Boron-Mediated Plant Somatic Embryogenesis: A Provocative Model

A central question in plant regeneration biology concerns the primary driving forces invoking the acquisition of somatic embryogenesis. Recently, the role of micronutrient boron (B) in the initiation and perpetuation of embryogenesis has drawn considerable attention within the scientific community. This interest may be due in part to the bewildering observation that the system-wide induction of embryogenic potential significantly varied in response to a minimal to optimal supply of B (minimal ≤ 0.1？mM, optimal = 0.1？mM). At the cellular level, certain channel proteins and cell wall-related proteins important for the induction of embryogenesis have been shown to be transcriptionally upregulated in response to minimal B supply suggesting the vital role of B in the induction of embryogenesis. At the molecular level, minimal to no B supply increased the endogenous level of auxin, which subsequently influenced the auxin-inducible somatic embryogenesis receptor kinases, suggesting the role of B in the induction of embryogenesis. Also, minimal B concentration may “turn on” other genetic and/or cellular transfactors reported earlier to be essential for cell-restructuring and induction of embryogenesis. In this paper, both the direct and indirect roles of B in the induction of somatic embryogenesis are highlighted and suggested for future validation. 1. Introduction 1.1. Somatic Embryogenesis in Plants In plants, somatic embryogenesis is a multistep and complex regeneration process which begins with the formation of proembryonic mass followed by somatic embryo initiation, maturation, and, ultimately, entire plantlet regeneration [1]. Mostly, it refers to the developmental plasticity characteristic of the differentiated cells to regain their totipotency and convert into embryos. In theory, each somatic cell has the potential to convert itself into a somatic embryo, though very few somatic cells are capable of undergoing such complicated morphological transformation under culture conditions. In fact, only certain plant taxa and selected explants types have been shown to be capable of inducing embryogenic potential with in vitro cultures. As a result of these complicating factors, knowledge of how to “switch on” all somatic cells with such embryogenic potential is quite limited. Since the initial descriptions of in vitro somatic embryogenesis [2, 3], one important characteristic of somatic embryos is the continuous growth resulting from the absence of developmental arrest [4]. In general, the process has three different stages of embryo development: globular,