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Guo-qing Song, Aaron E. Walworth, and Wayne H. Loescher

amphipolyploid. Movement of Phytohormones Phytohormones play essential roles in plant growth and development. Many phytohormones (e.g., auxin, cytokinin, gibberellin, abscisic acid, jasmonic acid, and salicylic acid) are phloem-mobile signals, able to move from

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Mason T. MacDonald, Rajasekaran R. Lada, Jeff Hoyle, and A. Robin Robinson

possible signal is abscisic acid (ABA). ABA is a natural phytohormone that is known to regulate various plant growth and developmental processes associated with biotic and abiotic stresses, including drought. Endogenous levels of ABA increase in parallel to

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Bo Meyering, Adam Hoeffner, and Ute Albrecht

plant internal conditions ( Simpson and Dean, 2002 ). Bioactive gibberellins (GA) are a class of endogenous phytohormones that positively direct seed germination, stem elongation, and floral initiation and significantly interact with the auxin

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Ju Ding, Kai Shi, Yan-Hong Zhou, and Jing-Quan Yu

phytohormones that are structurally similar to animal and insect steroid hormones. They control a broad range of processes, including seed germination, stem elongation, cell division and expansion, xylem differentiation, plant growth, and apical dominance

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Jingyi Lv, Yonghong Ge, Canying Li, Mengyuan Zhang, and Jianrong Li

., 2007 ). Recent discoveries showed that JAZ transcriptional repressors are key regulators of JA signaling and that they function as crucial interfaces to mediate the synergy and antagonism between JA and other phytohormones, such as ethylene and

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Henry E. DeVries II, Joseph P. Lardner, and Kenneth W. Mudge

To test the hypothesis that phytohormone production is related to mycorrhizae formation 29 isolates of ectomycorrhizal fungi have been evaluated for production of ethylene and auxin in pure culture and for their ability to form ectomycorrhizae with Pinus mugo. The fungi tested included a series of monokaryotic isolates of Laccaria bicolor and dikaryotic isolates of Amanita muscaria, Hebeloma crustuliniforme, L. laccata, L. proxima, Pisolithus tinctorius, Rhizopogon ellenae, and R. vinicolor. Inoculated root systems were rated for mycorrhizae formation, root/shoot ratio and root morphology. P. tinctorius isolates which formed abundant mycorrhizae produce no ethylene in vitro, but did produce the auxin IAA. L. bicolor isolates produced ethylene as well as auxins putatively identified as IAA, and IBA. Correlations between phytohormone production and mycorrhizogenicity will be presented.

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A. Smigocki and F. Hammerschlag

Immature `Redhaven' peach (Prunus persica L. Batsch) embryos were infected with Agrobacterium tumefaciens strain tms328::Tn5 carrying the functional cytokinin gene. Shoots were regenerated from callus grown on MS medium without added phytohormones and subsequently rooted on half-strength MS medium with 2.8 -naphthaleneacetic acid. These plants exhibited an increased frequency of branching in vitro. Low levels of cytokinin gene transcripts were detected in these cells by Northern analysis, and using an ELISA assay, the cytokinins zeatin and zeatinriboside were determined to be on the average 30-fold higher. From these results, the expression of the cytokinin gene appears to promote growth of cells in the absence of phytohormones thus serving as a marker for transformation and a promoter of morphogenesis without a 2,4-dichlorophenoxyacetic acid inductive step.

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D.M. Glenn and R. Scorza

In reciprocal grafts of tall (`Elberta' and `Loring') and dwarf (`Empress' and `Juseito') peach (Prunus persica Batsch.) phenotypes, we measured dry-matter partitioning, resistance to root system water flow, and phytohormone content of xylem exudate. Scion characteristics determined the phenotype and growth characteristics of the tree irrespective of the rootstock. Tall phenotypes had higher dry weight and lower root resistance to water flow than dwarf phenotypes. Cytokinin-like activity and auxin levels in xylem sap were higher in dwarf than in tall phenotypes; whereas gibberellin-like activity was unaffected by either rootstock or scion. The scion of peach influenced phytohormone levels and resistance to water flow in the root system in addition to root and shoot growth.

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K.A. Malik, Christena Visser, and praveen K. saxena

In vitro regeneration by shoot organogenesis and-or somatic embryogenesis is accomplished by culturing the explants on a nutrient medium supplemented with phytohormones. Auxins in general, and 2,4-D in particular, have been shown to induce somatic embryogenesis whereas shoot regeneration is stimulated by cytokinins. In studying the morphoregulatory role of thidiazuron (TDZ) - a substituted urea with cytokinin-like activity - we found that it induces a high frequency of both organogenesis and somatic embryogenesis depending upon the plant species. For instance, whole seedlings of peanut developed somatic embryos and those of bean and pea produced shoots in response to culture on TDZ (1-40 μM)-supplemented media. In cultured explants of geranium, the use of TDZ (0.2-1 μM) effectively replaced the requirement of 2,4-D or BAP and IAA for obtaining somatic embryos. The frequency of regeneration was two to ten times higher than that achieved with auxin-cytokinin combinations. While no direct evidence is currently available to establish a relationship between TDZ and endogenous phytohormones, our results suggest that it may act by establishing endogenously the auxin:cytokinin ratio permissive of induction and expression of morphogenically competent cells.

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Mark E. Lewis and Fred A. Bliss

Ten common bean (Phaseolus vulgaris L.) lines—including cultivars, breeding lines, and one wild line—were evaluated for susceptibility to Agrobacterium tumefaciens strain C58 by stab-inoculating intact shoot tips of germinating seeds. Significant differences for tumor frequency and size were found on the resulting 3-week-old seedlings. UW 325, a wild bean, had the highest rate of tumorigenesis; `Olathe', a dry bean cultivar, had the lowest. Uninoculated excised shoot tips cultured in media with BA or BA plus NAA exhibited differences in phytohormone sensitivity, as evidenced by callusing and root initiation. The cultivar Montcalm seemed to be highly sensitive, while `Olathe' was relatively insensitive. Fluorometric GUS assays of shoot tips from germinating seeds inoculated with the disarmed GUS-containing A. tumefaciens strain C58C1(pGV3850/pKIWI105) showed that UW 325 had the highest level of GUS activity. `Montcalm' had a high rate of tumorigenesis but a low level of GUS activity; this anomaly was attributed to its high phytohormone sensitivity. The use of the virulence-inducing compound acetosyringone in the inoculum culture medium did not alter genotypic differences (ranks) in susceptibility. Histochemical GUS assays of inoculated UW 325 shoot tips showed that 60% of the apexes exhibited one or more transformation events. Chemical names used: β-glucuronidase (GUS); α-naphthaleneacetic acid (NAA); N-(phenylmethyl)-1H-purin-6-amine (BA).