Tim D. Davis

Effect of Paclobutrazol on the Activities of some Enzymes of Activated Oxygen Metabolism and Lipid Peroxidation in Senescing Soybean Leaves

Summary Nineteen-day-old soybean [Glycine max (L.) Merr. cv. A 2] plants were treated with soil-applied paclobutrazol, an antigibberellin growth inhibitor, at the rate of 125 µg per 10 cm pot. The plants were then grown for 14 days under a 14 hr photoperiod after which they were transferred to the dark for senescence induction. At the end of the 14 day period in the light, treated plants exhibited higher chlorophyll (Chl) content and higher activities of catalase and glycolate oxidase compared to controls. In contrast, control leaves had higher activities of superoxide dismutase and a higher content of malondialdehyde (MDA), a product of lipid peroxidation. Upon transfer to dark, Chl and protein content declined in both control and treated plants, but the decline was much faster in controls. The activity of catalase declined markedly in controls while remaining constant in reated plants. Peroxidase and superoxide dismutase activities and MDA content increased in controls in the dark but remained relatively constant in treated plants. These results suggest that paclobutrazol delays dark-induced senescence in attached soybean leaves and that this delay is associated with the maintenance of catalase activity and the prevention of the senescence-linked rise in peroxidase activity and lipid peroxidation.

Biology of adventitious root formation

Special Lecture: The Origin, Diversity, and Biology of Shoot-Borne Roots P.W. Barlow. The Commercial Importance of Adventitious Rooting: Commercial Application of Adventitious Rooting to Forestry G.A. Ritchie. Infrastructure for Research: Model Systems for Studying Adventitious Root Formation S.G. Ernst. Setting the Stage for Rooting: Novel Experimental Systems for Determining Cellular Competence and Determination D. Mohnen. Induction of Rooting: Auxin Metabolism and Adventitious Root Initiation D. Blakesley. Root Development and Plant Growth: Modeling Root System Morphology in Rice S. Morita, J. Abe. Special Chapter: A Historical Evaluation of Adventitious Rooting Research to 1993 B.E. Haissig, T.D. Davis. Epilogue: Research on Adventitious Rooting W.P. Hackett, J.R. Murray. 12 additional articles. Index.

A Historical Evaluation of Adventitious Rooting Research to 1993

Vegetative propagation of plants by rooting of cuttings (cuttage) was successfully used hundreds of years before there was any study, much less understanding, of the underlying biological processes. For some species, cuttage was old practice even in antiquity, as evidenced in the writings of Aristotle (384–322 B.C.), Theophrastus (371–287 B.C.) and Pliny the Elder (23–79 A.D.). But cuttage was never successful enough to fulfill all then-current public and commercial demands and it still is not [e.g, see chapter by Howard in this volume]. In addition, organ formation has long been a study area within plant morphogenesis (Went and Thimann, 1937), which has made adventitious rooting of academic botanical interest. Hence research on the fundamental biology of adventitious rooting began and continues.

COMMERCIAL IMPORTANCE OF ADVENTITIOUS ROOTING TO HORTICULTURE

Vegetative or clonal reproduction is the most important propagation method used for the commercial production of many, if not most, horticultural crops (ornamentals, fruits, nuts and vegetables). One of the major advances in early agriculture was the discovery that important food crops such as figs, grapes and olives could be regenerated by inserting the base of their woody stems into the ground to induce the formation of adventitious roots and, hence, new plants. Because clonal reproduction may not occur naturally or at least easily in various plant species, the history of horticulture has evolved to a large extent around the development of new technologies for vegetative propagation. More elaborate propagation facilities and commercial asexual propagation techniques have continued to evolve to extend the process to more and more plants at lower costs per unit of production.

Thidiazuron-induced in vitro shoot formation from roots of intact seedlings of Albizzia julibrissin

Seedlings of silktree (Albizzia julibrissin Durrazz.) were grown in vitro on MS-media containing B5 vitamins, 3% sucrose, 0.25% phytagel and various concentrations (0.1–10 μM) of thidiazuron (TDZ). Addition of TDZ to the culture medium greatly reduced shoot and root elongation but did not influence shoot production from the cotyledonary node or apex. Within 8–10 days the seedling roots split open, formed large masses of callus, and developed green patches which eventually grew into normal shoots while still within the culture medium containing TDZ at 0.1–1.0 μM. Such callus and shoot formation did not occur in control cultures lacking TDZ. At higher TDZ concentrations (2.5–10 μM), the green patches formed in the callus did not further develop into shoots. Addition of other cytokinins (kinetin, benzylaminopurine, zeatin) to the culture medium also induced some shoot formation from the roots, but higher concentrations than TDZ were required to induce regeneration. Isopentenyladenine failed to induced shoot formation. Following excision and transfer to MS media with or without 4.9 μM IBA, the shoots induced by kinetin or benzylaminopurine rooted 4–7 days earlier than those induced by TDZ, but all excised shoots developed into normal rooted plantlets within 3 weeks.

In vitro regeneration of silktree (Albizzia julibrissin) from excised roots

Root segments (1 cm long) were excised from 15–20 day old seedlings of silktree (Albizzia julibrissin) grown on B5 medium. About 50% of the control (no growth regulators added) root explants formed shoot buds within 15 days after placement on the culture medium. After 30 days, there were about 4 shoots per control explant. Addition of low levels of various auxins (0.5 μM) did not influence the formation of shoot buds from the explants. Higher concentrations (5μM), however, decreased shoot regeneration. Kinetin and 2iP did not influence shoot regeneration at the concentrations tested (1 & 10 μM). Addition of benzyladenine, Zeatin, or thidiazuron to the culture medium increased both the percentage of explants that formed shoots and the number of shoots per explant. Thidiazuron was highly effective in stimulating shoot formation at low concentrations (<1 μM). At 0.05 μM thidiazuron, 95% of the explants produced shoots and about 10 shoots were formed per explant. Compared to TDZ, higher concentrations (10 μM) of benzyladenine and Zeatin were required to enhance shoot formation. Upon excision and transfer to B5 medium, regenerated shoots developed into normal rooted plantlets.

Promotion ofin vitro shoot formation from excised roots of silktree (Albizzia julibrissin) by an oxime ether derivative and other ethylene inhibitors.

SummaryThis report describes the regeneration response of excised seedling roots of silktree (Albizzia julibrissin) to added ethylene precursors/generators (1-amino-cyclopropane-1-carboxylic acid [ACC], 2-chloroethylphosphonic acid [CEPA]), biosynthesis inhibitors (aminoethoxyvinylglycine [AVG], an oxime ether derivative [OED={[(ispropylidene)-amino]oxy}-acetic acid-2-(methoxy)-2-oxoethyl ester], CoCl2 [Co++]), and an ethylene action inhibitor (AgNO3 [Ag+]). When placed on B5 medium, about 50% of the control explants formed shoot buds within 15 days. Addition of ACC or CEPA (1–10 µM) to the culture medium decreased both the percentage of cultures forming shoots and the number of shoots formed per culture. In contrast, AVG and OED (1–10 µM) increased shoot formation to almost 100% and increased the number of shoots formed per culture. Likewise, both Co++ and Ag+ (1–10 µM) increased shoot regeneration, but the number of shoots produced after 30 days was less than with AVG or OED. The inhibitors of ethylene biosynthesis were partially effective in counteracting the inhibitory effect of ACC on shoot formation. These results suggest that modulation of ethylene biosynthesis and/or action can strongly influence the formation of adventitious shoots from excised roots of silktree.

Influence of growth regulators on somatic embryogenesis, plantlet regeneration, and post-transplant survival of Echinochloa frumentacea.

After placement on Murashige and Skoogs basal medium supplemented with 3–5 mg/l 2,4-D, immature inflorescence expiants of Echinochloa frumentacea gave rise to three distinct types of callus: a) loosely arranged and soft; b) compact and translucent; c) compact, sticky and mucilaginous. Somatic embryo formation occurred in type ‘b’ callus in about 18–24 d. Callus types ‘a’ and ‘c’ did not produce somatic embryos. The highest percentage of cultures exhibiting somatic embryogenesis occurred on the medium containing 5 mg/l 2,4-D and 0.5 mg/l kinetin. Somatic embryos also formed directly on the inflorescence (without intervening callus formation) in about 15% of the expiants placed on this medium. The addition of paclobutrazol or uniconazole (0.25 or 1 mg/l) to the medium had no influence on the percentage of cultures exhibiting direct somatic embryogenesis, but paclobutrazol slightly increased the mean number of somatic embryos per culture. Many of the callus-derived somatic embryos germinated when subcultured on basal MS medium supplemented with kinetin. Addition of paclobutrazol or uniconazole to the culture medium at 0.25 or 1 mg/l decreased somatic embryo germination and shoot elongation but increased root length and leaf width. Both paclobutrazol and uniconazole increased survival of the plantlets following transplanting to soil. Increased post-transplant survival was accompanied by reduced water loss from plantlets produced on culture media containing triazoles.

Hydrogen peroxide-scavenging enzymes and antioxidants in Echinochloa frumentacea as affected by triazole growth regulators.

Paclobutrazol (PBZ)- and uniconazole (UCZ)-treated plants of Echinochloa frumentacea were shorter but had much wider leaves than untreated controls 10 days after treatment. Leaves of treated plants had a slightly higher concentration of soluble protein than the controls and exhibited enhanced activities of ascorbate peroxidase, monodehydroascorbate (MDHA) reductase, and glutathione (GSH) reductase. The triazoles did not influence the activity of dehydroascorbate (DHA) reductase. The leaves of treated plants had increased concentrations of water-soluble sulfhydryls and ascorbic acid. In contrast, the concentration of malondialdehyde (MDA), a by-product of lipid peroxidation, was lower in the leaves of treated plants than in controls. These results suggest that triazole growth regulators increased the activity of the endogenous H2O2-scavenging system in E. frumentacea.

Anatomy ofZea mays andGlycine max seedlings treated with triazole plant growth regulators

Soil drenches containing 250 μg of paclobutrazol or uniconazol (50 ml of a 17 μM solution) reduced the height of both corn (Zea mays L. cv. How.Sweet It Is) and soybean (Glycine max (L.) Merr. cv. A2) seedlings. With corn, uniconazol was considerably more active than paclobutrazol in reducing height whereas with soybean both compounds had similar dwarfing effects. The compounds increased foliar chlorophyll content and leaf thickness in soybean but had no effect on these parameters in corn. The increase in leaf thickness with soybean was due primarily to an increase in the thickness of the palisade cell layer. Chloroplast size and ultrastructure of both species were unaffected by the compounds. The growth regulators increased root diameter in both corn and soybean because of increased size of cortical parenchyma cells and particularly in soybean because of radial rather than longitudinal growth of the first few layers of the cortical parenchyma.