Hormone Signalling Crosstalk in Plant Growth Regulation Stephen Depuydt, Christian S. Hardtke  Current Biology  Volume 21, Issue 9, Pages R365-R373 (May 2011) DOI: 10.1016/j.cub.2011.03.013 Copyright © 2011 Elsevier Ltd Terms and Conditions

Figure 1 Examples of auxin, gibberellin or brassinosteroid involvement in growth phenomena across the plant life cycle. Auxin signalling is essential for embryogenesis and defines the later growth axes of the plant. In the root meristem, auxin action is shaped through polar auxin transport and determines root system growth and its branching pattern. In the shoot apex, auxin accumulation at the sites of primordia is essential for lateral organ formation. Auxin activity is also observed in floral organ primordia, ovule primordia and zygotes. Gibberellins have a major influence on germination, plant growth in general (mainly via cell expansion), floral development and flowering time. Brassinosteroids have been reported in almost all plant tissues, with highest levels found in seeds, pollen and young growing tissues. Brassinosteroids act largely post-embryonically with pronounced effects on general plant growth via cell elongation, vascular differentiation, and reproductive development. Current Biology 2011 21, R365-R373DOI: (10.1016/j.cub.2011.03.013) Copyright © 2011 Elsevier Ltd Terms and Conditions

Figure 2 Schematic working model of the spatio-temporal regulatory interactions between hormone pathways in root meristem growth, based on [82–85]. A complex network of regulatory interactions occurring across the transition zone balances cell division and differentiation/elongation. At early stages (3 days after germination, dag), the physiological conditions, notably high gibberellin levels required for germination, favour PIN gene expression and thereby polar auxin transport (PAT) and meristem growth. This process eventually comes to a halt around 5 dag as feedback regulation of hormone biosynthesis (not indicated) switches the regulatory interactions in favour of suppressing PAT and thereby promoting cell differentiation. See text for more details. Current Biology 2011 21, R365-R373DOI: (10.1016/j.cub.2011.03.013) Copyright © 2011 Elsevier Ltd Terms and Conditions

Figure 3 Overview of the evolution of the auxin, gibberellin and brassinosteroid pathways on the basis of the available analyses. The occurrence of functional hormone receptor pathways is depicted according to their appearance during land plant evolution. Hormones are represented by circles, while the plant's ability to actively respond to a particular hormone is shown as a star. ID and LRR22 refer to the island domain and the leucine-rich repeat 22, respectively, of BRI1, which are essential for brassinosteroid binding. Current Biology 2011 21, R365-R373DOI: (10.1016/j.cub.2011.03.013) Copyright © 2011 Elsevier Ltd Terms and Conditions