Tsune Kosuge

Bio: Tsune Kosuge is an academic researcher from University of California, Davis. The author has contributed to research in topics: Tryptophan & Phenylalanine. The author has an hindex of 15, co-authored 23 publications receiving 814 citations.

The rôle of indole-3-acetic acid accumulation by alpha methyl tryptophan-resistant mutants of Pseudomonas savastanoi in gall formation on oleanders

Abstract: Indole-3-acetic acid (IAA) accumulates in the culture medium of wild-type isolates of Pseudomonas savastanoi, the olive and oleander knot pathogen, and has been implicated as a factor in gall formation. Mutants of P. savastanoi were obtained by selecting for resistance to α-methyl tryptophan (MT), an analogue of tryptophan, which in other bacteria is known to be a feedback inhibitor of anthranilate synthetase, a regulatory enzyme in the tryptophan biosynthetic pathway. Thus, MT-resistant mutants may exhibit altered synthesis of tryptophan and/or IAA which is a product of tryptophan metabolism. The mutants we selected either failed to accumulate IAA or accumulated about twice the quantity accumulated by wildtype parent isolates. Mutants which did not accumulate IAA lacked tryptophan oxidative decarboxylase activity in cell-free preparations. All parent and mutant isolates exhibited similar growth patterns when infiltrated into oleander leaves. However, dissimilar symptom development was observed on oleanders inoculated with these isolates. Isolates that accumulated IAA caused formation of galls, while those which did not accumulate IAA failed to incite gall formation.

Secretion of Zeatin, Ribosylzeatin, and Ribosyl-1'' -Methylzeatin by Pseudomonas savastanoi: Plasmid-Coded Cytokinin Biosynthesis.

Abstract: Cytokinin production by strains of the phytopathogenic bacterium Pseudomonas syringae pv savastanoi was measured by immunoaffinity chromatography of the culture medium on immobilized anti-cytokinin antibodies, followed by high performance liquid chromatography, radioimmunoassay and mass spectrometry. P. savastanoi strain PB213-2 secretes zeatin (80 nanograms per milliliter) and ribosylzeatin (80 nanograms per milliliter). Even higher levels of zeatin (400 nanograms per milliliter) are produced by the olive-specific strain EW1006, which also produces 180 nanograms per milliliter of the recently identified cytokinin, ribosyl-1″ -methylzeatin. The amounts secreted were approximately 1000 times greater than those secreted by Agrobacterium tumefaciens (DA Regier, RO Morris 1982 Biochem Biophys Res Commun 104: 1560-1566). Examination of cytokinin production by plasmid deletion mutants of PB213-2 and EW1006 indicated that cytokinin biosynthesis was specified, at least in part, by plasmid-borne genes. A fragment of the 105 kilobase pair plasmid from EW1006 was cloned into Escherichia coli where its expression resulted in dimethylallyl transferase activity and the secretion of zeatin.

Bacterial volatiles promote growth in Arabidopsis.

Abstract: Several chemical changes in soil are associated with plant growth-promoting rhizobacteria (PGPR). Some bacterial strains directly regulate plant physiology by mimicking synthesis of plant hormones, whereas others increase mineral and nitrogen availability in the soil as a way to augment growth. Identification of bacterial chemical messengers that trigger growth promotion has been limited in part by the understanding of how plants respond to external stimuli. With an increasing appreciation of how volatile organic compounds signal plants and serve in plant defense, investigations into the role of volatile components in plant–bacterial systems now can follow. Here, we present chemical and plant-growth data showing that some PGPR release a blend of volatile components that promote growth of Arabidopsis thaliana. In particular, the volatile components 2,3-butanediol and acetoin were released exclusively from two bacterial strains that trigger the greatest level of growth promotion. Furthermore, pharmacological applications of 2,3-butanediol enhanced plant growth whereas bacterial mutants blocked in 2,3-butanediol and acetoin synthesis were devoid in this growth-promotion capacity. The demonstration that PGPR strains release different volatile blends and that plant growth is stimulated by differences in these volatile blends establishes an additional function for volatile organic compounds as signaling molecules mediating plant–microbe interactions.

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

Abstract: 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

Bacterial biosynthesis of indole-3-acetic acid

Abstract: 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.

Auxin biosynthesis by the YUCCA flavin monooxygenases controls the formation of floral organs and vascular tissues in Arabidopsis

Abstract: Auxin biosynthesis in plants has remained obscure although auxin has been known for decades as a key regulator for plant growth and development. Here we define the YUC gene family and show unequivocally that four of the 11 predicted YUC flavin monooxygenases (YUC1, YUC2, YUC4, and YUC6) play essential roles in auxin biosynthesis and plant development. The YUC genes are mainly expressed in meristems, young primordia, vascular tissues, and reproductive organs. Overexpression of each YUC gene leads to auxin overproduction, whereas disruption of a single YUC gene causes no obvious developmental defects. However, yuc1yuc4, yuc2yuc6, all of the triple and quadruple mutants of the four YUC genes, display severe defects in floral patterning, vascular formation, and other developmental processes. Furthermore, inactivation of the YUC genes leads to dramatically reduced expression of the auxin reporter DR5-GUS in tissues where the YUC genes are expressed. Moreover, the developmental defects of yuc1yuc4 and yuc1yuc2yuc6 are rescued by tissue-specific expression of the bacterial auxin biosynthesis gene iaaM, but not by exogenous auxin, demonstrating that spatially and temporally regulated auxin biosynthesis by the YUC genes is essential for the formation of floral organs and vascular tissues.

The shikimate pathway and aromatic amino acid biosynthesis in plants.

Abstract: L-tryptophan, L-phenylalanine, and L-tyrosine are aromatic amino acids (AAAs) that are used for the synthesis of proteins and that in plants also serve as precursors of numerous natural products, such as pigments, alkaloids, hormones, and cell wall components. All three AAAs are derived from the shikimate pathway, to which ≥30% of photosynthetically fixed carbon is directed in vascular plants. Because their biosynthetic pathways have been lost in animal lineages, the AAAs are essential components of the diets of humans, and the enzymes required for their synthesis have been targeted for the development of herbicides. This review highlights recent molecular identification of enzymes of the pathway and summarizes the pathway organization and the transcriptional/posttranscriptional regulation of the AAA biosynthetic network. It also identifies the current limited knowledge of the subcellular compartmentalization and the metabolite transport involved in the plant AAA pathways and discusses metabolic engineering efforts aimed at improving production of the AAA-derived plant natural products.