Induced systemic resistance (ISR) in plants: mechanism of action

Plants possess a range of active defense apparatuses that can be actively expressed in response to biotic stresses (pathogens and parasites) of various scales (ranging from microscopic viruses to phytophagous insect). The timing of this defense response is critical and reflects on the difference between coping and succumbing to such biotic challenge of necrotizing pathogens/parasites. If defense mechanisms are triggered by a stimulus prior to infection by a plant pathogen, disease can be reduced. Induced resistance is a state of enhanced defensive capacity developed by a plant when appropriately stimulated. Systemic acquired resistance (SAR) and induced systemic resistance (ISR) are two forms of induced resistance wherein plant defenses are preconditioned by prior infection or treatment that results in resistance against subsequent challenge by a pathogen or parasite. Selected strains of plant growth-promoting rhizobacteria (PGPR) suppress diseases by antagonism between the bacteria and soil-borne pathogens as well as by inducing a systemic resistance in plant against both root and foliar pathogens. Rhizobacteria mediated ISR resembles that of pathogen induced SAR in that both types of induced resistance render uninfected plant parts more resistant towards a broad spectrum of plant pathogens. Several rhizobacteria trigger the salicylic acid (SA)-dependent SAR pathway by producing SA at the root surface whereas other rhizobacteria trigger different signaling pathway independent of SA. The existence of SA-independent ISR pathway has been studied in Arabidopsis thaliana, which is dependent on jasmonic acid (JA) and ethylene signaling. Specific Pseudomonas strains induce systemic resistance in viz., carnation, cucumber, radish, tobacco, and Arabidopsis, as evidenced by an enhanced defensive capacity upon challenge inoculation. Combination of ISR and SAR can increase protection against pathogens that are resisted through both pathways besides extended protection to a broader spectrum of pathogens than ISR/SAR alone. Beside Pseudomonas strains, ISR is conducted by Bacillus spp. wherein published results show that several specific strains of species B. amyloliquifaciens, B. subtilis, B. pasteurii, B. cereus, B. pumilus, B. mycoides, and B.sphaericus elicit significant reduction in the incidence or severity of various diseases on a diversity of hosts.     

A conserved mechanism for nitrile metabolism in bacteria and plants

Pseudomonas fluorescens SBW25 is a plant growth-promoting bacterium that efficiently colonises the leaf surfaces and rhizosphere of a range of plants. Previous studies have identified a putative plant-induced nitrilase gene ( pinA ) in P. fluorescens SBW25 that is expressed in the rhizosphere of sugar beet plants. Nitrilase enzymes have been characterised in plants, bacteria and fungi and are thought to be important in detoxification of nitriles, utilisation of nitrogen and synthesis of plant hormones. We reveal that pinA is a NIT4-type nitrilase that catalyses the hydrolysis of β-cyano- l -alanine, a nitrile common in the plant environment and an intermediate in the cyanide detoxification pathway in plants. In plants cyanide is converted to β-cyano- l -alanine, which is subsequently detoxified to aspartic acid and ammonia by NIT4. In P. fluorescens SBW25 pinA is induced in the presence of β-cyano- l -alanine, and the β-cyano- l -alanine precursors cyanide and cysteine. pinA allows P. fluorescens SBW25 to use β-cyano- l -alanine as a nitrogen source and to tolerate toxic concentrations of this nitrile. In addition, pinA is shown to complement a NIT4 mutation in Arabidopsis thaliana , enabling plants to grow in concentrations of β-cyano- l -alanine that would otherwise prove lethal. Interestingly, over-expression of pinA in wild-type A. thaliana not only resulted in increased growth in high concentrations of β-cyano- l -alanine, but also resulted in increased root elongation in the absence of exogenous β-cyano- l -alanine, demonstrating that β-cyano- l -alanine nitrilase activity can have a significant effect on root physiology and root development.     

Glutamate in plants: metabolism, regulation, and signalling

Glutamate occupies a central position in amino acid metabolism in plants. The acidic amino acid is formed by the action of glutamate synthase, utilizing glutamine and 2-oxoglutarate. However, glutamate is also the substrate for the synthesis of glutamine from ammonia, catalysed by glutamine synthetase. The alpha-amino group of glutamate may be transferred to other amino acids by the action of a wide range of multispecific aminotransferases. In addition, both the carbon skeleton and alpha-amino group of glutamate form the basis for the synthesis of gamma-aminobutyric acid, arginine, and proline. Finally, glutamate may be deaminated by glutamate dehydrogenase to form ammonia and 2-oxoglutarate. The possibility that the cellular concentrations of glutamate within the plant are homeostatically regulated by the combined action of these pathways is examined. Evidence that the well-known signalling properties of glutamate in animals may also extend to the plant kingdom is reviewed. The existence in plants of glutamate-activated ion channels and their possible relationship to the GLR gene family that is homologous to ionotropic glutamate receptors (iGluRs) in animals are discussed. Glutamate signalling is examined from an evolutionary perspective, and the roles it might play in plants, both in endogenous signalling pathways and in determining the capacity of the root to respond to sources of organic N in the soil, are considered.     

Vitamin E: action, metabolism and perspectives

Natural vitamin E includes four tocopherols and four tocotrienols. RRR-alpha-tocopherol is the most abundant form in nature and has the highest biological activity. Although vitamin E is the main lipid-soluble antioxidant in the body, not all its properties can be assigned to this action. As antioxidant, vitamin E acts in cell membranes where prevents the propagation of free radical reactions, although it has been also shown to have pro-oxidant activity. Non-radical oxidation products are formed by the reaction between alpha-tocopheryl radical and other free radicals, which are conjugated to glucuronic acid and excreted through the bile or urine. Vitamin E is transported in plasma lipoproteins. After its intestinal absorption vitamin E is packaged into chylomicrons, which along the lymphatic pathway are secreted into the systemic circulation. By the action of lipoprotein lipase (LPL), part of the tocopherols transported in chylomicrons are taken up by extrahepatic tissues, and the remnant chylomicrons transport the remaining tocopherols to the liver. Here, by the action of the "alpha-tocopherol transfer protein", a major proportion of alpha-tocopherol is incorporated into nascent very low density lipoproteins (VLDL), whereas the excess of alpha-tocopherol plus the other forms of vitamin E are excreted in bile. Once secreted into the circulation, VLDL are converted into IDL and LDL by the action of LPL, and the excess of surface components, including alpha-tocopherol, are transferred to HDL. Besides the LPL action, the delivery of alpha-tocopherol to tissues takes place by the uptake of lipoproteins by different tissues throughout their corresponding receptors. Although we have already a substantial information on the action, effects and metabolism of vitamin E, there are still several questions open. The most intriguing is its interaction with other antioxidants that may explain how foods containing small amounts of vitamin E provide greater benefits than larger doses of vitamin E alone.     

Vitamin E: action, metabolism and perspectives

Natural vitamin E includes four tocopherols and four tocotrienols. RRR-α-tocopherol is the most abundant form in nature and has the highest biological activity. Although vitamin E is the main lipid-soluble antioxidant in the body, not all its properties can be assigned to this action. As antioxidant, vitamin E acts in cell membranes where prevents the propagation of free radical reactions, although it has been also shown to have pro-oxidant activity. Non-radical oxidation products are formed by the reaction between α-tocopheryl radical and other free radicals, which are conjugated to glucuronic acid and excreted through the bile or urine. Vitamin E is transported in plasma lipoproteins. After its intestinal absorption vitamin E is packaged into chylomicrons, which along the lymphatic pathway are secreted into the systemic circulation. By the action of lipoprotein lipase (LPL), part of the tocopherols transported in chylomicrons are taken up by extrahepatic tissues, and the remnant chylomicrons transport the remaining tocopherols to the liver. Here, by the action of the “α-tocopherol transfer protein”, a major proportion of α-tocopherol is incorporated into nascent very low density lipoproteins (VLDL), whereas the excess of α-tocopherol plus the other forms of vitamin E are excreted in bile. Once secreted into the circulation, VLDL are converted into IDL and LDL by the action of LPL, and the excess of surface components, including α-tocopherol, are transferred to HDL. Besides the LPL action, the delivery of α-tocopherol to tissues takes place by the uptake of lipoproteins by different tissues throughout their corresponding receptors. Although we have already a substantial information on the action, effects and metabolism of vitamin E, there are still several questions open. The most intriguing is its interaction with other antioxidants that may explain how foods containing small amounts of vitamin E provide greater benefits than larger doses of vitamin E alone.     

Acetylcholine receptor: effects of proteolysis on receptor metabolism

Previous studies (Miskin, R., T. G. Easton, and E. Reich, 1970, Cell. 15:1301-1312) have shown that sarcoma virus transformation and tumor promoters reduced the cell surface concentration of acetylcholine receptors (AChR) in differentiating chick embryo myogenic cultures. Both of these agents also induced high rates of plasminogen activator (PA) synthesis in myogenic cultures (Miskin, R., T. G. Easton, A. Maelicke, and E. Reich, 1978, Cell. 15:1287-1300), and the present work was performed to establish whether proteolysis might significantly affect receptor metabolism. Proteolysis in myogenic cultures was modulated by one or more of the following: stimulation of PA synthesis, direct addition of plasmin, removal of plasminogen, or addition of plasmin inhibitors. The results were: (a) When the rates of proteolysis were raised either by addition of plasmin or by stimulating PA synthesis in the presence of plasminogen, both the steady-state concentration and the half-life of surface AChR decreased, but the rate of receptor synthesis was unaffected. (b) The magnitude of these effects, and their dependence on added plasminogen, indicated that proteolysis initiated by plasminogen activation could account almost entirely for the reduction in receptor half-life produced by sarcoma virus transformation and phorbol ester. (c) The rate of receptor synthesis, which is also reduced by viral transformation and tumor promoters, was not modified by proteolysis; hence plasmin action may be responsible for a large part, but not all of the change in surface receptor under these conditions. (d) The plasmin catalysed changes in receptor parameters appear to occur in response to modified membrane metabolism resulting from proteolysis of surface components other than AChR itself.     

Dynamin: characteristics,mechanism of action and function

Dynamin - a member of the GTP-ase protein family - is essential for many intracellular membrane trafficking events in multiple endocytic processes. The unique biochemical features of dynamin - especially its propensity to assemble - enable severing the nascent vesicles from the membrane. The mechanism of dynamin's action is still a subject of debate - whether it functions as a mechanochemical enzyme or a regulatory GTPase. The GTPase domain of dynamin contains three GTP-binding motifs. This domain is very conservative across the species, including that recently cloned by us in the unicellular eukaryote Paramecium. Dynamin interacts with a number of partners such as endophilin and proteins involved in coordination of endocytosis with motor molecules. A growing body of evidence indicates that dynamin and dynamin-related proteins are involved both in pathology and protection against human diseases. The most interesting are dynamin-like Mx proteins exhibiting antiviral activity.     

Acetylcholine and oxidative metabolism in septum and hippocampus in vitro

Regulation of acetylcholine metabolism varied in brain slices from hippocampus and septum which have different proportions of cholinergic nerve cell bodies and nerve endings. Anoxia (0% oxygen) inhibited acetylcholine synthesis (-77%) and its calcium-dependent release (-87%) from hippocampal slices but had no effect on synthesis or release by septal slices. [1,5-14C]Citrate incorporation into acetylcholine was higher in septum than in hippocampus, which suggested that citrate metabolism differs regionally. (-)Hydroxycitrate, a specific inhibitor of ATP citrate (pro3S)-lyase (EC 4.1.3.8), reduced [U-14C]glucose incorporation into acetylcholine more in septal than in hippocampal slices. 14CO2 production from glucose or citrate was similar in control and experimental conditions in the two regions. These findings indicate that acetylcholine metabolism varies regionally, which may partially explain the selective vulnerability of certain brain areas to anoxia and other metabolic insults.     

Herbivory-induced signalling in plants: perception and action

Plants and herbivores have been interacting for millions of years. Over time, plants have evolved mechanisms to defend against herbivore attacks. Herbivore-challenged plants reconfigure their metabolism to produce compounds that are toxic, repellant or anti-digestive for the herbivores. Some compounds are volatile signals that attract the predators of herbivores. All these responses are tightly regulated by a signalling network triggered by the plant's perception machinery. Several compounds that specifically elicit herbivory-induced responses in plants have been isolated from herbivore oral secretions and oviposition fluids. Elicitor perception is rapidly followed by cell membrane depolarization, calcium influx and mitogen-activated protein kinase (MAPK) activation; plants also elevate the concentrations of reactive oxygen and nitrogen species, and modulate phytohormone levels accordingly. In addition to these reactions in the herbivore-attacked regions of a leaf, defence responses are also mounted in unattacked parts of the attacked leaf and as well in unattacked leaves. In this review, we summarize recent progress in understanding how plants recognize herbivory, the involvement of several important signalling pathways that mediate the responses to herbivore attack and the signals that transduce local into systemic responses.     

Iron and heme metabolism in Plasmodium falciparum and the mechanism of action of artemisinins

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