Somatic embryogenesis (SE) is a means by which plants can regenerate bipolar structures from a somatic cell. During the process of cell differentiation, the explant responds to endogenous stimuli, which trigger the induction of a signaling response and, consequently, modify the gene program of the cell. SE is probably the most studied plant regeneration model, but to date it is the least understood due to the unclear mechanisms that occur at a cellular level. In this review, the authors seek to emphasize the importance of signaling on plant SE, highlighting the interactions between the different plant growth regulators (PGR), mainly auxins, cytokinins (CKs), ethylene and abscisic acid (ABA), during the induction of SE. The role of signaling is examined from the start of cell differentiation through the early steps on the embryogenic pathway, as well as its relation to a plant’s tolerance of different types of stress. Furthermore, the role of genes encoded to transcription factors (TFs) during the embryogenic process such as the LEAFY COTYLEDON (LEC), WUSCHEL (WUS), BABY BOOM (BBM) and CLAVATA (CLV) genes, Arabinogalactan-proteins (AGPs), APETALA 2 (AP2) and epigenetic factors is discussed.

Coffee is a very important commodity in the world. It is a perennial plant and as a consequence its genetic improvement had been difficult. The biotechnology, in particular somatic embryogenesis (SE) has helped to the propagation of commercial coffee cultivars. Along almost 50 years, different research groups around the world have developed uncountable SE processes to produce an efficient, reproducible, and reliable SE process. Many of the factors that affect the embryogenic response have been studied. It has been demonstrated that the genetic modification of coffee is possible. This transformation can lead to the improvement of varieties with very attractive agronomic traits, such as low caffeine content, resistant to diseases and pests. IntroductionCoffee is one of the two major commodities in the world. The cultivation of coffee gives employment to more than 80 million people, and every year, more than 11 million hectares are cultivated around the world.All coffee species are trees or perennial woody bushes. The coffee plants differ among themselves in different aspects such as morphology, size, and ecological adaptation. Although there are more than 124 species of the Coffea genus, only two are used to produce coffee commercially, Coffea arabica and C. canephora, which accounts for 60 and 40 % of world coffee production, respectively. C. canephora (2n = 2x = 22) is an outcrossing diploid, allogamous, consisting of polymorphic populations of strongly heterozygous individuals and with high caffeine content. C. arabica is an allotetraploid (2n = 4x = 44) derived from a spontaneous

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Cytokinins (CK) are plant growth regulators involved in multiple physiological processes in plants. One less studied aspect is CK homeostasis (HM). The primary genes related to HM are involved in biosynthesis (IPT), degradation (CKX), and signaling (ARR). This paper demonstrates the effect of auxin (Aux) and CK and their cross talk in a Coffea canephora embryogenic system. The transcriptome and RT-qPCR suggest that Aux in pre-treatment represses biosynthesis, degradation, and signal CK genes. However, in the induction, there is an increase of genes implicated in the CK perception/signal, indicating perhaps, as in other species, Aux is repressing CK, and CK are inducing per se genes involved in its HM. This is reflected in the endogenous concentration of CK; pharmacology experiments helped study the effect of each plant growth regulator in our SE system. We conclude that the Aux–CK balance is crucial to directing somatic embryogenesis in C. canephora.

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