Light exaggerates apical hook curvature through phytochrome actions in tomato seedlings

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

The apical hook of tomato seedlings is exaggerated by phytochrome actions, while in other species such as bean, pea and Arabidopsis, the hook is exaggerated by ethylene and opens by phytochrome actions. The present study was aimed to clarify mainly whether ethylene is responsible for the phytochrome-mediated hook exaggeration of tomato seedlings. Dark-grown 5-day-old seedlings were subjected to various ways of ethylene application in the dark as well as under the actions of red (R) or far-red light (FR). The ethylene emitted by seedlings was also quantified relative to hook exaggeration. The results show: Ambient ethylene, up-to about 1.0 μL L^-1, suppressed (opened) the hooks formed in the dark as well as the ones exaggerated by R or FR, while at 3.0-10 μL L^-1 it enhanced (closed) the hook only slightly as compared with the most-suppressed level at about 1.0 μL L^-1. Treatment with 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene biosynthesis, did not enhance the hook, only mimicking the suppressive effects of ambient ethylene. The biosynthesis inhibitor, CoCl₂ or aminoethoxyvinylglycine, enhanced hook curvature, and the enhancement was canceled by supplement of ethylene below 1.0 μL L^-1. Auxin transport inhibitor, N-1-naphthylphthalamic acid, by contrast, suppressed curvature markedly without altering ethylene emission. The effects of the above-stated treatments did not differentiate qualitatively among the R-, FR-irradiated seedlings and dark control so as to explain phytochrome-mediated hook exaggeration. In addition, ethylene emission by seedlings was affected neither by R nor FR at such fluences as to cause hook exaggeration. In conclusion, (1) ethylene suppresses not only the light-exaggerated hook, but also the dark-formed one; (2) ethylene emission is not affected by R or FR, and also not correlated with the hook exaggerations; thus ethylene is not responsible for the hook exaggeration in tomato; and (3) auxin is essential for the maintenance and development of the hook in tomato as is the case in other species lacking phytochrome-mediated hook exaggeration. A possible mechanism of phytochrome action for hook exaggeration is discussed.

Identification and functional characterization of two HOOKLESS genes in Tomato (Solanum lycopersicum)

The apical hook of dark-grown dicotyledonous plants results from asymmetric growth of the inner and outer sides of the upper part of the hypocotyl. This protective structure prevents damage of the shoot apical meristem and the young leaves as the seedling pushes through the soil. HOOKLESS (HLS1) of Arabidopsis was recognised as an ethylene response gene whose product is required for hook formation. We cloned two cDNAs from tomato, Sl-HLS1 and Sl-HLS2, and showed through the complementation of the Arabidopsis hls1 mutant that the encoded proteins are functional orthologs of At-HLS1. The genomic clones of Sl-HLS1 and Sl-HLS2 showed similar structure with two introns and three exons. While the data indicate complete functional redundancy between the two tomato HLS genes, only the expression of Sl-HLS2 is regulated by ethylene and auxin and the ethylene-induced expression of Sl-HLS2 is localised in the outer side of the hook while the auxin-responsive expression is not restricted to a specific side of the hook. 1-MCP, an inhibitor of ethylene signalling, inhibited auxin-enhanced accumulation of Sl-HLS2 mRNA in the apical hook suggesting that regulation of Sl-HLS2 by auxin is ethylene-dependent. The overexpression of tomato Sl-HLS1 and Sl-HLS2 in Arabidopsis confers hypersensitivity to ethylene. The data presented bring further insight into the complex ethylene-auxin interplay in hook formation.

Hormonal networks involved in apical hook development in darkness and their response to light

In darkness, the dicot seedlings produce an apical hook as result of differential cell division and extension at opposite sides of the hypocotyl. This hook protects the apical meristem from mechanical damage during seedling emergence from the soil. In darkness, gibberellins act via the DELLA-PIF (PHYTOCHROME INTERACTING FACTORs) pathway, and ethylene acts via the EIN3/EIL1 (ETHYLENE INSENSITIVE 3/EIN3 like 1)-HLS1 (HOOKLESS 1) pathway to control the asymmetric accumulation of auxin required for apical hook formation and maintenance. These core pathways form a network with multiple points of connection. Light perception by phytochromes and cryptochromes reduces the activity of PIFs and (COP1) CONSTITUTIVE PHOTOMORPHOGENIC 1-both required for hook formation in darkness-, lowers the levels of gibberellins, and triggers hook opening as a component of the switch between heterotrophic and photoautotrophic development. Apical hook opening is thus a suitable model to study the convergence of endogenous and exogenous signals on the control of cell division and cell growth.

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

Progression of the apical hook of tomato, Solanum lycopersicum, exaggerated by phytochrome mediation at the early germination stage is followed in detail macroscopically and anatomically, and its proposed significance, i.e. survival by securing the seed coat release in the field, is reinforced by new findings. Furthermore, after self-release or artificial removal of the seed coat and the endosperm, no hook exaggeration occurs any more. Similar light-induced hook exaggeration (LIHE) is also found in carrot, parsley, and Cryptotaenia japonica, which share some seed characteristics with tomato. These findings also support the above-stated significance.