Light exaggerates apical hook curvature through phytochrome actions in tomato seedlings

Abstract

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) micromol 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.

Ethylene Is Not Responsible for Phytochrome-Mediated Apical Hook Exaggeration in Tomato

“…Ponte-Rosa, Sekaiichi and Seikoh No.17, were purchased, respectively, from Ishihara Seed Co. (Sakai, Japan), Marutane Seed Co. (Kyoto, Japan) and Watanabe Seed Co. (Misatomachi, Miyagi, Japan). The three cultivars behaved similarly in phytochrome-mediated hook exaggeration (Shichijo et al, 2010a,b; Shichijo and Hashimoto, 2013), and were used arbitrarily in the present experiments. Seeds were sterilized for 10–20 min with 25-times diluted solution of commercial bleach, followed by thorough washing with running tap water for 30–90 min, and soaked in sterilized distilled water until full imbibition.…”

“…In tomato and some other dicotyledonous species characterized by seeds bearing the hard-to-split tough seed coat and the rich endosperm, the apical hook is markedly exaggerated by exposure to light, mediated by phytochromes through the very low, low and high irradiance responses, and hence activated by red (R) as well as far-red light (FR). The hook exaggeration and its subsequent opening motion facilitate the release of the seed coat, and contribute to the survival of the seedling above the ground (Shichijo et al, 2010a,b; Shichijo and Hashimoto, 2013). …”

Significance of light-induced hook exaggeration as reinforced by the concomitant anatomical change of germinating tomato seeds

“…1A-6). These light actions are fluence ratedependent, 4 and under the conditions for Figure 1B the hook angle reached more than 300° and the upper arm became about 1.5-fold as long as the lower arm at some developmental stage before the vigorous elongation of the lower arm (numeral data not shown; see Fig. 1B-2).…”

“…In the light, however, the seedling responds to light coming through soil by LIHE and the upper arm extension (Fig. 1B-2), which bring the seed part into a deeper depth, eventually allow the time enough (additionally one day, 4 cf. Fig.…”

“…4 Based on some simulation experiments for germination in the field, we have proposed that LIHE may play a role, not in breaking through compacted soil surface, but in securing to release the seed coat at some depth of soil in response to light coming through soil gaps. 4 Here we present additional observations and experimental data not published yet and reinforce the proposed significance of the recently discovered photo-response.…”