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Richard L. Bell

Discs of cambial tissue were excised from actively growing shoots of `Bartlett' pear, and explanted directly on regeneration induction media. The basal medium was 1/2 strength MS macro-nutrients, MS micro-nutrients and organics, 8 g/l agar, and 30 g/l sucrose. Phytohormone treatments consisted of a factorial design of NAA (0 and 5μM) and TDZ (1, 2, 3, 4, and 5μM). After 4 weeks incubation in the dark, the explants were transferred to auxin-free media with identical concentrations of TDZ. There was an absolute requirement for auxin in the induction medium, as all discs on auxin-free initial media died without callusing. Maximum shoot regeneration 4 weeks after transfer to expression media was obtained with an initial medium containing 5μM NAA and 3μM TDZ, from which 30% of the explants produced one or more adventitious shoots. This rate of regeneration is similar to that obtained in some experiments with in vitro leaf explants, and provides an alternative system for regeneration of pear.

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Jinq-Tian Ling, N. Nito, and M. Iwamasa

Protoplasts were isolated from embryogenic calli of Citrus reticulata cv. Ponkan and Citropsis gabunensis, and fused in electric fields. The maximal fusion efficiency was obtained by application of AC at 75 V/cm (1.0 MHz) and DC square-wave pulse at 1.125 KV/cm for 40 usec. Fusion-treated protoplasts were cultured on MT medium without phytohormone, solidified with 0.6% agar. Colonies from the protoplasts were proliferated on MT medium with zeatin 1 mg/l and 0.9% agar. Selection of somatic hybrid callus was based on chromosome count and isoenzyme analysis. The somatic hybrids were tetraploid (2n=36). C. reticulata and C. gabunensis were both homozygous at Got-1 locus, but distinguishable easily because band of the latter migrated faster than that of the former. In zymogram of somatic hybrid, both parent bands were retained and a new hybrid band was also evident between them. Embryos from somatic-hybrid callus regenerated intact plant. The hybrid plants showed intermediate morphological characteristics of the parents.

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George Lazarovits

Plant growth-promoting rhizobacteria (PGPR) enhance plant development by many mechanisms. Indirect growth effects result from PGPR activities that displace soilborne pathogens and thereby reduce disease. Direct effects include improved nutrition, reduced disease due to activation of host defenses, and bacterial production of phytohormones. An understanding of the mode of action is essential for exploitation of PGPR for field use. For instance, bacteria that act as biological control agents can only be of benefit at locations where disease occurs. PGPR that stimulate plant growth directly will likely have more universal uses and greater impacts. Thus, we have been developing model systems for identifying PGPR with such traits. In this presentation, the effects of bacterization of tissue culture-grown plants, plug transplants, and seed with a growth-promoting Pseudomonas sp. (PsJN) will be described. Potential uses for this and other PGPR will also be identified. The talk will consider the advantages and limitations of: a) screening methods used for selection of PGPR, b) model systems available for studying the mechanisms of action, and c) why transplants offer an ideal delivery system for rhizobacteria. Results from field trials with PGPR with different modes of action will be presented and their future role in agriculture considered.

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William R. Woodson

The senescence of carnation (Dianthus caryophylus) flower petals is regulated by the phytohormone ethylene and is associated with the expression of a number of senescence-related genes. These genes encode enzymes in the ethylene biosynthetic pathway, including both ACC synthase and ACC oxidase. Members of these gene families are differentially regulated in floral organs, with specific members responsible for the increase in ethylene biosynthesis that leads to petal senescence. Pollination often serves as the external signal to initiate the senescence cascade. Following pollination, a rapid increase in ethylene production by the pistal occurs, which is subsequently followed by increased ethylene in the petal. This response is mediated by pollen–pistil interaction(s) that occurs only in compatible pollinations. Recent data indicate that the signal transduction cascade following this cell-cell communication involves protein phosphorylation, as pollination-induced ethylene is sensitive to protein kinase and phosphatase inhibitors. To date, our lab has cloned and characterized a number of senescence-related genes that are believed to play a role in the process of senescence. These include genes that encode enzymes involved in cell wall dissolution (b-galactosidase), protein degradation (cysteine proteinase) and detoxification of breakdown products (glutathione s-transferase). Many of these senescence-related genes are under the transcriptional regulation of ethylene, which has been characterized at the molecular level. A number of biotechnology approaches to controlling the senescence of flowers have been explored. These include the down-regulation of ethylene biosynthetic genes, the expression of a dominant-negative mutation of the ethylene receptor gene, and the expression of genes that lead to increased cytokinin levels in tissues. These will be discussed in relation to the potential for delaying senescence through genetic engineering.

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Wei Li, Rongcai Yuan, Jacqueline K. Burns, L.W. Timmer, and Kuang-Ren Chung

Colletotrichum acutatum J. H. Simmonds infects citrus flower petals, causing brownish lesions, young fruit drop, production of persistent calyces, and leaf distortion. This suggests that hormones may be involved in symptom development. To identify the types of hormones, cDNA clones encoding proteins related to ethylene and jasmonate (JA) biosynthesis, indole-3-acetic acid (IAA) regulation, cell-wall modification, signal transduction, or fruit ripening were used to examine differential gene expressions in calamondin (Citrus madurensis Lour) and/or `Valencia' sweet orange (Citrus sinensis Osbeck) after C. acutatum infection. Northern-blot analyses revealed that the genes encoding 1-aminocyclopropane-1-carboxylate (ACC) oxidase and 12-oxophytodienoate required for ethylene and JA biosynthesis, respectively, were highly up-regulated in both citrus species. Both gene transcripts increased markedly in petals, young fruit and stigmas, but not in calyces. The transcripts of the genes encoding IAA glucose transferase and auxin-responsive GH3-like protein, but not IAA amino acid hydrolyase, also markedly increased in both species 5 days after inoculation. The expansin and chitinase genes were slightly up-regulated, whereas the senescence-induced nuclease and ß-galactosidase genes were down-regulated in calamondin. No differential expression of transcripts was detected for the genes encoding expansin, polygalacturonase, and serine-threonine kinase in sweet orange. As compared to the water controls, infection of C. acutatum increased ethylene and IAA levels by 3- and 140-fold. In contrast, abscisic acid (ABA) levels were not significantly changed. Collectively, the results indicate that infection by C. acutatum of citrus flowers triggered differential gene expressions, mainly associated with IAA, ethylene, and JA production and regulation, and increased hormone concentrations, consistent with the hypothesis of the involvement of phytohormones in postbloom fruit drop.

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Xunzhong Zhang, E.H. Ervin, and R.E. Schmidt

A variety of organic materials such as humic substances, seaweed extracts (SWE), organic matter, and amino acids are being used as fertilizer supplements in commercial turfgrass management. Among them, SWE and humic acid (HA) are widely used in various biostimulant product formulations. These compounds have been reported to contain phytohormones and osmoprotectants such as cytokinins, auxins, polyamines, and betaines. Manufacturer claims are that these products may supplement standard fertility programs by reducing mineral nutrient requirements while improving stress tolerance. There is a lack of season-long, field-based evidence to support these claims. This study was conducted to investigate the influence of monthly field applications of SWE, HA, and high and low seasonal fertilization regimes on the physiological health of fairway-height creeping bentgrass (Agrostis stolonifera L.). Plots were treated monthly with SWE at 16 mg·m-2 and HA (70% a.i.) at 38 mg·m-2 alone, or in combination, and were grown under low (20 kg·ha-1/month) or high nitrogen (50 kg·ha-1/month) fertilization regimes during 1996 and 1997. Endogenous antioxidant superoxide dismutase (SOD) activity, photochemical activity (PA), and turf quality were measured in July of each year. Superoxide dismutase activity was increased by 46% to 181%, accompanied by a PA increase of 9% to 18%, and improved visual quality of bentgrass in both years. There was no significant fertilization × supplement interaction. Although not part of our original objectives, it was noted that significantly less dollar spot (Sclerotinia homoeocarpa F.T. Bennett) disease incidence occurred in supplement-treated bentgrass. Our results indicate that increased SOD activity in July due to SWE and/or HA applications improved overall physiological health, irrespective of fertilization regime. This suggests that these compounds may be beneficial supplements for reducing standard fertilizer and fungicide inputs, while maintaining adequate creeping bentgrass health.

Open access

Yanxia Zhao, Guimei Qi, Fengshan Ren, Yongmei Wang, Pengfei Wang, and Xinying Wu

Abscisic acid (ABA) is an essential phytohormone that regulates plant growth and development, particularly in response to abiotic stress. The ABA receptor PYR/PYL/RCAR (PYL) family has been identified from some plant species. However, knowledge about the PYL family (VvPYLs) in grape (Vitis vinifera) is limited. This study aims to conduct genome-wide analyses of VvPYLs. We successfully identified eight PYL genes from the newest grape genome database. These VvPYLs could be divided into three subfamilies. Exon-intron structures were closely related to the phylogenetic relationship of the genes, and PYL genes that clustered in the same subfamily had a similar number of exons. VvPYL1, VvPYL2, VvPYL4, VvPYL7, and VvPYL8 were relatively highly expressed in roots. VvPYL1, VvPYL3, VvPYL7, and VvPYL8 were expressed in response to cold, salt, or polyethylene glycol stress. VvPYL6 was up-regulated by cold stress for 4 hours, and the expression of VvPYL2 was 1.74-fold greater than that of the control under cold stress. VvPYL8 was up-regulated 1.64-, 1.83-, and 1.90-fold compared with the control when treated with salt, PEG, or cold stress after 4 hours, respectively. Additionally, abiotic stress-inducible elements exist in VvPYL2, VvPYL3, VvPYL7, and VvPYL8, indicating that in these four genes, the response to abiotic stress may be regulated by cis-regulatory elements. The transcriptional levels of VvPYL1 and VvPYL8 significantly increased from fruit set to the ripening stage and decreased in the berry when treated by exogenous ABA. The eight VvPYL genes have diverse roles in grape stress responses, berry ripening, or development. This work provides insight into the role of VvPYL gene families in response to abiotic stress and berry ripening in grape.

Free access

Bruce W. Wood

one or more seed-associated phloem mobile phytohormones in regulation of floral initiation (i.e., the production of meristems of clearly recognizable flower primordia and includes all preceding reactions that are required if flowers are to be initiated

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Sanalkumar Krishnan and Emily B. Merewitz

suppression and to improve turfgrass quality ( Ervin and Koski, 2001 ). Therefore, the objectives of this study were to evaluate the effects of exogenous applications of Spd and Spm on growth physiology and accumulation patterns of phytohormones including GA

Free access

Yang Yang, Runfang Zhang, Pingsheng Leng, Zenghui Hu, and Man Shen

physiological and biochemical responses during cold acclimation, including the modification of membrane lipid composition, synthesis of protective proteins, increase of compatible compounds, and regulation of phytohormones ( Guy, 1990 ; Thomashow, 1999