Somatic embryogenesis is the process by which somatic cells, under induction conditions, generate embryogenic cells, which go through a series of morphological and biochemical changes that result in the formation of a somatic embryo. Somatic embryogenesis differs from zygotic embryogenesis in that it is observable, its various culture conditions can be controlled, and a lack of material is not a limiting factor for experimentation. These characteristics have converted somatic embryogenesis into a model system for the study of morphological, physiological, molecular and biochemical events occurring during the onset and development of embryogenesis in higher plants; it also has potential biotechnological applications. The focus of this review is on embryo development through somatic embryogenesis and especially the factors affecting cell and embryo differentiation.

The stationary life of plants has led to the evolution of a complex gridded antioxidant defence system constituting numerous enzymatic components, playing a crucial role in overcoming various stress conditions. Mainly, these plant enzymes are superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), glutathione peroxidase (GPX), glutathione reductase (GR), glutathione S-transferases (GST), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), and dehydroascorbate reductase (DHAR), which work as part of the antioxidant defence system. These enzymes together form a complex set of mechanisms to minimise, buffer, and scavenge the reactive oxygen species (ROS) efficiently. The present review is aimed at articulating the current understanding of each of these enzymatic components, with special attention on the role of each enzyme in response to the various environmental, especially abiotic stresses, their molecular characterisation, and reaction mechanisms. The role of the enzymatic defence system for plant health and development, their significance, and cross-talk mechanisms are discussed in detail. Additionally, the application of antioxidant enzymes in developing stress-tolerant transgenic plants are also discussed.

Histocytological analysis carried out on leaf explants of Coffea arabica undergoing somatic embryogenesis revealed that, using a culture method involving a single Gelrite-containing semisolid medium, the entire region surrounding the edge of the plant-derived leaf explants showed the differentiation of organized structures with little or no callusing. Histological examination of embryogenesis without callus formation (direct somatic embryogenesis) revealed that at approximately 1 week after the explant had been placed in culture, the development of the embryo began in the form of a small, isodiametric, densely cytoplasmic cell that underwent a series of organized divisions. In embryogenesis from callus (indirect somatic embryogenesis), however, the embryogenic cell was observed within the first week. Our histological observations indicate that both direct and indirect somatic embryos of coffee that form on explanted leaf segments and callus, respectively, have a unicellular origin.

Cell differentiation depends on the proper and sequential expression of key genes required for morphogenesis. Several aspects of control are required for this which include: chromatin modifications, DNA methylation, correct amount of particular transcription factors, proper nuclear arrangement, etc. During the last few years the homeobox transcription factor WUSCHEL (WUS) has been shown to cause dedifferentiation when expressed on somatic cells followed by a production of new stem cells that can lead to somatic embryogenesis or organogenesis. We found that expression of WUS in coffee plants can induce calli formation as well as a 400% increase somatic embryo production. The results show that transgenic expression of the transcription factor WUS can be useful to increase somatic embryogenesis in heterologous systems. However, a critical developmental stage and additional hormonal requirements are required for the induction of embryogenesis by WUS in Coffea canephora.

A cDNA corresponding to 16 kDa of the maize cyclin D2 N-terminus was cloned and this polypeptide was overexpressed to produce homologous antibodies. This antibody recognized a 38 kDa protein in extracts from maize embryonic axes which corresponds to the predicted size for cyclin D2 protein. Expression of cyclin D2 was followed at the transcriptional and protein levels, and the effect of cytokinins and abscisic acid (ABA) was followed during maize germination. Cytokinins importantly stimulated cyclin D2 gene expression at late germination times and sucrose was necessary for stimulation, whereas the effect of ABA was not different from that in controls. However, cyclin D2 protein levels in control axes reached a peak at 6 h germination, declining thereafter, and neither cytokinins nor ABA modified this behavior. Two cyclic-dependent kinase A (Cdk-A)-type proteins and proliferating cell nuclear antigen (PCNA) were found co-immunoprecipitating with cyclin D2, and these immunoprecipitates were able to phosphorylate both histone H1 and the maize retinoblastoma-related protein (RBR). This protein kinase activity differed from the pattern of protein accumulation during germination, and the activity was not modified by either cytokinins or ABA. We discuss these findings in terms of the importance of the cell cycle for the germination process.

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Fusarium species belonging to the Fusarium fujikuroi species complex (FFSC) are associated with maize in northern Mexico and cause Fusarium ear and root rot. In order to assess the diversity of FFSC fungal species

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