Somatic embryogenesis (SE), resembling zygotic embryogenesis, is a very efficient and fast method of vegetative plant propagation. Somatic embryos, the final products of this process, are immediately or after drying and/ or encapsulation used as artificial seeds. SE efficiency is not only dependent on the quantity, but also on quality of somatic embryos. Although SE has been recorded for species across many genera and from a variety of plant tissues, regeneration of plants from somatic embryos is often a significant problem for some plant species. Many efforts have been made to obtain higher levels of germination and conversion of somatic embryos to plants. This review will focus on the present knowledge about enhancing the vigor of somatic embryos with special attention paid to the effect of plant hormones (gibberellins, ethylene, abscisic acid), germination and conversion of somatic embryos and their influence on the storage reserves content (starch, oligosaccharides), their hydrolytic products (raffinose, sucrose, glucose), and on the activity of hydrolytic enzymes (alpha-amylase).
This review focuses on the present knowledge about beneficial free-living soil bacteria that associate closely with plant as plant growth-promoting rhizobacteria (PGPR). Growth promotion can occur mainly by two mechanisms (1) directly by phytohormone production (e.g. gibberelin, auxin and cytokinin) or enzymatic lowering of plant ethylene levels (ACC deaminase), nitrogen fixation, iron chelating by siderophores, phosphorum solubilization or (2) indirectly by the reduction or prevention of the action of plant pathogens. The properties of PGPR offer a great promise for agronomic applications. This review presents examples of its application in practice.
Plants can develop local and systemic wide-spectrum resistance induced by pathogens or by some chemical products, a phenomenon known as systemic acquired resistance (SAR). Non-pathogenic microbes or some elicitors can also induce resistance called induced systemic resistance (ISR). Plant growth regulators (e.g., jasmonates, salicylates, ethylene) play an important role in induction of these systemic resistance types. Jasmonic acid (JA) and its methyl ester (MeJA) as well as ethylene are thought to be important components of the signaling pathway regulating ISR response in plants, while salicylic acid (SA) and its methyl ester (MeSA) are required for induction of SAR. This review will focus on the present knowledge about the role of jasmonates in the induction of plant resistance induction against pathogens and their relation to ethylene and salicylates in the process.
The effect of salicylic acid (SA) applied during the proliferation phase of cell suspension in vitro regeneration system of Medicago sativa L. on cell suspension growth, production of somatic embryos and their following development, germination and conversion were examined. SA is a potent inhibitor of the above processes. It is suggested that endogenous jasmonates are of some importance in regulation of somatic embryogenesis.
The biosynthesis of ethylene in plants and its regulation by manipulating the expression of ACC synthase or ACC oxidase genes are discussed. Ethylene synthesis can be reduced by the introduction of antisense ACC synthase or antisense ACC oxidase genes. Expression genes of SAM hydrolase from bacteriofage T3, which catalyze the conversion of SAM to methylothioadenosine, also diminished ACC availability. Another possibility of ethylene biosynthesis control is the expression of gene encoding ACC deaminase from Pseudomonas.
Somatic embryogenesis, resembling zygotic embryogenesis in vivo, is considered to be an efficient method of in vitro propagation of a number of agronomically important plant species including Medicago sativa L. and it offers an in vitro experimental system for studying the embryo development. Artificial seed technology is one of the important applications of the process. Induction of embryogenesis, embryo development and induction desiccation tolerance are affected by plant growth regulators. The review will focus on the effect of plant growth regulators: auxins, cytokinins, gibberellins, abscisic acid, jasmonates, ethylene and inhibitors on the synthesis and action of different phases of somatic embryogenesis in Medicago sativa L.
In this study we examined the effect of 2,4-D, NAA and picloram at 1.5, 3.0 and 4.5 mg/l in MS medium on callus induction and somatic embryogenesis in Asparagus densiflorus cv. Sprengeri and Asparagus plumosus. The callus formation of both shoot tips and nodal explants from A. densiflorus after 8 weeks of culture on MS medium supplemented with NAA and picloram (1.5, 3.0 mg/l) was observed. However, callus on medium containing 2,4-D was occasionally induced. On medium supplemented with picloram shoot tip explants of Asparagus plumosus showed callus inducing capacity. After four weeks of callus culture on hormone-free medium the globular embryos were achieved. On callus induced NAA organogenesis was observed.
The review article presents data on the ethylene emanation by bacteria, the two different pathways of its biosynthesis in these microorganisms and the role of ethylene in plant pathogenesis. ACC deaminase from Pseudomonas and Enterobacter spp., which catalyses the hydrolytic cleavage of ACC in higher plants was also discussed.
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