Abstract
Auxin and cytokinin signaling participates in regulating a large spectrum of developmental and physiological processes in plants. The shoots and roots of plants have specific and sometimes even contrary responses to these hormones. Recent studies have clearly shown that establishing the spatiotemporal distribution of auxin and cytokinin response signals is central for the control of shoot apical meristem (SAM) induction in cultured tissues. However, little is known about the role of these hormones in root apical meristem (RAM) initiation. Here, we found that the expression patterns of several regulatory genes critical for RAM formation were correlated with the establishment of the embryonic root meristem during somatic embryogenesis in Arabidopsis. Interestingly, the early expression of the WUS-RELATED HOMEOBOX 5 (WOX5) and WUSCHEL genes was induced and was nearly overlapped within the embryonic callus when somatic embryos (SEs) could not be identified morphologically. Their correct expression was essential for RAM and SAM initiation and embryonic shoot–root axis establishment. Furthermore, we analyzed the auxin and cytokinin response during SE initiation. Notably, cytokinin response signals were detected in specific regions that were correlated with induced WOX5 expression and subsequent SE formation. Overexpression of the ARABIDOPSIS RESPONSE REGULATOR genes ARR7 and ARR15 (feedback repressors of cytokinin signaling), disturbed RAM initiation and SE induction. These results provide new information on auxin and cytokinin-regulated apical–basal polarity formation of shoot–root axis during somatic embryogenesis.
Highlights
The most critical event during embryogenesis appears to be the formation of the shoot apical meristem (SAM) and root apical meristem (RAM), from which almost the entire plant is postembryonically established (Meinke, 1991; Scheres, 2007)
Green primary somatic embryos (PSEs) can be generated from explants, and disk-like embryonic calli are produced from PSEs in embryonic callus-inducing medium (ECIM) containing 2,4-dichlorophenoxyacetic acid (2,4-D)
To analyze the spatiotemporally regulated formation of the root stem cell niche at the early stages of somatic embryogenesis, we examined the expression patterns of genes that play critical roles in root stem cell specification
Summary
The most critical event during embryogenesis appears to be the formation of the shoot apical meristem (SAM) and root apical meristem (RAM), from which almost the entire plant is postembryonically established (Meinke, 1991; Scheres, 2007). In the SAM of Arabidopsis, WUSCHEL (WUS) is a critical regulator, and encodes a homeodomain protein that is required for stem cell formation and maintenance (Laux et al, 1996). An element required in the QC to maintain columella stem cells is WUS-RELATED HOMEOBOX 5 (WOX5), a putative homeodomain transcription factor (Haecker et al, 2004). In the QC, WOX5 acts in a similar way to WUS in the organizing center (OC) of the SAM, highlighting molecular and developmental similarities between the stem cell niches of both root and shoot meristems (Perilli et al, 2012). PLETHORA (PLT) genes, which belong to the AP2-type transcription factor family, play a key role in the specification and maintenance of root stem cells from early embryogenesis onward (Aida et al, 2004; Galinha et al, 2007). The SCARECROW (SCR)/SHORTROOT (SHR) transcription factors are required to maintain stem cell activity within the RAM (Di Laurenzio et al, 1996; Helariutta et al, 2000; Sabatini et al, 2003)
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