Many plant-associated bacteria synthesize the phytohormone indoleacetic acid (IAA). While IAA produced by phytopathogenic bacteria, mainly by the indoleacetamide pathway, has been implicated in the induction of plant tumors, it is not clear whether IAA synthesized by beneficial bacteria, usually via the indolepyruvic acid pathway, is involved in plant growth promotion. To determine whether bacterial IAA enhances root development in host plants, the ipdc gene that encodes indolepyruvate decarboxylase, a key enzyme in the indolepyruvic acid pathway, was isolated from the plant growth-promoting bacterium Pseudomonas putida GR12-2 and an IAA-deficient mutant constructed by insertional mutagenesis. The canola seedling primary roots from seeds treated with wild-type P. putida GR12-2 were on average 35 to 50% longer than the roots from seeds treated with the IAA-deficient mutant and the roots from uninoculated seeds. In addition, exposing mung bean cuttings to high levels of IAA by soaking them in a suspension of the wild-type strain stimulated the formation of many, very small, adventitious roots. Formation of fewer roots was stimulated by treatment with the IAA-deficient mutant. These results suggest that bacterial IAA plays a major role in the development of the host plant root system.

Role of Pseudomonas putida Indoleacetic Acid in Development of the Host Plant Root System

Polyphenol oxidases in plants

We have developed a new assay that differentiates between indoleacetic acid (IAA)-producing and -nonproducing bacteria on a colony plate lift. Medium supplemented with 5 mM L-tryptophan is inoculated with isolates of interest, overlaid with a nitrocellulose membrane, and then incubated until bacterial colonies reach 1 to 2 mm in diameter. The membrane is removed to a filter paper saturated with Salkowski reagent and incubated until distinct red haloes form around the colonies. The colorimetric reaction to IAA is limited to a region immediately surrounding each colony, is specific to isolates producing IAA, occurs within 1 h after the membrane is placed in the reagent, and is sensitive to as little as 50 pmol of IAA in a 2-mm2 spot. We have used this assay for quantifying epiphytic and endophytic populations of IAA-producing isolates of Pseudomonas syringae subsp. savastanoi and for detecting IAA-producing colonies of other pseudomonads and Erwinia herbicola. The assay provides a rapid and convenient method to screen large numbers of bacteria. Images

Rapid In Situ Assay for Indoleacetic Acid Production by Bacteria Immobilized on a Nitrocellulose Membrane

Cytochrome c oxidase is discussed as a possible photoacceptor when cells are irradiated with monochromatic red to near-IR radiation. Four primary action mechanisms are reviewed: changes in the redox properties of the respiratory chain components following photoexcitation of their electronic states, generation of singlet oxygen, localized transient heating of absorbing chromophores, and increased superoxide anion production with subsequent increase in concentration of the product of its dismutation, H2O2. A cascade of reactions connected with alteration in cellular homeostasis parameters (pHi, [Cai], cAMP, Eh, [ATP] and some others) is considered as a photosignal transduction and amplification chain in a cell (secondary mechanisms).

Primary and secondary mechanisms of action of visible to near-IR radiation on cells.

Polyphenol oxidases and peroxidases are among the most studied enzymes in fruits and vegetables. Owing to the deleterious effects of discoloration and off-flavor formation induced by their actions, these enzymes have not ceased to be a matter of concern to food technologists, while their versatility as catalyst and their diversity as protein present a challenge to the biochemist. This article gives an account on the present state of knowledge in this field. The occurrence of polyphenol oxidases and peroxidases in food and food raw materials, and their role and importance in food processing are briefly outlined. Results of biochemical research including catalytic properties, substrate specificity, susceptibility towards pH and temperature, action of inhibitors, isolation, purification, and characteristics of the enzymes are given, with special emphasis on recent achievements based on high resolution separation and isoenzyme techniques. Finally, the behavior of polyphenol oxidase and peroxidase in selected major groups of fruits and vegetables is discussed. Some contradictions found in the literature are pointed out and some questions that have not been given the necessary attention by researchers so far are mentioned.

Polyphenol oxidase and peroxidase in fruits and vegetables.

The effect of bilateral irradiation with white light (1000 Meter Candle Sec) on the basipetal transport of auxin has been investigated. Illumination of either the intact shoot or the excised coleoptile tip of the Zea seedling, decreased the amount of diffusible auxin obtained from the tip, and decreased Avena curvature response to unilaterally applied indoleacetic acid. Irradiation of the intact Zea seedling did not affect the absorption of 14C-labeled indoleacetic acid from an agar block subsequently placed on the decapitated coleoptile. However, light caused a significant decrease in the amount of labeled auxin basipetally transported, without affecting materially the velocity of that transport. These and other observations are interpreted as support for the hypothesis that the primary hormonal phenomenon in first-positive phototropism is a light-induced impairment in the basipetal transport of auxin.

Auxin Transport in Zea mays Coleoptiles II. Influence of Light on the Transport of Indoleacetic Acid-2-14C

The phytohormone auxin is widely recognized as a regulator of growth and morphogenesis. Since retarded growth and altered developmental patterns in plants are known to follow exposure to ionizing radiation, Skoog (26) investigated the inactivation of auxin by xand ?-rays. His data indicated that both native auxin preparations and pure 3-indoleacetic acid (IAA) in solution were highly labile when irradiated. A marked reduction in auxin levels also occurred when plant tissues were given moderate doses of x-radiation. Auxin thus appeared to be radiosensitive in vitro and in vivo. Skoog pointed out the parallel effects of decreased auxin levels and low doses of radiation in growth and correlation phenomena. His results were used subsequently by other workers to interpret various radiation responses on the basis of auxin radiosensitivity (10, 16, 27). However, certain considerations discussed below cast doubt on the implied direct sensitivity of the auxin molecule, and prompted our reinvestigation of auxin radiosensitivity. Concomitantly, a more precise evaluation of IAA inactivation kinetics was obtained. The present work deals with the destruction of auxin in aqueous solution by ionizing radiation. Auxin sensitivity in the plant, and the biochemical mechanisms whereby auxin levels are lowered in vivo, will be treated in subsequent papers of this series.

Studies on the Mechanism of Phytohormone Damage by Ionizing Radiation. I. The Radiosensitivity of Indoleacetic Acid.

The photoinduction of fruiting bodies by light of defined wavelengths was studied in the fungus Schizophyllum commune Fr. Several properties of the induction were established. (1) The exposure-response relationship for induced fruiting was determined for light of 448 nm. (2) The Bunsen-Roscoe Law of Reciprocity was found to hold for the photoinduction of fruiting bodies for the interval 36 to 2000 sec with light of 448 nm. (3) Light of wavelengths from 320 nm to 525 nm induced fruiting bodies. Although the photoreceptor is unknown, it may be a flavin rather than a carotenoid, because light in the near ultraviolet (350 nm-400 nm) was inductive. (4) Neither red light (660 nm) nor far-red light (730 nm) induced fruiting bodies or affected the sensitivity of the fungus toward photoinduction by blue light (448 nm).

http://www.plantphysiol.org/content/plantphysiol/44/12/1712.full.pdf

Morphogenesis in Schizophyllum commune. II. Effects of Monochromatic Light

The effect of phototropic stimulation of Zea coleoptile tips on the distribution of both endogenous indoleacetic acid (IAA) and applied C(14)-labeled IAA was determined. The tips rested on bisected agar blocks. More IAA was found in the blocks under the shaded side of the coleoptile tips than those under the irradiated side. However, no significant difference was observed between the total amounts of IAA, endogenous or labeled, in the irradiated and shaded sides of the experimental system. In addition, less endogenous auxin was found in the shaded tissues than in their irradiated counterparts. It is suggested that phototropism following unilateral irradiation with first positive radiant densities might be a consequence of lateral inequalities in the ability of the irradiated and shaded tissues to transport auxin basipetally.

Hormonal Relations in the Phototropic Response: III. The Movement of C14-labeled and Endogenous Indoleacetic Acid in Phototropically Stimulated Zea Coleoptiles

Solon A. Gordon

Papers

Auxin Transport in Zea mays Coleoptiles II. Influence of Light on the Transport of Indoleacetic Acid-2-14C

Studies on the Mechanism of Phytohormone Damage by Ionizing Radiation. I. The Radiosensitivity of Indoleacetic Acid.

Morphogenesis in Schizophyllum commune. II. Effects of Monochromatic Light

Hormonal Relations in the Phototropic Response: III. The Movement of C14-labeled and Endogenous Indoleacetic Acid in Phototropically Stimulated Zea Coleoptiles

Intracellular Localization of the Tryptophan-indoleacetate Enzyme System.