c a way, auxin response factors (ARFs), which are inhibited by Aux/IAA, are released to activate the transcription of auxin-responsive genes. A negative-feedback loop is active since auxin negatively regulates Aux/IAA abundance while their respective genes are themselves auxin-induced (Fig. 14) (Benjamins and Scheres 2008). Auxin action has also been linked to changes in cellular redox status (De Tullio et al. 2009; Eckardt 2010; Jiang et al. 2010), as well as alternative auxin-responsive pathways, and an even more complex signaling has been described (Benjamins and Scheres 2008; Lau et al. 2008; Strader et al. 2008). However, detailed reviewing of this mechanism is beyond the scope of the current chapter.
As recently demonstrated, from the earliest phases of embryogenesis, cytokinins act antagonistically to auxin in controlling root pattern through to postembryonic development (Muller and Sheen 2008; Moubayidin et al. 2009; Perilli et al 2009; Werner and Schmulling 2009). In particular, during embryogenesis, cytokinin/ auxin interplay has an essential role for the specification of the first root stem-cell niche, which is marked by an auxin concentration maximum occurring in a single cell (i.e., hypophysis) (Fig. 9). As mentioned above (see Sect. 4.3.1), a high level of auxin at the level of the hypophysis activates the expression of two negative regulators of cytokinin signaling, thus suppressing cytokinin output (Muller and Sheen 2008).
As far as postembryonic root development is concerned, cytokinins promote cell differentiation at the boundaries between the division and elongation zones (i.e., the transition zone) by suppressing auxin signaling and transport, while auxin promotes cell division by suppressing cytokinin signaling (Blilou et al. 2005; Dello Ioio et al 2007, 2008; Perilli et al. 2009; Ruzicka et al. 2009). In particular, increased
cytokinin levels reduce root meristem size and inhibit root growth, by modulating PIN expression and therefore auxin distribution. This interplay relies on the convergence of both hormone classes on the same target gene, SHORTHYPOCOTYL (SHY2), which encodes an IAA-class repressor protein of the auxin signaling pathway (Fig. 15) (Benjamins and Scheres 2008). In particular, SHY2 prevents the activation of auxin-responsive genes by negatively regulating PIN 1, 3, and 7 and therefore repressing auxin export at the vascular transition zone (Dello Ioio et al. 2008). Note that auxin and cytokinins control SHY2 in opposite ways; auxin drives SHY2 protein degradation, through a ubiquitin-ligase complex (SCFTIR1), thus stabilizing PIN expression level and auxin distribution. Conversely, cytokinins promote SHY2 expression, via a two-component signaling pathway (AHK3/AA1), which activates ARR1, a primary type-B cytokinin response factor that binds to the SHY2 promoter specifically at the vascular tissue transition zone (Fig. 15) (Benjamin and Scheres 2008; Dello Ioio et al. 2008; Moubayidin et al. 2009). Thus, in the presence of cytokinins, auxin transport is limited, auxin-dependent cell division is antagonized, and cell elongation occurs. Recently, it has been shown in maize that an ARR-mediated cytokinin signal is inactive in the QC and such repression could be related to the maintenance of the QC (Jiang et al. 2010). Finally, it must be underlined that a negative feedback control is active, since SHY2 protein downregulates ISOPENTENYLTRANSFERASE (AtlPT), which encodes an enzyme involved in a rate-limiting step of cytokinin biosynthesis (Fig. 15) (Sakakibara 2006; Dello Ioio et al. 2008). Note also that cytokinins antagonize auxin signaling and distribution during lateral root formation (Fukaki and Tasaka 2009). However, cytokinin-induced inhibition of root primordia development remains unaltered in auxin mutants, and auxin cannot reverse the cytokinin-induced inhibitory effect, suggesting that auxin and cytokinins likely control lateral root initiation through discrete pathways (Werner and Schmiilling 2009)
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