Aromatic Amino Acid-Dependent Expression of Indole-3-Pyruvate Decarboxylase Is Regulated by TyrR in Enterobacter cloacae UW5

Abstract

The plant growth-promoting rhizobacterium Enterobacter cloacae UW5 synthesizes the plant growth hormone indole-3-acetic acid (IAA) via the indole-3-pyruvate pathway utilizing the enzyme indole-3-pyruvate decarboxylase that is encoded by ipdC. In this bacterium, ipdC expression and IAA production occur in stationary phase and are induced by an exogenous source of tryptophan, conditions that are present in the rhizosphere. The aim of this study was to identify the regulatory protein that controls the expression of ipdC. We identified a sequence in the promoter region of ipdC that is highly similar to the recognition sequence for the Escherichia coli regulatory protein TyrR that regulates genes involved in aromatic amino acid transport and metabolism. Using a tyrR insertional mutant, we demonstrate that TyrR is required for IAA production and for induction of ipdC transcription. TyrR directly induces ipdC expression, as was determined by real-time quantitative reverse transcription-PCR, by ipdC promoter-driven reporter gene activity, and by electrophoretic mobility shift assays. Expression increases in response to tryptophan, phenylalanine, and tyrosine. This suggests that, in addition to its function in plant growth promotion, indolepyruvate decarboxylase may be important for aromatic amino acid uptake and/or metabolism.Auxins are an important class of phytohormones that are essential for many aspects of plant growth and development, including organogenesis; tropic responses; cellular processes such as cell expansion, division, and differentiation; and gene regulation (1,9,21,25,60). The predominant natural auxin is indole-3-acetic acid (IAA) (8,21,60). In addition to synthesis in plant tissues, many plant-associated bacteria also produce and secrete IAA that can influence the health of host plants. Production of IAA by some phytopathogenic bacteria causes plant diseases such as gall tumor formation by Agrobacterium tumefaciens, Erwinia herbicola pv. gypsophilae, and Pseudomonas syringae pv. savastanoi and necrotic lesions caused by Pseudomonas syringae pv. syringae (15,16,35,38,42,63). Loss of the ability to synthesize IAA, through mutagenesis, reduces the virulence of these pathogens (2, 16).Paradoxically, IAA produced by plant growth-promoting rhizobacteria (PGPR) has been found to enhance host root system development. Plant roots colonized with the PGPR species Azospirillum brasilense Sp6, Enterobacter cloacae UW5, and Pseudomonas putida GR12-2 displayed increases in root hair formation, the number and length of lateral roots, and/or primary root length that were dependent on bacterial IAA production. Mutants that were unable to synthesize IAA did not increase root proliferation (7,23,45,58,65). Well-developed root systems are important for natural nutrient uptake and for anchoring plants in soil. The differences in the effect of IAA produced by these two groups of bacteria may be due to differences in the levels of IAA production in planta or other contributing factors (46,57,65).A number of IAA biosynthetic ...

Auxin and Plant-Microbe Interactions

“…TyrR is a regulatory protein involved in transport and metabolism of aromatic amino acids. TyrR is directly involved in the induction of ipdC gene expression in E. cloacae in response to tryptophan, phenylalanine, and tyrosine, suggesting a broader function of indole-3-pyruvate decarboxylases (Ryu and Patten 2008).…”

Klebsiella variicola and Klebsiella quasipneumoniae with capacity to adapt to clinical and plant settings

“…32 This molecule also functions as an important signaling molecule in the regulation of plant growth, expansion, cell differentiation and cell division regulation. 33 K. variicola and K. pneumoniae isolates were capable of producing indole acetic acid (table II).…”

Salkowski’s Reagent Test as a Primary Screening Index for Functionalities of Rhizobacteria Isolated from Wild Dipterocarp Saplings Growing Naturally on Medium-Strongly Acidic Tropical Peat Soil

“…Since it is known that available cations of essential metals, including Fe, Cu, Mo, and Zn, are absolutely poor in fume-rich, woody peat soil, 12) the Fe-solubilizing activities of the rhizobacteria from Fe-chelates are important in the peatland ecosystem. Among 29 of bacterial isolates tested by CAS assay for iron-solubilizing activity (Table 3), four isolates, Pseudomonas sp.…”

“…10) Particularly, TRP transaminase, which catalyzes an oxidative deamination on TRP to yield indole-3-pyruvic acid (IPyA), was first reported in Enterobacter cloacae, which showed a remarkable capability of IAA production. 11,12) Hence Salkowski's reagent test, specified for the detection of IAA and its precursory compound IPyA, is often used for primary screening of IAA-producing rhizobacteria.…”