Considerable progress has recently been made in understanding adventitious root formation using physiological studies. It is recognized that rooting is a process consisting of distinct phases, each with its own requirements. In this review, the successive phases in the rooting process are described and the possible roles of wounding-related compounds, auxin, ethylene and phenolic compounds during these specific phases are discussed. Recent results are assisting the development of advanced rooting treatments. Molecular studies on rooting are underway and will be essential in revealing the mechanisms underlying adventitious root formation.

Review the formation of adventitious roots: New concepts, new possibilities

The broad applications of somatic embryogenesis, both in basic and applied research, have stimulated studies on the determination of in vitro conditions for the induction of somatic embryos and their conversion into plants. As a result, efficient protocols on SE induction and plant regeneration have recently become available for many plant species, including Arabidopsis thaliana (L.) Heynh., a model plant in genetics and embryogenesis. Studies on factors controlling in vitro plant morphogenesis are highly desirable not only for the development of improved regeneration systems, but also for the analysis of molecular mechanisms underlying plant embryogenesis. This review focuses on the conditions influencing the induction of embryogenic potential in in vitro cultured plant cells. The roles of explant type, endo- and exogenous plant growth regulators and stress factors in the induction of somatic embryogenesis are especially emphasized. Possible mechanisms by which different factors induce or modify embryogenic competence in cultured plant cells are also discussed. Since the production of genetically solid and true-to-type plants is desired, especially for transformation and micropropagation practice, the problem of the genetic characteristics of regenerants, in terms of their chimerism and somaclonal variation, is discussed in some detail. Special consideration is given to A. thaliana– a major model plant species for classical genetics and genomics. Recent availability of efficient embryogenic cultures in this organism makes it possible to benefit from advanced genomic research of Arabidopsis to study plant embryogenesis on the molecular level.

Factors influencing somatic embryogenesis induction and plant regeneration with particular reference to Arabidopsis thaliana (L.) Heynh

Allelopathic compounds are metabolites released from plants that might be beneficial or detrimental to the growth of receptor plants. These compounds are involved in the environmental complex of managed or natural ecosystems. Allelopathic compounds have been shown to play important roles in the determination of plant diversity, dominance, succession, and climax of natural vegetation and in the plant productivity of agroecosystems. The overuse of synthetic agrochemicals often causes environmental hazards, an imbalance of soil microorganisms, nutrient deficiency, and change of soil physicochemical properties, resulting in a decrease of crop productivity. The incorporation of allelopathic substances into agricultural management may reduce the use of synthetic herbicides, fungicides, and insecticides and lessen environmental deterioration. Scientists in many different habitats around the world have demonstrated the above examples previously. It is known that most volatile compounds, such as terpenoids, are re...

Roles of allelopathy in plant biodiversity and sustainable agriculture

Phenolic substances are an important constituent of forest leaf material. Within living plant tissue they occur as free compounds or glycosides in vacuoles or are esterified to cell wall components (Harborne 1980). Major classes of phenolic compounds in higher plants are summarized in Table 1. ‘Phenolics’ are chemically defined as substances that possess an aromatic ring bearing a hydroxyl substituent, including functional derivatives (esters, methyl esters, glycosides, etc.). ‘Polyphenols’ are those phenols with two or more hydroxyl-groups. The term ‘polymeric phenol’ will be used for compounds composed of two or more rings (e.g. condensed tannins, flavenoids). Phenolics in plant litter are to a small extent water soluble, are released by rain water and can be detected in small amounts in throughfall and stem flow (Bruckert et al. 1971). However, their main origin is their release from decomposing leaf litter deposited on the soil surface and from decomposing fine roots. Phenolics act as intermediates in the formation of humic compounds in the soil environment (Stevenson 1982). By condensation and polymerization reactions mediated by soil microbes (Bailly & Raboanary 1986), simple low molecular weight phenolic compounds together with amino acids and proteins are chemically altered into the more stable higher molecular weight fulvic acids, humic acids and finally into humins (Martin & Haider 1980). After reaching the soil, the effects of phenolics on the functioning of forest ecosystems are many and drastic. Soil phenolic substances directly affect bacteria, the development of mycelia and spore germination of saprotrophic and mycorrhizal fungi, and the germination and growth of higher plants (Hartley & Whitehead 1985). The process of humification is also largely directed by the polyphenol content of litter (Duchaufour 1983). Thus, site quality characteristics such as mineralization rate, nitrification rate and soil productivity are highly influenced by phenolics, released from living, senescent and decomposing plant tissue.

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Role of phenolic substances from decomposing forest litter in plant–soil interactions

Thidiazuron (TDZ) has gained a considerable attention during past decades due to its efficient role in plant cell and tissue culture. Wide array of physiological responses were observed in response to TDZ-application in different plant species. TDZ has shown both auxin and cytokinin like effects, although, chemically, it is totally different from commonly used auxins and cytokinins. A number of biological (physiological and biochemical) events in cells are induced or enhanced by TDZ, but the mode of action of TDZ is yet unknown. However, varieties of underlying mechanisms were revealed by reports showing how morphogenic events were induced by application of TDZ. Other reports showed that TDZ may modify endogenous plant growth regulators, either directly or indirectly and produce reactions in cell/tissue, necessary for its division/regeneration. Other possibilities include modification in cell membrane, energy levels, nutrient absorption, transport and assimilation, etc. In this review, recent advancements in TDZ application in plant sciences are discussed.

	 

	Key words: Thidiazuron, plant growth regulators, somatic embryogenesis, regeneration, cell cultures, metabolism.

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Thidiazuron: A multi-dimensional plant growth regulator

SummaryShoot cultures of Quercus robur have been established and multiplied in vitro using original material from both juvenile seedlings and mature trees. The juvenile explants were embryos and shoot apices from 3–4-month-old seedlings. Shoot tips from the new growth of stump sprouts were used to establish initial cultures of mature trees. Of the various macronutrient formulae tried, the most satisfactory were Heller’s medium with 1 mM of (NH4)2SO4, and Gresshoff and Doy’s. A dose of 6-benzylaminopurine at 1 mg l−1 was used in initial cultures and at 0.1 mg l−1 in shoot multiplication cultures. A rooting rate of 83% was achieved with juvenile material and 63% with adult material by briefly dipping the basal ends of shoots regenerated in vitro in concentrated solutions of IBA.

In vitro plantlet regeneration from juvenile and mature Quercus robur, L.

Cold storage of shoot cultures and alginate encapsulation of shoot tips of Camellia japonica L. and Camellia reticulata Lindley

This work describes experiments demonstrating the feasibility of long-term conservation of Castanea sativa germplasm through cryopreservation of embryonic axes or somatic embryo clumps. Between 93 % and 100 % of excised embryonic axes of recalcitrant chestnut seeds survived storage in liquid nitrogen (LN) following desiccation in a laminar flow cabinet to moisture contents of 20-24 % (on a fresh weight basis), and some 63 % subsequently developed as whole plants. Desiccation to moisture contents less than 19 % produced damage resulting in loss of organized plant development after cryostorage, allowing only root growth. When 6-8 mg clumps of globular or heart-shaped somatic embryos were precultured for 7 days on high-sucrose medium and then desiccated to a moisture content of 25 % before storage in LN, the embryogenesis resumption level after thawing was 33 %. When the embryo clumps were precultured for 3 days on high-sucrose medium followed by 60 min application of PVS2 vitrification solution before cryostorage, the post-storage embryogenesis resumption level was 68 %.

Cryopreservation of zygotic embryo axes and somatic embryos of European chestnut.

The aim of this study was to micropropagate mature Quercus robur L. trees when material retaining physiologically juvenile characteristics (stump sprouts, epicormic shoots) is not available. Branch segments from 70–300 year-old trees were force-flushed and the flushed, partially rejuvenated or reinvigorated shoots were used as a source of explants for establishment of cultures. In vitro establishment and multiplication was achieved with seven of the eight selected trees. The proliferation capacity of cultures of vertically placed explants declined after several subcultures, but efficient shoot multiplication was achieved by culturing decapitated shoots placed horizontally on GD medium supplemented with 0.89 μM of 6-benzyladenine. Reculturing the same horizontal explant several times allowed both higher multiplication rates and a shorter subculture cycle (2 weeks). An initial dark period of 5 days generally improved rooting capacity, which ranged, depending on clone, from 15 to 46%.

Foreed flushing of branch segments as a method for obtaining reactive explants of mature Quercus robur trees for micropropagation

Somatic embryogenesis and plantlet regeneration were achieved from immature and mature zygoticCamellia japonica embryos cultured on Murashige & Skoog's mineral medium without growth regulators or with various combinations of IBA and BAR The dependence of embryogenesis rates on growth regulator levels was not clear, though high concentrations such as 4 mg 1-1BAP plus 2 mg 1-1IBA were definitely inhibitory. BAP at 1 or 2 mg 1-1 did appear to determine the formation of ‘bud-like embryos’. By far the most responsive initial explants were immature embryonic axes collected in September, 94% of which produced somatic embryos as against only 20% for embryonic axes from mature seeds collected in October. Cotyledon explants were also embryogenic. Somatic embryos differentiating directly on the hypocotyl of the embryonic axes or the surface of cotyledons passed through typical stages of embryogenesis. Indirect somatic embryogenesis via callus was also evident. Embryogenic potential was maintained by secondary embryogenesis through the successive generations of embryos.

Ana M. Vieitez

Papers

In vitro plantlet regeneration from juvenile and mature Quercus robur, L.

Cold storage of shoot cultures and alginate encapsulation of shoot tips of Camellia japonica L. and Camellia reticulata Lindley

Cryopreservation of zygotic embryo axes and somatic embryos of European chestnut.

Foreed flushing of branch segments as a method for obtaining reactive explants of mature Quercus robur trees for micropropagation

Somatic embryogenesis and plant regeneration from embryonic tissues of Camellia japonica L.