In Vitro Embryogenesis in Higher Plants

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H 2 O 2 is a well-known product of oxidative stress and plays multiple roles in plant physiology. H 2 O 2 belongs to the class of ROS produced in photosynthetic tissues, mitochondria, and also in the cytosol under certain stress conditions, such as cold, drought, salt stress or pathogen attack. Kovtun et al. Constitutively active ANP1 mimics the H 2 O 2 effect and initiates the mitogen-activated protein kinase MAPK cascade that induces specific stress-responsive genes, but it blocks the action of auxin, a plant mitogen and growth hormone.

These observations provide a molecular link between oxidative stress and auxin signal transduction. NO stimulates the activation of cell division and embryogenic cell formation in leaf-protoplast cells of alfalfa, in the presence of auxins. So cells regain the ability to divide during dedifferentiation, and then to re-differentiate into embryogenic cells. However, they did not find that NO was required for the progression of the embryogenic pathway. Some aspects of the downstream events of the putative NO-dependent signalling cascade, leading to mitotic activation of auxin, needs to be investigated.

Pasternak et al. Similar results were derived from other oxidative stress-inducing agents such as copper, menadione, paraquat or alloxan; applied at non-lethal concentrations these agents appeared to promote both, cell division and embryogenic cell formation under non-embryogenic conditions Pasternak et al. They postulated that parallel activation of auxin and stress signalling may be a key event in cellular adaptation, reprogramming of gene expression patterns, cellular metabolism and physiology, resulting in totipotency and embryogenic competence acquisition of plant somatic cells.

The hypothesis that SE is a specific form of stress response related to an adaptation process is supported by experiments which show that a heat-shock system is involved in the developmental reprogramming. Furthermore, some common elements of auxin and heat-shock response are predicted from molecular studies Pitto et al. Recent evidence supports the idea that stress response, oxidative stress and SE may be related. For example, Ganesan and Jayabalan showed that addition of haemoglobin to the culture medium increased SE efficiency in cotton, especially in cultures treated with mgl -1 of haemoglobin, by increasing the oxygen level and inducing stress in the growing tissues.

Increased levels of antioxidant enzyme activity, such as SOD and peroxidase, indicate that excess oxygen uptake and stressed condition of plant tissue resulted from haemoglobin supplementation. This increased oxygen uptake and haemoglobin-mediated stress apparently accelerated SE in cotton.

MAPK cascades may link auxin signalling to oxidative stress responses and cell cycle regulation Hirt, ; Neil et al. It thus appears likely that the primary function of downstream regulatory proteins, such as MAPKs, is to bridge the gap in embryogenesis induction of different types of cells. Stress factors have been observed to trigger reprogramming of microspores into embryos Maraschin et al.

The authors postulated that many stress factors may be responsible for reprogramming microspores into embryos, suggesting that initiation of androgenesis might be induced by converging signalling pathways. Although different stress signals may trigger the same downstream pathway, analogous situations may also be found during the induction of SE.

DNA methylation. In , Phillips et al. The relevant mechanism may be described as a programmed loss of cellular control. The most commonly observed plant tissue culture-imposed changes include: chromosome rearrangements, DNA methylation, and mutations.

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These authors associated 2,4-D and higher levels of other auxins, such as 1-naphthaleneacetic acid NAA and Indoleacetic acid IAA , with increases in DNA methylation throught increased levels of methylated cytosines. Leljak-Levanic et al. They suggested that the presence of 2,4-D as well as a low concentration of NH 4 Cl the sole source of nitrogen in the PGR-free Murashige and Skoog MS medium used in the culture system, might cause stress in the embryogenic culture and also that SE could be induced by stressful conditions through methylation changes.

High levels of DNA methylation and early embryo development were not dependent exclusively on the presence or absence of exogenous auxin. However, the changes in chromatin structure were largely due to stresses caused by the in vitro conditions, and related especially to the PGRs. It has also been suggested that the embryogenic effect of 2,4-D probably derives from its methylating action on the nuclear DNA De Klerk et al.

In fact, large amounts of 2,4-D are known to increase DNA methylation levels in carrot cultures Kaeppler et al. Sharma et al. According to Dudits et al. In a study with soybean SE, Thibaud-Nissen , immature cotyledons were placed on high levels of the auxin 2,4-D. Somatic embryos developed from the adaxial side of the cotyledon, whereas the abaxial side evolved into a callus. Using a 9,cDNA clone array, they compared by steady-state RNA the adaxial side somatic embryos from the abaxial callus formation , at five time points over the course of the 4 weeks.

The genes found were grouped according to the similarity of their expression profiles. Their results indicate that the appearance of somatic embryos is preceded by dedifferentiation of the cotyledon during the first 2 weeks on auxin. The changes in mRNA abundance of genes characteristic of oxidative stress and genes indicative of cell division were found in the adaxial side of the cotyledons suggest that the arrangement of the new cells into organized structures might depend on a genetically controlled balance between cell proliferation and cell death.


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If MAPK phosphorylation cascades link oxidative stress responses to auxin signalling and cell cycle regulation, then other types of stress-oxidative-responses may also be associated with cell reprogramming. Alternative oxidase lowers mitochondria ROS formation in plants cells.

Thus the existence of a direct correlation between alternative oxidase AOX gene expression, stress responses and cellular reprogramming through the embryogenic pathway must be considered Arnholdt-Schmitt et al. Although fragmentary and difficult to unify, the experimental results covered in this review highlight some suggestions to explore the function of stress as a required condition for the induction of plant SE. Based on the examples presented, SE could be considered as a cell response to exogenously applied stressors.

The review also links the embryogenic pathway induced by stress with oxidative burst and changes in cell metabolism, and suggests that the new embryogenic program could be facilitated through the establishment of less repressed chromatin structure. Nevertheless, further work will be required in specific areas as genomics, proteomics and metabolomics to clarify the global role of stress in the cellular mechanisms conducting to SE induction in plants.

We like to thank especially Dr. Krystyna Klimaszewska Canadian Forest Service, Laurentian Forestry Centre, Quebec for encouragement to publish this review and also for her critical reading and suggestions.

REGULATION OF SOMATIC EMBRYOGENESIS IN CROPS: A REVIEW

Sincere appreciation is extended to Dr. Betaine a novel candidate for rapid induction of somatic embryogenesis in tea Camellia sinensis L. Plant Growth Regulation , March , vol. Efficient embryogenesis in the callus of tea Camellia sinensis enhanced by the osmotic stress or antibiotics treatment. Plant Biotechnology , January , vol. Reactive oxygen species: metabolism, oxidative stress, and signal transduction. Annual Review of Plant Biology, June , vol.

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AOX - a functional marker for efficient cell reprogramming under stress? Trends in Plant Science , June , vol. Radiation induced alterations in Vigna radiata during in vitro somatic embryogenesis. International Journal of Radiation Biology , December , vol. Detection of DNA methylation changes during somatic embryogenesis of Siberian ginseng Eleuterococcus senticosus.

Plant Science , July , vol. Induction of adventitious shoots or somatic embryos on in vitro cultured zygotic embryos of Helianthus annuus : Variation of endogenous hormone levels. Plant Physiology and Biochemistry , October , vol. Gene expression during somatic embryogenesis - recent advances. Current Science , September , vol.

Nitric oxide modulates the expression of cell cycle regulatory genes during lateral root formation in tomato. Journal of Experimental Botany , February , vol. Auxin and heat shock activation of a novel member of the calmodulin like domain protein kinase gene family in cultured alfalfa cells. Regeneration of roots, shoots and embryos: physiological, biochemical and molecular aspects.

Biologia Plantarum , January , vol. Zygotic embryogenesis versus somatic embryogenesis. Journal of Experimental Botany , August , vol. Molecular biology of somatic embryogenesis.

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In vitro Embryogenesis in Plants. Transition of somatic plant cells to an embryogenic state. Why somatic plant cells start to form embryos?

In: Mujid, Abdul and Samaj, Josef. Somatic Embryogenesis. Acta Biologica Szegediensis, , vol. GAJ, Malgorzata D. Factors influencing somatic embryogenesis induction and plant regeneration with particular reference to Arabidopsis thaliana L.

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Plant Growth Regulation , May , vol. Evaluation of haemoglobin erythrogen : for improved somatic embryogenesis and plant regeneration in cotton Gossypium hirsutum L.


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