MicroRNAs (miRNAs) are endogenous ∼23 nt RNAs that play important gene-regulatory roles in animals and plants by pairing to the mRNAs of protein-coding genes to direct their posttranscriptional repression. This review outlines the current understanding of miRNA target recognition in animals and discusses the widespread impact of miRNAs on both the expression and evolution of protein-coding genes.

MicroRNAs: Target Recognition and Regulatory Functions

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.

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Signaling Overview of Plant Somatic Embryogenesis.

Successful regeneration of genetically modified plants from cell culture is highly dependent on the species, genotype, and tissue-type being targeted for transformation. Studies in some plant species have shown that when expression is altered, some genes regulating developmental processes are capable of triggering plant regeneration in a variety of plant cells and tissue-types previously identified as being recalcitrant to regeneration. In the present research, we report that developmental genes encoding GROWTH-REGULATING FACTORS positively enhance regeneration and transformation in both monocot and dicot species. In sugar beet (Beta vulgaris ssp. vulgaris), ectopic expression of Arabidopsis GRF5 (AtGRF5) in callus cells accelerates shoot formation and dramatically increases transformation efficiency. More importantly, overexpression of AtGRF5 enables the production of stable transformants in recalcitrant sugar beet varieties. The introduction of AtGRF5 and GRF5 orthologs into canola (Brassica napus L.), soybean (Glycine max L.), and sunflower (Helianthus annuus L.) results in significant increases in genetic transformation of the explant tissue. A positive effect on proliferation of transgenic callus cells in canola was observed upon overexpression of GRF5 genes and AtGRF6 and AtGRF9. In soybean and sunflower, the overexpression of GRF5 genes seems to increase the proliferation of transformed cells, promoting transgenic shoot formation. In addition, the transformation of two putative AtGRF5 orthologs in maize (Zea mays L.) significantly boosts transformation efficiency and resulted in fully fertile transgenic plants. Overall, the results suggest that overexpression of GRF genes render cells and tissues more competent to regeneration across a wide variety of crop species and regeneration processes. This sets GRFs apart from other developmental regulators and, therefore, they can potentially be applied to improve transformation of monocot and dicot plant species.

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Overexpression of the Transcription Factor GROWTH-REGULATING FACTOR5 Improves Transformation of Dicot and Monocot Species.

http://science.sciencemag.org/content/sci/343/6174/1025.full.pdf

Cell Surface ABP1-TMK Auxin-Sensing Complex Activates ROP GTPase Signaling

Chromatin Accessibility Dynamics and a Hierarchical Transcriptional Regulatory Network Structure for Plant Somatic Embryogenesis.

Plants have a high plasticity for cell differentiation that not only allow them to regenerate damaged parts of their body but under certain conditions, plant cells can regain totipotency and change their developmental fate to re-differentiate into a somatic embryo and finally a whole plant. The ability to change their cell fate and generate somatic embryos is restricted to a discrete group of cells. Somatic embryogenesis (SE) is a complex phenomenon, and it is poorly understood. SE can start in various ways and the differential response among cells can be stimulated by several factors, among them highlight the genotype, the physiological state and the origin of the explants, as well as the medium culture or the plant growth regulators (PGRs) used for in vitro culture. However, several stress treatments such as low or high temperature, heavy metals, osmotic shock, among others, might play a crucial role in SE induction, even in the absence of exogenous PGRs. Here, we will provide an overview about the role of stresses and their influence on SE induction.

The Relationship Between Stress and Somatic Embryogenesis

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.

Somatic Embryogenesis in Coffea spp.

Auxins are involved in almost every aspect of plant physiology. For instance, auxins play a central role in the differentiation process during the development of plants. Furthermore, the homeostasis of auxins involves biosynthesis and degradation as well as their conjugation with amino acids and carbohydrates, and the hydrolysis of some of these conjugates liberates indole-3-acetic acid (IAA). The balance in the IAA concentration triggers its own signal transduction pathway and produces a molecular and biochemical response. This response begins with the sensing of the IAA concentration through the construction of a co-receptor complex that includes an F-box protein from the TRANSPORT INHIBITOR RESPONSE 1 (TIR1)/AUXIN SIGNALING F-BOX PROTEIN (AFB) family and a member of the AUXIN/IAA-INDUCIBLE (AUX/IAA) family of transcriptional repressors. This complex allows the expression of auxin response genes. Most of the auxin-regulated processes are tightly regulated. Several differentially expressed miRNAs, which alter the auxin response, have been identified in Arabidopsis thaliana somatic embryogenesis development. Also, during the stress response in soybean roots, auxin-responsive cis-elements in the promoters of many salt-responsive miRNAs have been found. These findings suggest that miRNAs may be regulated by auxins. In this chapter, we analyze developing research related to the interaction between auxins and miRNAs.

The Role of MiRNAs in Auxin Signaling and Regulation During Plant Development

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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Johny R. Avilez-Montalvo

Papers

Signaling Overview of Plant Somatic Embryogenesis.

The Relationship Between Stress and Somatic Embryogenesis

Somatic Embryogenesis in Coffea spp.

The Role of MiRNAs in Auxin Signaling and Regulation During Plant Development

Auxin-Cytokinin Cross Talk in Somatic Embryogenesis of Coffea canephora