The Role of Auxin and Cytokinin Signaling Components in de novo Shoot Organogenesis

Abstract

De novo shoot organogenesis (DNSO) is a widely used procedure for obtaining regenerated plant shoots for biotechnological, industrial and conservation purposes. Particular morphogenic events leading to shoot regeneration are induced by plant hormones auxin and cytokinin that are exogenously supplemented to the regenerating explants through the nutrient media. It was shown, in Arabidopsis and in other plant species, that the early stages of DNSO are crucially dependent on auxin signaling and include the development of pluripotent primordia that are developmentally identical to lateral root primordia, whereas in the later stages, these primordia acquire the shoot identity and develop a shoot apical meristem (SAM) in a process that is governed mostly by cytokinin, but is also influenced by auxin signaling. In this chapter, we discuss the current state of knowledge about the particular components of auxin and cytokinin signaling that are specifically involved in the phytohormonal regulation of DNSO. Throughout DNSO, the auxin signaling multicellular domains are generated through the action of auxin influx and efflux carriers, whereas AUXIN RESPONSE FACTORs (ARFs), that are modulated by the Aux/IAA repressors, regulate gene expression during various stages of DNSO. Conversely, the cytokinin signals relevant for DNSO are perceived overwhelmingly through the receptor ARABIDOPSIS HISTIDINE KINASE4 (AHK4) and effected through the type-B ARABIDOPSIS RESPONSE REGULATORs (ARRs), whereas type-A ARRs act as attenuators of the cytokinin signals. Throughout DNSO, auxin and cytokinin signals enter mutual crosstalk, and crosstalk with other phytohormones, sugars and other developmental signals, for a complex and fine-tuned regulation of shoot regeneration.