+) loss that returned to the initial level 10 min after treatment. Phenolic acids slightly increased Ca(2+) influx immediately after treatment, while other metabolites tested resulted in transient Ca(2+) efflux from the root. In the long-term (24 h) treatment, all metabolites tested significantly reduced K(+) uptake and the adverse effects of phenolic acids were smaller than for monocarboxylic acids and Mn(2+). Treatment with monocarboxylic acids for 24 h shifted H(+) from net efflux to net influx, while all three phenolic acids did not cause significant effects compared with the control. Based on results of pharmacological experiments and membrane potential measurements, a model explaining the effects of secondary metabolites on membrane transport activity is proposed. We also suggest that plant tolerance to these secondary metabolites could be considered a useful trait in breeding programs.     

Amino acids regulate salinity-induced potassium efflux in barley root epidermis

The amino acid content increases substantially in salt-stressed plants. The physiological relevance of this phenomenon remains largely unknown. Using the MIFE ion flux measuring technique, we studied the effects of physiologically relevant concentrations of 26 amino acids on NaCl-induced K⁺ flux from barley root epidermis. We show that 21 (of 26) amino acids caused a significant mitigation of the NaCl-induced K⁺ efflux, while valine and ornithine substantially enhanced the detrimental effects of salinity on K⁺ homeostasis. Our results suggest that physiologically relevant concentrations of free amino acids might contribute to plant adaptive responses to salinity by regulating K⁺ transport across the plasma membrane, thus enabling maintenance of an optimal K⁺/Na⁺ ratio as opposed to being merely a symptom of plant damage by stress. Investigating the specific mechanisms of such amelioration remains a key issue for future studies.     

Evolutionary origins of Brassicaceae specific genes in Arabidopsis thaliana

Background  

All sequenced genomes contain a proportion of lineage-specific genes, which exhibit no sequence similarity to any genes outside the lineage. Despite their prevalence, the origins and functions of most lineage-specific genes remain largely unknown. As more genomes are sequenced opportunities for understanding evolutionary origins and functions of lineage-specific genes are increasing.     

Salinity-induced ion flux patterns from the excised roots of <Emphasis Type="Italic">Arabidopsis sos</Emphasis> mutants

The SOS signal-transduction pathway is known to be important for ion homeostasis and salt tolerance in plants. However, there is a lack of in planta electrophysiological data about how the changes in signalling and ion transport activity are integrated at the cellular and tissue level. In this study, using the non-invasive ion flux MIFE technique, we compared net K⁺, H⁺ and Na⁺ fluxes from elongation and mature root zones of Arabidopsis wild type Columbia and sos mutants. Our results can be summarised as follows: (1) SOS mutations affect the function of the entire root, not just the root apex; (2) SOS signalling pathway is highly branched; (3) Na⁺ effects on SOS1 may by-pass the SOS2/SOS3 complex in the root apex; (4) SOS mutation affects H⁺ transport even in the absence of salt stress; (5) SOS1 mutation affects intracellular K⁺ homeostasis with a plasma membrane depolarisation-activated outward-rectifying K⁺ channel being a likely target; (6) H⁺ pump also may be a target of SOS signalling. We provide an improved model of SOS signalling and discuss physiological mechanisms underlying salt stress perception and signalling in plants. Our work shows that in planta studies are essential for understanding the functional genomics of plant salt tolerance.     

The synthesis of short- and medium-chain-length poly(hydroxyalkanoate) mixtures from glucose- or alkanoic acid-grown <Emphasis Type="Italic">Pseudomonas oleovorans</Emphasis>

Pseudomonas oleovorans NRRL B-778 accumulated mixtures of poly-3-hydroxybutyrate (PHB) and medium-chain-length poly(hydroxyalkanoates) (mcl-PHAs) when grown on glucose, octanoic acid or oleic acid, whereas growth on nonanoic acid or undecanoic acid resulted in copolymers of poly-3-hydroxybutyrate-co-3-hydroxyvalerate (PHB-co-HV). Acetone fractionation verified the presence of PHB/mcl-PHA mixtures. The acetone-insoluble (AIS) fractions of the polymers derived from glucose (PHA-glucose), octanoic acid (PHA-octanoic) and oleic acid (PHA-oleic) were exclusively PHB while the acetone-soluble (AS) fractions contained mcl-PHA composed of differing ratios of 3-hydroxy-acid monomer units, which ranged in chain length from 6 to 14 carbon atoms. In contrast, both the AIS and AS fractions from the polymers derived from nonanoic acid (PHA-nonanoic) and undecanoic acid (PHA-undecanoic) were composed of comparable ratios of 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV). The unfractionated PHA-glucose, PHA-octanoic and PHA-oleic polymers had melting temperatures (T _m) between 177 and 179°C, enthalpies of fusion (ΔH _f) of 20 cal/g and glass transition temperatures (T _g) of 3–4°C. This was due to the large PHB content in the polymer mixtures. On the other hand, the PHA-nonanoic and PHA-undecanoic polymers had thermal properties that supported their copolymer nature. In both cases, the T _m values were 161°C, ΔH _f values were 7cal/g and T _g values were −3°C. Journal of Industrial Microbiology & Biotechnology (2002) 28, 147–153 DOI: 10.1038/sj/jim/7000231 Received 30 July 2001/ Accepted in revised form 04 November 2001     

Evolution of anaerobic ciliates from the gastrointestinal tract: phylogenetic analysis of the ribosomal repeat from Nyctotherus ovalis and its relatives

The 18S and 5.8S rDNA genes and the internal transcribed spacers ITS-1 and ITS-2 of ciliates living in the hindgut of frogs, millipedes, and cockroaches were analyzed in order to study the evolution of intestinal protists. All ciliates studied here belong to the genus Nycrotherus. Phylogenetic analysis revealed that these ciliates from a monophyletic group that includes the distantly related anaerobic free-living heterotrichous ciliates Metopus palaeformis and Metopus contortus. The intestinal ciliates from the different vertebrate and invertebrate hosts are clearly divergent at the level of their rDNA repeats. This argues for the antiquity of the associations and a predominantly vertical transmission. This mode of transmission seems to be controlled primarily by the behavior of the host. The different degrees of divergence between ciliates living in different strains of one and the same cockroach species most likely reflect the different geographical origins of the hosts. In addition, host switches must have occurred during the evolution of cockroaches, since identical ciliates were found only in distantly related hosts. These phenomena prevent the reconstruction of potential cospeciation events.      

Mechanisms of thaxtomin A-induced root toxicity revealed by a thaxtomin A sensitive <Emphasis Type="Italic">Arabidopsis</Emphasis> mutant (<Emphasis Type="Italic">ucu2</Emphasis>-<Emphasis Type="Italic">2/gi</Emphasis>-<Emphasis Type="Italic">2</Emphasis>)

Key message

The Arabidopsis mutant ( ucu2 - 2/gi - 2 ) is thaxtomin A, isoxaben and NPA-sensitive indicated by root growth and ion flux responses providing new insights into these compounds mode of action and interactions.

Abstract

Thaxtomin A (TA) is a cellulose biosynthetic inhibitor (CBI) that promotes plant cell hypertrophy and cell death. Electrophysiological analysis of steady-state K⁺ and Ca²⁺ fluxes in Arabidopsis thaliana roots pretreated with TA for 24 h indicated a disturbance in the regulation of ion movement across the plant cell membrane. The observed inability to control solute movement, recorded in rapidly growing meristematic and elongation root zones, may partly explain typical root toxicity responses to TA treatment. Of note, the TA-sensitive mutant (ucu2-2/gi-2) was more susceptible with K⁺ and Ca²⁺ fluxes altered between 1.3 and eightfold compared to the wild-type control where fluxes altered between 1.2 and threefold. Root growth inhibition assays showed that the ucu2-2/gi-2 mutant had an increased sensitivity to the auxin 2,4-D, but not IAA or NAA; it also had increased sensitivity to the auxin efflux transport inhibitor, 1-naphthylphthalamic acid (NPA), but not 2,3,5- Triiodobenzoic acid (TIBA), when compared to the WT. The NPA sensitivity data were supported by electrophysiological analysis of H⁺ fluxes in the mature (but not elongation) root zone. Increased sensitivity to the CBI, isoxaben (IXB), but not dichlobenil was recorded. Increased sensitivity to both TA and IXB corresponded with higher levels of accumulation of these toxins in the root tissue, compared to the WT. Further root growth inhibition assays showed no altered sensitivity of ucu2-2/gi-2 to two other plant pathogen toxins, alternariol and fusaric acid. Identification of a TA-sensitive Arabidopsis mutant provides further insight into how this CBI toxin interacts with plant cells.