Cover illustration: Much of the past 100 years plus of intensive research on phototropism in plants owes much of it impetus to Charles (and Francis) Darwin's seminal book, 'The power of movement in plants' (1880). The top photograph shows dark-grown Arabidopsis thaliana seedlings exhibiting phototropic curvature towards a unidirectional blue light given from the right. Below the text is shown Darwin's depiction of canary grass (Phalaris canariensis) phototropism from 'The power of movement in plants' to illustrate the 'state-of-the art' in 1880 [image reproduced with permission from John van Wyhe, ed. The complete works of Charles Darwin online (http://darwin-online.org.uk/)]. The photographs and diagrams in the bottom twothirds are meant to show how far the field has progressed since Darwin, especially within the past two decades. Left, top three panels, phot1-gfp fusion protein signals (green colour) in the same hypocotyl cells of a dark-grown Arabidopsis seedling 1 min (top), 5 min (middle), and 20 min (bottom) after exposure to a pulse of blue laser light; note that the signal moves from wholly plasmamembrane-localized to a fair proportion being cytoplasmically localized (U Pedmale, RB Celaya, E Liscum, unpublished data). Top right, ribbon diagram of the crystal structure of the photosensory LOV2 domain from oat (PDB ID: 2v1a; Halavaty AS, Moffat K. 2007. N- and C-terminal flanking regions modulate light-induced signal transduction in the Lov2 domain of the blue light sensor phototropin 1 from Avena sativa. Biochemistry 46, 14001-14009). Second right image, chemical structure of indole-3-acetic acid (auxin), the 'influence' Darwin proposed moved from one portion of the plant to another to affect the growth response. Third right image, in situ hybridization of a hypocotyl section from dark-grown Brassica oleracea probed with a BHLH (AtBHLH134) transcription factor mRNA that has been shown to be differentially expressed in response to tropic stimulation; note the intense reddish colour in the epidermal cell layer on the 'shaded' side as compared to the equivalent cell layer on the 'lit' side (A Esmon, E Liscum, unpublished data). The bottom images show a cellular model for phototropic signal-response in a cross-section of an Arabidopsis seedling hypocotyl. Phot1 activation is depicted by the intensity of the blue coloration (see right-hand key) in the plasma membrane of the epidermal and the first cortical layer of cells; auxin distribution is depicted by the intensity of the yellow coloration within the same two cell layers in which phot1 activity is shown; the differential transcriptional response, regulated by the auxin-responsive transcription factor ARF7, is depicted by the intensity of red coloration in the nuclei in the outer two cell layers; the endodermal cell layer separating the cortical cells and the vasculature is shown bordered in green. (Layout by E Liscum.)
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