Indole-3-acetic acid (IAA) is an important phytohormone with the capacity to control plant development in both beneficial and deleterious ways. The ability to synthesize IAA is an attribute that many bacteria including both plant growth-promoters and phytopathogens possess. There are three main pathways through which IAA is synthesized; the indole-3-pyruvic acid, indole-3-acetamide and indole-3-acetonitrile pathways. This chapter reviews the factors that effect the production of this phytohormone, the role of IAA in bacterial physiology and in plant–microbe interactions including phytostimulation and phytopathogenesis.

Indole-3-acetic acid in plant–microbe interactions

It is proposed that abiotic synthesis of organic compounds occurs in metastable states. These states are permitted by kinetic barriers which inhibit the approach to stable equilibrium in the C-H-O-N system. Evidence for metastable equilibrium among organic compounds in sedimentary basins is reviewed, and further evidence is elucidated from hydrous pyrolysis experiments reported in the literature. This analysis shows that at hydrothermal conditions, organic compounds are formed or destroyed primarily through oxidation/reduction reactions, and that the role of temperature is to lower the kinetic barriers to these reactions. These lines of evidence allow the development of a scenario in which abiotic synthesis can occur at hydrothermal conditions through the reduction of CO2 and N2. This scenario can be tested quantitatively with distribution of species calculations as functions of temperature, pressure, hydrogen fugacity (fH2) and initial composition. One example of such a test is given for an early, sudden outgassing of the Earth, in which CO2, H2O, and N2 are transported from the mantle to the atmosphere by hydrothermal solutions. Activities of metastable aqueous organic species which form as a consequence of this process are evaluated at conditions appropriate for seafloor hydrothermal systems, and are found to maximize at about 200 °C and between the oxidation states set by two mineral assemblages common in the oceanic crust.

Geochemical constraints on the origin of organic compounds in hydrothermal systems

Crystalline surfaces of common rock-forming minerals are likely to have played several important roles in life’s geochemical origins. Transition metal sulfides and oxides promote a variety of organic reactions, including nitrogen reduction, hydroformylation, amination, and Fischer-Tropsch-type synthesis. Fine-grained clay minerals and hydroxides facilitate lipid self-organization and condensation polymerization reactions, notably of RNA monomers. Surfaces of common rock-forming oxides, silicates, and carbonates select and concentrate specific amino acids, sugars, and other molecular species, while potentially enhancing their thermal stabilities. Chiral surfaces of these minerals also have been shown to separate left- and right-handed molecules. Thus, mineral surfaces may have contributed centrally to the linked prebiotic problems of containment and organization by promoting the transition from a dilute prebiotic “soup” to highly ordered local domains of key biomolecules.

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Mineral Surfaces, Geochemical Complexities, and the Origins of Life

Publisher Summary This chapter discusses the evidence that in Avena coleoptiles and pea stem tissues the hormone auxin induces cell enlargement, by activating proton extrusion, and that the resulting acidification of the wall leads to enzymic cell wall loosening and thus cell enlargement. It emphasizes the point that auxin itself does not act directly on the wall but acts at the cell surface or within the cytoplasm. When a plant cell enlarges, most of the increase in volume is due to the uptake of water into an expanding–centrally located vacuole. The direction of the cell expansion depends on the molecular architecture of the wall and can be primarily in one direction or equally in all directions. Cell enlargement can be initiated in one of the two ways: by an increase in the osmotic concentration of the cell or by an increase in cell wall extensibility (cell wall loosening). In most systems, cell enlargement is initiated by wall loosening. If a section is removed from the elongation zone of a pea stem and placed in water, the cells enlarge, but only at a slow rate. Rapid enlargement can be induced by only four agents: the group of hormones called auxins, hydrogen ions, CO, and the phytotoxin fusicoccin. If hydrogen ions are excreted from auxin-treated Avena coleoptile cells, and if the pH of the wall region falls below 5.8, the walls would undergo cell wall loosening and growth would occur.

Control of plant cell enlargement by hydrogen ions.

Marine benthic diatoms excrete large quantities of extracellular polymeric substances (EPS), both as a function of their motility system and as a response to environmental conditions. Diatom EPS consists predominantly of carbohydrate-rich polymers and is important in the ecology of cells living on marine sediments. Production rates, production pathways, and monosaccharide composition of water-soluble (colloidal) carbohydrates, EPS, and intracellular storage carbohydrate (glucans) were investigated in the epipelic (mud-inhabiting) diatoms Cylindrotheca closterium (Ehrenburg), Navicula perminta (Grun.) in Van Heurck, and Amphora exigua Greg. under a range of experimental conditions simulating aspects of the natural environment. Cellular rates of colloidal carbohydrate, EPS, and glucan production were significantly higher during nutrient-replete compared with nutrient-limited growth for all three species. The proportion of EPS in the extracellular carbohydrate pool increased significantly (to 44%-69%) as cells became nutrient limited. Cylindrotheca closterium produced two types of EPS differing in sugar composition and production patterns. Nutrient-replete cells produced a complex EPS containing rhamnose, fucose, xylose, mannose, galactose, glucose, and uronic acids. Nutrient-limited cells produced an additional EPS containing mannose, galactose, glucose, and uronic acids. Both EPS types were produced under illuminated and darkened conditions. 1 4 C-labeling revealed immediate production of 1 4 C-glucan and significant increases in 1 4 C-EPS between 3 and 4 h after addition of label. The glucan synthesis inhibitor 2,6-dichlorobenzonitrile significantly reduced 1 4 C-colloidal carbohydrate and 1 4 C-EPS. The glucanase inhibitor P-nitrophenyl β-D-gluco-pyranoside resulted in accumulation of glucan within cells and lowered rates of 1 4 C-colloidal and 1 4 C-EPS production. Cycloheximide prevented glucan catabolism, but glucan production and EPS synthesis were unaffected.

Environmental effects on exopolymer production by marine benthic diatoms: Dynamics, changes in composition, and pathways of production

Several cell wall-bound glycosidases present in Avena sativa coleoptiles were assayed by following the hydrolysis of p-nitrophenyl-glycosides. Particular emphasis was placed on characterizing some parameters affecting the activity of beta-galactosidase. The pH optimum of this enzyme is 4.5 to 5.5; it is sensitive to copper ions and p-chloromercuribenzoate treatment and apparently has an exceptionally low turnover rate. Indoleacetic acid treatment enhanced in vivo beta-galactosidase activity of coleoptile segments by 36% over control after 60 minutes. This enhancement was prevented by abscisic acid and cycloheximide. High buffer strengths and low pH reduced the indoleacetic acid-enhanced increase in enzyme activity. These data lend support to the following proposed model of indoleacetic acid action. Indoleacetic acid enhances the release of hydrogen ions into the cell wall which promote the activities of cell wall glycosidases, some of which may participate in the cell extension process.

Activation of Avena Coleoptile Cell Wall Glycosidases by Hydrogen Ions and Auxin

β-FeOOH · Cln (akaganéite) and Fe1-xO (wüstite) in hot brine from the Atlantis II Deep (Red Sea) and the uptake of amino acids by synthetic β-FeOOH · Cln

The possibility of an auxin effect on the permeability of pea (Pisum sativum L. ev. Alaska) segments to tritium-labeled water has been investigated by three separate laboratories, and the combined results are presented. We were unable to obtain any indication of a rapid effect of indoleacetic acid on the efflux of 3HHO when pea segments previously “loaded” for 90 minutes with 3HHO were transferred to unlabeled aqueous medium with indoleacetic acid. We were able to confirm that segments pretreated with 3HHO plus indoleacetic acid for 60 to 90 minutes can show an enhanced 3HHO release as compared with minus indoleacetic acid controls. However, this phenomenon appears to be due to an increased uptake of 3HHO during the prolonged indoleacetic acid pretreatment, and therefore we conclude that auxin does not alter the permeability of pea segments to 3HHO in either short term or long term tests. We confirm previous reports that the uptake of 3HHO in pea segments proceeds largely through the cut surfaces, and that the cuticle is a potent barrier to 3HHO flux.

Auxin Does Not Alter the Permeability of Pea Segments to Tritium-labeled Water

b-Feooh (Akaganeite) in Red Sea brine

Laboratory analysis of Red Sea Atlantis II Deep brine solutions revealed a significant thiocyanate concentration (2.4 × 10−5M) Amino acid analysis revealed large quantities of glycine (about 1 micromole per liter) and no other dissolved free amino acids. This brine and the underlying sediment constitute a reducing, acidic, clay rich, anhydrous to hydrous gradient system that can vary in temperature from 1100°C underlying magma to 63°C water. Since these conditions occur along a sea floor spreading zone there may be an important link between the origin and evolution of the Earth’s crust and chemical evolution.

Michael J. Dowler

Papers

Activation of Avena Coleoptile Cell Wall Glycosidases by Hydrogen Ions and Auxin

β-FeOOH · Cln (akaganéite) and Fe1-xO (wüstite) in hot brine from the Atlantis II Deep (Red Sea) and the uptake of amino acids by synthetic β-FeOOH · Cln

Auxin Does Not Alter the Permeability of Pea Segments to Tritium-labeled Water

b-Feooh (Akaganeite) in Red Sea brine

Chemical Evolution and Plate Tectonics