Abnormalities in somatic embryogenesis caused by 2,4-D: an overview

Somatic embryogenesis is a morphogenetic event where somatic cells have the ability to produce embryos without gamete fusion. It is used as a technique for plant mass propagation. It is a process that has six well defined steps such as induction, expression, development, maturation, germination and plant conversion. These steps are characterized by distinct physiological, morphological and molecular events. Although somatic embryogenesis has been established in several plant species, there remains many problems to be solved. The main problem in somatic embryogenesis is the large number of abnormal embryos produced which cannot germinate nor convert into normal plants. Abnormalities in somatic embryos (SE) can be generated by genetic or epigenetic changes in the DNA. These changes in the DNA can be influenced by external factors such as the use of plant growth regulators and mutagenic substances or stress factors applied to the plant tissue such as high and low temperatures, drought, salinity, and heavy metals. Abnormalities generated by genetic changes in the DNA are hardly reversible; however, abnormalities generated by epigenetic changes may be reversible and the abnormal embryos are able to produce normal plants in most cases. This review focuses on the identification of the main factors that can cause abnormal SE development in different plant species, suggest how SE abnormalities are related to somaclonal variations and identify which genes may be involved with embryo abnormalities. Zygotic embryo abnormalities in Arabidopsis thaliana mutants are listed with the aim to understand the main genetic mechanisms involved in embryo aberrations. The abnormalities in somatic embryos are related to the use of 2,4-D in most of the published protocols, this sintetic auxin disrupts the endogenous auxin balance and the auxin polar transportation interfering with the embryo apical-basal polarity.