These authors could also show that the two auxin biosynthesis genes CYP79B2 and CYP79B3, which are rate limiting in a Trp-dependent pathway ( Zhao et al., 2002), are active in the root meristem, further supporting that biosynthesis takes place in this high-auxin-response region. (2005) could demonstrate a peak in biosynthesis rate in the most apical 0.5 mm of the root tip, suggesting both auxin transport from the aerial part of the seedlings and local auxin biosynthesis contribute to the high auxin levels of Arabidopsis thaliana root tips. By measuring the auxin biosynthesis rate in young seedlings, Ljung et al. For example, in the root, auxin levels peak in the root tip, including the meristem ( Sabatini et al., 1999). Auxin biosynthesis occurs both at distant sites as a source for PAT and at local auxin response sites.
However, recent data also suggest that other aspects of auxin homeostasis play critical roles in determining the size and position of local auxin peaks/gradients. These local auxin peaks/gradients largely depend on tightly regulated timing and direction of polar auxin transport (PAT) mediated by, for example, members of the PIN family of auxin efflux facilitators, and disrupted functions of the PIN proteins result in defects in the establishment of embryo polarity, organ positioning, and organ development ( Friml et al., 2002, 2003 Marchant et al., 2002 Benková et al., 2003 Reinhardt et al., 2003 Heisler et al., 2005). Auxin responses are largely dependent on the auxin concentration perceived by individual cells, resulting from tight homeostatic control of auxin levels (reviewed in Woodward and Bartel, 2005), and evidence for the involvement of local auxin gradients or peaks in several of the above-mentioned processes have been highlighted during the last few years ( Friml et al., 2002, 2003 Benková et al., 2003 Heisler et al., 2005 Scarpella et al., 2006). The plant hormone auxin plays critical roles during plant growth and development and has been proposed to act in pathways controlling embryo axis formation, organ phyllotaxis, organ and tissue differentiation, root meristem organization, and tropic responses ( Went, 1974 Mattsson et al., 1999 Sabatini et al., 1999 Reinhardt et al., 2000 Benková et al., 2003 Friml et al., 2003). Furthermore, the lack of a shoot apical meristem in seedlings carrying a fusion construct between STY1 and a repressor domain, SRDX, suggests that STY1, and other SHI/STY members, has a role in the formation and/or maintenance of the shoot apical meristem, possibly by regulating auxin levels in the embryo. This suggests that the SHI/STY family members are essential regulators of auxin-mediated leaf and flower development. Our results suggest that STY1, and most likely other SHI/STY members, are DNA binding transcriptional activators that target genes encoding proteins mediating auxin biosynthesis. Here, we further examine the function of STY1 by analyzing its DNA and protein binding properties.
We have also shown previously that SHI/STY family members redundantly affect development of flowers and leaves. We have shown previously that the induction of the SHORT-INTERNODES/STYLISH ( SHI/STY) family member STY1 results in increased transcript levels of the YUCCA ( YUC) family member YUC4 and also higher auxin levels and auxin biosynthesis rates in Arabidopsis seedlings. The disruption of normal auxin biosynthesis in mouse-ear cress ( Arabidopsis thaliana) leads to severe abnormalities, suggesting that spatiotemporal regulation of auxin biosynthesis is fundamental for normal growth and development. The establishment and maintenance of auxin maxima in vascular plants is regulated by auxin biosynthesis and polar intercellular auxin flow.
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