Production of the phytohormone indole-3-acetic acid (IAA) is widespread among bacteria that inhabit the rhizosphere of plants. Several different IAA biosynthesis pathways are used by these bacteria, with a single bacterial strain sometimes containing more than one pathway. The level of expression of IAA depends on the biosynthesis pathway; the location of the genes involved, either on chromosomal or plasmid DNA, and their regulatory sequences; and the presence of enzymes that can convert active, free IAA into an inactive, conjugated form. The role of bacterial IAA in the stimulation of plant growth and phytopathogenesis is considered.

Bacterial biosynthesis of indole-3-acetic acid

The plant hormones, auxins and cytokinins, are involved in several stages of plant growth and development such as cell elongation, cell division, tissue differentiation, and apical dominance. The biosynthesis and the underlying mechanism of auxins and cytokinins action are subjects of intense investigation. Not only plants but also microorganisms can synthesize auxins and cytokinins. The role of phytohormone biosynthesis by microorganisms is not fully elucidated: in several cases of pathogenic fungi and bacteria these compounds are involved in pathogenesis on plants; auxin and cytokinin production may also be involved in root growth stimulation by beneficial bacteria and associative symbiosis. The genetic mechanism of auxin biosynthesis and regulation by Pseudomonas, Agrobacterium, Rhizobium, Bradyrhizobium, and Azospirillum, are well studied; in these bacteria several physiological effects have been correlated to the bacterial phytohormones biosynthesis. The pathogenic bacteria Pseudomonas and Agrobacterium produce indole-3-acetic acid via the indole-3-acetamide pathway, for which the genes are plasmid borne. However, they do possess also the indole-3-pyruvic acid pathway, which is chromosomally encoded. In addition, they have genes that can conjugate free auxins or hydrolyze conjugated forms of auxins and cytokinins. In Agrobacterium there are also several genes, located near the auxin and cytokinin biosynthetic genes, that are involved in the regulation of auxins and cytokinins sensibility of the transformed plant tissue. Symbiotic bacteria Rhizobium and Bradyrhizobium synthesize indole-3-acetic acid via indole-3-pyruvic acid; also the genetic determinants for the indole-3-acetamide pathway have been detected, but their activity has not been demonstrated. In the plant growth-promoting bacterium Azospirillum, as in Agrobacterium and Pseudomonas, both the indole-3-pyruvic acid and the indole-3-acetamide pathways are present, although in Azospirillum the indole-3-pyruvic acid pathway is of major significance. In addition, biochemical evidence for a tryptophan-independent indole-3-acetic acid pathway in Azospirillum has been presented.

Synthesis of phytohormones by plant-associated bacteria

This paper will exemplify molecular communications in the rhizosphere, especially between plants and bacteria, and between bacteria and bacteria. More specifically, we describe signalling pathways that allow bacteria to sense a wide diversity of plant signals, plants to respond to bacterial infection, and bacteria to coordinate gene expression at population and community level. Thereafter, we focus on mechanisms evolved by bacteria and plants to disturb bacterial signalling, and by bacteria to modulate hormonal signalling in plants. Finally, the dynamics of signal exchange and its biological significance we elaborate on the cases of Rhizobium symbiosis and Agrobacterium pathogenesis.

Molecular communication in the rhizosphere

Tyramine poisoning is caused by the ingestion of food containing high levels of tyramine, a biogenic amine. Any foods containing free tyrosine are subject to tyramine formation if poor sanitation and low quality foods are used or if the food is subject to temperature abuse or extended storage time. Tyramine is generated by decarboxylation of the tyrosine through tyrosine decarboxylase (TDC) enzymes derived from the bacteria present in the food. Bacterial TDC have been only unequivocally identified and characterized in Gram-positive bacteria, especially in lactic acid bacteria. Pyridoxal phosphate (PLP)-dependent TDC encoding genes (tyrDC) appeared flanked by a similar genetic organization in several species of lactic acid bacteria, suggesting a common origin by a single mobile genetic element. Bacterial TDC are also able to decarboxylate phenylalanine to produce phenylethylamine (PEA), another biogenic amine. The molecular knowledge of the genes involved in tyramine production has led to the development of molecular methods for the detection of bacteria able to produce tyramine and PEA. These rapid and simple methods could be used for the analysis of the ability to form tyramine by bacteria in order to evaluate the potential risk of tyramine biosynthesis in food products.

/pdf/tyramine-and-phenylethylamine-biosynthesis-by-food-bacteria-1efegm1zdi.pdf

Tyramine and Phenylethylamine Biosynthesis by Food Bacteria

Microbial production of plant hormones

1. A strain of Bacillus cereus has been isolated that can produce tryptamine when grown in a broth containing tryptophan. 2. The conditions of culture under which this conversion is optimum, as well as the general pathways of tryptophan metabolism by this micro-organism, have been examined, and the information obtained has been used to obtain the first demonstration of cell-free tryptophan-carboxy-lyase activity. 3. The significance of these findings both to the current attempts to elucidate the pathways of metabolism of tryptophan in higher plants and to the published generalizations about the previously studied amino acid carboxy-lyases is discussed.

The production of tryptamine from tryptophan by Bacillus cereus (KVT).

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The production of tryptamine from tryptophan by Bacillus cereus (KVT).