Under appropriate in vivo or in vitro conditions, certain somatic plant cells have the capability to initiate embryogenic development (somatic embryogenesis). Somatic embryogenesis provides an unique experimental model to understand the molecular and cellular bases of developmental plasticity in plants. In the last few years, the application of modern experimental techniques, as well as the characterization of Arabidopsis embryogenesis mutants, have resulted in the accumulation of novel data about the acquisition of embryogenic capabilities by somatic plant cells. In this review, we summarize relevant experimental observations that can contribute to the description and definition of a transitional state of somatic cells induced to form totipotent, embryogenic cells. During this somatic-to-embryogenic transition, cells have to dedifferentiate, activate their cell division cycle and reorganize their physiology, metabolism and gene expression patterns. The roles of stress, endogenous growth regulators and chromatin remodelling in the coordinated reorganization of the cellular state are especially emphasized.

http://www.cas.miamioh.edu/~meicenrd/pdevlmnt/feheretal2003.pdf

Transition of somatic plant cells to an embryogenic state

Somatic embryogenesis is defined as a process in which a bipolar structure, resembling a zygotic embryo, develops from a non-zygotic cell without vascular connection with the original tissue. Somatic embryos are used for studying regulation of embryo development, but also as a tool for large scale vegetative propagation. Somatic embryogenesis is a multi-step regeneration process starting with formation of proembryogenic masses, followed by somatic embryo formation, maturation, desiccation and plant regeneration. Although great progress has been made in improving the protocols used, it has been revealed that some treatments, coinciding with increased yield of somatic embryos, can cause adverse effects on the embryo quality, thereby impairing germination and ex vitro growth of somatic embryo plants. Accordingly, ex vitro growth of somatic embryo plants is under a cumulative influence of the treatments provided during the in vitro phase. In order to efficiently regulate the formation of plants via somatic embryogenesis it is important to understand how somatic embryos develop and how the development is influenced by different physical and chemical treatments. Such knowledge can be gained through the construction of fate maps representing an adequate number of morphological and molecular markers, specifying critical developmental stages. Based on this fate map, it is possible to make a model of the process. The mechanisms that control cell differentiation during somatic embryogenesis are far from clear. However, secreted, soluble signal molecules play an important role. It has long been observed that conditioned medium from embryogenic cultures can promote embryogenesis. Active components in the conditioned medium include endochitinases, arabinogalactan proteins and lipochitooligosaccharides.

https://link.springer.com/content/pdf/10.1023/A:1015673200621.pdf

Developmental pathways of somatic embryogenesis

▪ Abstract The nature of cell wall proteins is as varied as the many functions of plant cell walls. With the exception of glycine-rich proteins, all are glycosylated and contain hydroxyproline (Hyp). Again excepting glycine-rich proteins, they also contain highly repetitive sequences that can be shared between them. The majority of cell wall proteins are cross-linked into the wall and probably have structural functions, although they may also participate in morphogenesis. On the other hand, arabinogalactan proteins are readily soluble and possibly play a major role in cell-cell interactions during development. The interactions of these proteins between themselves and with other wall components is still unknown, as is how wall components are assembled. The possible functions of cell wall proteins are suggested based on repetitive sequence, localization in the plant body, and the general morphogenetic pattern in plants.

Plant cell wall proteins

A floral transmitting tissue-specific glycoprotein attracts pollen tubes and stimulates their growth

Plant cells are surrounded by cell walls, which are dynamic structures displaying a strictly regulated balance between rigidity and flexibility. Walls are fairly rigid to provide support and protection, but also extensible, to allow cell growth, which is triggered by a high intracellular turgor pressure. Wall properties regulate the differential growth of the cell, resulting in a diversity of cell sizes and shapes. The plant hormone auxin is well known to stimulate cell elongation via increasing wall extensibility. Auxin participates in the regulation of cell wall properties by inducing wall loosening. Here, we review what is known on cell wall property regulation by auxin. We focus particularly on the auxin role during cell expansion linked directly to cell wall modifications. We also analyze downstream targets of transcriptional auxin signaling, which are related to the cell wall and could be linked to acid growth and the action of wall-loosening proteins. All together, this update elucidates the connection between hormonal signaling and cell wall synthesis and deposition.

https://www.mdpi.com/1422-0067/19/4/951/pdf/1

The Role of Auxin in Cell Wall Expansion.

Arabinogalactan proteins (AGPs) are found in higher plants and in liverworts [5, 10]. The AGPs are proteoglycans occurring on cell membranes, in the extracellular matrix and in gum exudates. One of the best known AGPs is gum arabic which is excreted by Acacia senegal upon wounding. Already some centuries ago, gum arabic was produced in eastern Africa and traded by Arabs. Nowadays gums, containing similar AGPs as gum arabic or AGP-related compounds, are still being used in the food industry as additives for their aggregating and gelling capacity. AGPs do not have a structural function in that they are not covalently linked in the cell wall, and are often secreted in large amounts, for instance in gums. In the latter case they might act as a physical barrier by forming a gel upon wounding, thus preventing attacks by potential pathogens. Since AGPs occur in many different tissues and are not exclusively produced upon wounding, it seems likely that they do have other functions. In this review the role of AGPs in plant differentiation is discussed. Electrophoretic analysis, monoclonal antibodies and addition to cell cultures resulted in circumstantial and direct evidence that AGPs or AGP structures are involved in the development of cells.

Arabinogalactan proteins and plant differentiation

Two monoclonal antibodies (ZUM 15 and ZUM 18) directed against carrot (Daucus carota L.) seed arabinogalactan proteins (AGPs) were used to isolate specific AGP fractions. For both carrot and tomato (Lycopersicon esculentum Mill.) seed AGPs analyzed by crossedelectrophoresis, the ZUM 15 and ZUM 18 AGP fractions showed one identical peak. However, the Rf values for the two species were different: 0.82 for carrot seed AGPs and 0.52 for tomato seed AGPs. When the fractionated AGPs (carrot or tomato) were added to carrot cell lines they had a dramatic effect on the culture. One AGP fraction (ZUM 15 AGPs) was able to induce vacuolation of embryogenic cells. Those cells failed to produce embryos. The other AGP fraction (ZUM 18 AGPs) increased the percentage of embryognic cells from about 40% up to 80% within one week and this subsequently resulted in the formation of more embryos on hormone-free medium. This activity was higher than that of unfractionated carrot seed AGPs, while the optimum concentration was 50-fold lower. Since both ZUM 18 AGPs (carrot or tomato) yielded identical responses it can be concluded that neither the Rf value nor the source are essential for biological activity. The dose-response curve of ZUM 18 AGPs showed a sharp optimum. When the AGPs that also bound to the antibody ZUM 15 were removed, the dose-response curve of the remaining AGPs (containing only the ZUM 18 epitope, not the ZUM 15 epitope) resembled a saturation curve. Regardless of its concentration, the fraction in which AGP molecules contained both epitopes showed no appreciable embryogenesis-promoting activity. The biological activity of AGPs was therefore determined by the presence of embryogenesis-enhancing and-inhibiting epitopes. The inhibiting and enhancing epitopes can be located on separate molecules or one single AGP molecule.

Arabinogalactan-protein epitopes in somatic embryogenesis of Daucus carota L.

For the initiation of embryogenic cucumber (Cucumis sativus L.) cell lines, from excised radicles, directly in liquid medium, the culture regime, explant density and type and concentration of hormones were adjusted so that pro-embryogenic masses (PEMs) were formed within about 8 weeks. The established cucumber cell lines were maintained tor several years without loss of embryogenic and genetic stability. The ploidy level of somatic embryos from different cucumber eell lines was either diploid or tetraploid and depended on the ploidy level of Ihe cell line. Cucumber cell lines that produced only diploid embryos were obtained by selecting completely diploid explant material and growing it in the dark during the initiation phase. Mixoploid explains could lead to tetraploid or mixoploid ceil lines. Isolation and additional selection and subculturing of single PEMs resulted in either completely diploid or tetraploid cell lines, indicating that all cells of individual PEMs are either diploid or tetraploid. The ernbryogenic cucumber cell Imes, differing only in ploidy level, were indistinguishable in growth rate and embryogetiic potential and were genetically stable over several years.

Genetically stable cell lines of cucumber for the large-scale production of diploid somatic embryos

A method of stimulating the growth division or somatic embryogenesis of plant cells in in vitro culture by applying to the culture medium an effective amount of AGPs and culture media comprising such AGPs.

A method for stimulating growth, division or embryogenesis of plants in in vitro culture

Two monoclonal antibodies (ZUM 15 and ZUM 18) directed against carrot (Daucus carota L.) seed arabinogalactan proteins (AGPs) were used to isolate specific AGP fractions. For both carrot and tomato (Lyco persicon esculentum Mill.) seed AGPs analyzed by crossed electrophoresis, the ZUM 15 and ZUM 18 AGP fractions showed one identical peak. However, the Rf values for the two species were different: 0.82 for carrot seed AGPs and 0.52 for tomato seed AGPs. When the fractionated AGPs (carrot or tomato) were added to carrot cell lines they had a dramatic effect on the culture. One AGP fraction (ZUM 15 AGPs) was able to induce vacuolation of embryogenic cells. Those cells failed to produce embryos. The other AGP fraction (ZUM 18 AGPs) increased the percentage of embryogenic cells from about 40% up to 80% within one week and this subsequently resulted in the formation of more embryos on hormone-free medium. This activity was higher than that of unfractionated carrot seed AGPs, while the optimum concentration was 50-fold lower. Since both ZUM 18 AGPs (carrot or tomato) yielded identical responses it can be concluded that neither the Rf value nor the source are essential for biological activity. The dose response curve of ZUM 18 AGPs showed a sharp opti mum. When the AGPs that also bound to the antibody ZUM 15 were removed, the dose-response curve of the remaining AGPs (containing only the ZUM 18 epitope, not the ZUM 15 epitope) resembled a saturation curve. Regardless of its concentration, the fraction in which AGP molecules contained both epitopes showed no appreciable embryogenesis-promoting activity. The biological activity of AGPs was therefore determined by the presence of embryogenesis-enhancing and-inhibiting epitopes. The in hibiting and enhancing epitopes can be located on separ ate molecules or one single AGP molecule.

Arabinogalactan-protein epitopes in somatic embryogenesis of

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