We have identified the first molecular components that control lateral root founder cell identity in the Arabidopsis root. These include an IAA28-dependent auxin signaling module in the basal meristem region that regulates GATA23 expression and thereby lateral root founder cell specification and root branching patterns.

Glycogen synthase kinase 3 (GSK3) is a key regulator in signaling pathways in both animals and plants. Three Arabidopsis thaliana GSK3s are shown to be related to brassinosteroid (BR) signaling. In a phenotype-based compound screen we identified bikinin, a small molecule that activates BR signaling downstream of the BR receptor. Bikinin directly binds the GSK3 BIN2 and acts as an ATP competitor. Furthermore, bikinin inhibits the activity of six other Arabidopsis GSK3s. Genome-wide transcript analyses demonstrate that simultaneous inhibition of seven GSK3s is sufficient to activate BR responses. Our data suggest that GSK3 inhibition is the sole activation mode of BR signaling and argues against GSK3-independent BR responses in Arabidopsis. The opportunity to generate multiple and conditional knockouts in key regulators in the BR signaling pathway by bikinin represents a useful tool to further unravel regulatory mechanisms.

Gradients of the plant hormone auxin, which depend on its active intercellular transport, are crucial for the maintenance of root meristematic activity. This directional transport is largely orchestrated by a complex interaction of specific influx and efflux carriers that mediate the auxin flow into and out of cells, respectively. Besides these transport proteins, plant-specific polyphenolic compounds known as flavonols have been shown to act as endogenous regulators of auxin transport. However, only limited information is available on how flavonol synthesis is developmentally regulated. Using reduction-of-function and overexpression approaches in parallel, we demonstrate that the WRKY23 transcription factor is needed for proper root growth and development by stimulating the local biosynthesis of flavonols. The expression of WRKY23 itself is controlled by auxin through the AUXIN RESPONSE FACTOR 7 (ARF7) and ARF19 transcriptional response pathway. Our results suggest a model in which WRKY23 is part of a transcriptional feedback loop of auxin on its own transport through local regulation of flavonol biosynthesis.flavonoid | lateral root | WRKY P lant growth and development are characterized by recurrent organogenesis and continuous adaptation to environmental conditions. These intriguing features rely on the ability to establish and maintain meristematic activity. Both de novo induction and maintenance of root meristematic activity are governed by gradients of the plant hormone auxin (1-3). Although several plant tissues are able to synthesize auxin (4, 5), installation and maintenance of auxin maxima are mediated mainly by polar auxin transport (6). Besides the well-known auxin import (AUXIN RESISTANT 1/LIKE AUX1) and export (PIN-FORMED [PIN] and ABCB/P-GLYCOPROTEIN/MDR) proteins (7-10), additional regulators mediate the flow of auxin throughout the plant. For example, flavonols (plant-specific polyphenolic compounds), have been proposed to act as endogenous auxin transport regulators based on their competition with the synthetic auxin transport inhibitor 1-N-naphthylphthalamic acid (11). Although the molecular targets of flavonol regulation remain unknown, genetic and pharmacologic evidence clearly demonstrate a role for these secondary metabolites as negative regulators of auxin transport (12-18). Flavonol biosynthesis was recently shown to be induced by auxin through a TRANSPORT INHIBITOR RESPONSE 1 (TIR1) auxin receptor-dependent pathway (16). However, our understanding of how flavonol biosynthesis is fine-tuned during development and in response to internal and environmental signals is still limited. Here, we report on the functional characterization of a member of the WRKY family, a large, plant-specific class of transcription factors that has been associated with responses to pathogen attack, mechanical stress, and senescence (19). Our results suggest that proper expression of WRKY23 is essential for normal root development, and that misexpression of WRKY23 causes defects in meristem organization by inte...