Light exaggerates apical hook curvature through phytochrome actions in tomato seedlings
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Contrary to the established notion that the apical hook of dark-grown dicotyledonous seedlings opens in response to light, we found in tomato (Solanum lycopersicum L.) that the apical hook curvature is exaggerated by light. Experiments with several tomato cultivars and phytochrome mutants, irradiated with red and far-red light either as a brief pulse (Rp, FRp) or continuously (Rc, FRc), revealed: the hook-exaggeration response is maximal at the emergence of the hypocotyl from the seed; the effect of Rp is FRp-reversible; fluence–response curves to a single Rp or FRp show an involvement of low and very low fluence responses (LFR, VLFR); the effect of Rc is fluence-rate dependent, but that of FRc is not; the phyA mutant (phyA hp-1) failed to respond to an Rp of less than 10−2 μmol m−2 and to an FRp of all fluences tested as well as to FRc, thus indicating that the hook-exaggeration response involves phyA-mediated VLFR. The Rp fluence–response curve with the same mutant also confirmed the presence of an LFR mediated by phytochrome(s) other than phyA, although the phyB1 mutant (phyB1 hp-1) still showed full response probably due to other redundant phytochrome species (e.g., phyB2). Simulation experiments led to the possible significance of hook exaggeration in the field that the photoresponse may facilitate the release of seed coat when seeds germinate at some range of depth in soil. It was also observed that seed coat and/or endosperm are essential to the hook exaggeration.
KeywordsApical hypocotyl hook High-pigment-1 Hook curvature Mutant Phytochrome Solanum lycopersicum L.
- FR (FRp, FRc)
Far-red light (pulse of FR, continuous FR)
Low fluence response
FR-absorbing form of phytochrome
Ratio of Pfr to total phytochrome
- R (Rp, Rc)
Red light (pulse of R, continuous R)
Very low fluence response
We thank Professor Richard E. Kendrick (Wageningen Agricultural University, Wageningen, The Netherlands) for supplying seeds of tomato mutants, and Professors Tetsuro Mimura and Taisaku Amakawa (Kobe University, Japan), and Mr. Nobuyuki Kawashima (President, Kyokko Trading Co, Tokyo, Japan) for their kind cooperation.
Supplementary Movie S1: Time-lapse movie. Growth and hook curvature exaggeration in tomato seedlings under continuous WL, taken at 10-min intervals during 6 days (144 h). White fluorescent light, 17 μmol m−2 s−1. Cv. Ponte-Rosa Supplementary material 1 (MP4 4815 kb)
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