The Essential Role of Cytokinin Signaling in Root Apical Meristem Formation during Somatic Embryogenesis

A commentary on Establishment of embryonic shoot–root axis is involved in auxin and cytokinin response during Arabidopsis somatic embryogenesis Plant tissue culture and subsequent organogenesis have been regarded as part of the scientific roots of modern plant biotechnology (Sussex, 2008). Establishments of shoot apical meristem (SAM) and root apical meristem (RAM) are critical steps for somatic embryogenesis (Scheres, 2007). WUSCHEL (WUS), a homeodomain protein, controls stem cell differentiation and maintenance and its ectopic expression facilitated the transition from vegetative growth to embryogenesis in Arabidopsis (Zuo et al., 2002). Gain-of-function of WUS mutant can trigger cell pluripotence and re-establishing new meristems in the cortex, which further reinforced the essential role of WUS in the initiation and maintenance of meristem in planta (Xu et al., 2005). Nevertheless, the underlying mechanisms of how somatic cell maintains plasticity and triggers cellular switch during the sex-free embryogenesis are still elusive, which is one of the top 25 profound as-yet-unanswered scientific questions (Vogel, 2005). During early somatic embryogenesis, while auxin signaling cascade induces the expression of WUS (Su et al., 2011), little is known about the underlying mechanisms for the RAM establishment. The quiescent center (QC) is formed from apical descendant cell after asymmetric division of the hypophysis and essential in maintaining the identity of root stem cells (Möller and Weijers, 2009; Peris et al., 2010). WUSCHEL-RELATED HOMEOBOX 5 (WOX5) plays an analogous role in the QC of the RAM as WUS in organizing center (OC) of the SAM. PLETHORA1 (PLT1) and PLT2, two AP2-type transcription factors, are required to specify and maintain the stem–cell identity in the RAM (Galinha et al., 2007). In addition, transcription factors SCARECROW (SCR) and SHORTROOT (SHR) are necessary to maintain stem cell activity in the RAM (Sabatini et al., 2003). In this study, Dr Xian Sheng Zhang's lab exploited the marker lines of the above genes to investigate the specification and initiation of RAM during nongametic somatic embryogenesis. They detected almost simultaneously induced WOX5 and PLT2 expression in some regions of embryonic callus only 24 h after transferred onto 2,4-D free somatic embryo-inducing medium (SEIM), while the expression of SCR was detected later in 24–36 h. Further, the spatiotemporal expression patterns of WUS and WOX5 were examined by tagged with distinct fluorescence protein markers. Nearly identical expression patterns of them were found after 24 h induction. The WOX5 signals were then detected below the regions of WUS signals, suggesting that …