Phototropism involves a lateral gradient of growth inhibitors, not of auxin. A review

doi:10.1016/0098-8472(89)90036-1

Environmental and Experimental Botany

Volume 29, Issue 1, January 1989, Pages 25-36

https://doi.org/10.1016/0098-8472(89)90036-1 Get rights and content

Abstract

During phototropic curvature, indolyl-3-acetic acid (IAA) remains evenly distributed in the hypocotyl of sunflower (Helianthus annuus L.) and in the oat (Avena sativa L.) coleoptile. At the irradiated side, growth-inhibiting substances accumulate. In sunflower, basipetal movement of a growth factor is not involved, since the top of the seedling can be covered or removed without affecting the phototropic response; this response, moreover, is independent of the rate of elongation growth. The chemical nature of the growth-inhibiting substances is only partly known. In the hypocotyl they occur in the neutral fraction: in sunflower cis-xanthoxin is one of them, in radish (Raphanus sativus L.) cis- and trans-raphanusanins, and possibly raphanusamide, are involved. The inhibitor(s) in the oat coleoptile are acidic. During curvature, their amount remains rather constant but the distribution changes with an accumulation at the irradiated side. It is concluded that phototropic curvature is brought about by an accumulation, at the irradiated side, of growth-inhibiting substances that unilaterally reduce cell elongation even though the IAA distribution is uniform.

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Chapter 23 Phototropism in higher plants
2001, Comprehensive Series in Photosciences
Phototropism refers to the curvature movements of organs whose directionality is determined by the direction or uneven distribution of impinging light. Optimizing the acquisition of light energy in shade, and directing the growth of shoots away from shade, appear to be the most common roles played by phototropism. Seedling organs such as coleoptiles and hypocotyls are typically phototropic and have served as model systems in laboratory studies. Results with these organs have revealed the following basic aspects of the process of phototropism. The light signal is perceived by the photoreceptor which is most sensitive to blue light. The direction of movement is determined by the transverse light gradient generated within the photosensing zone. The phototropic curvature results from an asymmetry of growth, which is not caused by local light-growth response but by growth redistribution. The plant hormone auxin is laterally translocated in response to the transverse light gradient and mediates the growth redistribution. This chapter summarizes most of the results obtained during the last century, including the recent advances made in molecular genetic studies. The field has experienced controversies over some of its basic aspects. It will be noted that many of the apparently complex and conflicting results represent aspects of the overall phototropic system that is undoubtedly highly sophisticated.
Phototropic stimulation induces the conversion of glucosinolate to phototropism-regulating substances of radish hypocotyls
2000, Phytochemistry
The distribution of natural growth inhibitors, the raphanusanins (isomers of 3-(methylthio)methylene-2-pyrrolidinethione) and their precursors (4-methylthio-3-butenyl glucosinolate (MTBG) and 4-methylthio-3-butenyl isothiocyanate (MTBI)), between illuminated and shaded halves of radish hypocotyls during phototropic curvature was analyzed using a physicochemical assay. Phototropic stimulation rapidly decreased MTBG content, and abruptly increased contents of MTBI and raphanusanins in the illuminated halves of radish hypocotyls within 30 min after the onset of unilateral illumination. Content in the shaded halves was similar to that in dark controls. When MTBG, MTBI, and raphanusanins at endogenous levels were applied unilaterally to etiolated hypocotyls, MTBI and raphanusanins caused hypocotyls to bend but MTBG showed no activity. Blue illumination promoted myrosinase (thioglucosidase) activity, which releases MTBI from MTBG, in hypocotyls after 10 min, although enzyme activity in dark controls did not change. These results suggest that phototropic stimulation promotes myrosinase activity in the illuminated side of radish hypocotyls, releasing bioactive MTBI from inactive MTBG and simultaneously producing bioactive raphanusanins.
A new auxin-inhibiting substance, 4-Cl-6,7-dimethoxy-2-benzoxazolinone, from light-grown maize shoots
1996, Phytochemistry
Two auxin-inhibiting substances were isolated from light-grown maize shoots. One was a new compound and determined from its spectral data as 4-Cl-6,7-dimethoxy-2-benzoxazolinone (Cl-DMBOA). Another was identified from its spectral data as a known compound, 6,7-dimethoxy-2-benzoxazolinone (DMBOA). Above concentrations of 10⁻⁵ M, Cl-DMBOA inhibited the auxin-inducing elongation in the Avena coleoptile section test. Its inhibitory activity was almost comparable to that of DMBOA and higher than that of 6-methoxy-2-benzoxazolinone (MBOA) which we previously isolated.
Structure-activity relationships of benzoxazolinones with respect to auxin-induced growth and auxin-binding protein
1994, Phytochemistry
The structure-activity relationships of the naturally occurring auxin-inhibiting substance, 6-methoxy-2-benzoxazolinone, isolated from maize shoots, and its artificial analogues with respect to auxin activity and membrane-bound auxin-binding protein, were studied. 6-Isobutoxy-2-benzoxazolinone strongly inhibited auxin (1-naphthylacetic acid, NAA)-induced growth of etiolated maize coleoptile segments. 6-Isopropoxy-2-benzoxazolinone, 6-ethoxy-2-benzoxazolinone and 6-methoxy-2-benzoxazolinone showed significant inhibitory activity. However, 2-benzoxazolin-one and 6-benzyloxy-2-benzoxazolinone did not inhibit auxin-induced growth. On the basis of these data, it seemed reasonable to assume that the auxin-inhibitory activity was enhanced by increasing the side chain length at position C-6 on the 2-benzoxazolinone, although the attachment of a ring at C-6 destroyed activity. Competition by benzoxazolinones with an alkoxy group at C-6 with ³H-NAA at auxin-binding protein(s) isolated from endoplasmic reticulum (ER) membrane of maize shoots showed a positive correlation with their physiological effects. However, since the inhibitory activity of the benzoxazolinones for auxin-receptor(s) binding was small compared with their physiological activity, the benzoxazolinones may contribute to inhibition of auxin-induced growth through interference with other auxin-receptors.
Even Distribution of Endogenous Indole-3-acetic Acid in Phototropism of Pea Epicotyls
1992, Journal of Plant Physiology
The curvatures and the differential flank growth rates of etiolated and de-etiolated pea epicotyls were determined in response to various phototropic stimulations. The phototropic curvatures of pea epicotyls were caused only by growth inhibition at the lighted side, except in the first positive curvature of etiolated epicotyls, in which a slight growth promotion was observed at the shaded side next to the growth inhibition at the lighted side. De-etiolation magnified the response of the epicotyls to phototropic stimulation both at the first and second positive curvatures. The distribution of endogenous indole-3-acetic acid (IAA) was determined in relation to phototropism using an HPLC with fluorescence detector. Endogenous IAA was equally distributed over the lighted and shaded sides of peripheral cell layers of the epicotyl, both at the first and second positive curvatures of etiolated and de-etiolated pea seedlings. De-etiolation did not change the even distribution of IAA but induced a decrease in the amount of IAA in the peripheral cell layers. Apparently, as in hypocotyls and coleoptiles, the Cholodny-Went theory, which describes phototropic curvature in terms of a lateral gradient of auxin, does not apply to the phototropism of the pea epicotyl.
Revision of the structure of raphanusanins, phototropism-regulating substances of radish hypocotyls
1991, Tetrahedron Letters
The structures of raphanusanins A and B, growth inhibitors involved in phototropism of radish hypocotyls, were determined by X-ray crystallographic structure analysis and by spectral studies to be (3R^*,6R^*)- and (3R^*,6S^*)-3-[methoxy(methylthio)methyl]-2-pyrrolidinethione, respectively. Therefore, the previously proposed structure of these compounds of 2-piperidinethione skeleton should be revised.

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Environmental and Experimental Botany

Abstract

References (29)

Caprolactam, a light-promoted growth inhibitor in sunflower seedlings

Phytochemistry

A rapid, sensitive and accurate determination of indolyl-3-acetic acid

Phytochemistry

The combined determination of indolyl-3-acetic and abscisic acids in plant materials

Anal. Biochem.

Purification and lability of cis-xanthoxin, and its occurrence in phototropically stimulated hypocotyls of Helianthus annuus L.

Z. Pflanzenphysiol.

Identification of plant growth inhibitors produced by photolysis of violaxanthin

Phytochemistry

Licht und Wachstum—II

Z. Bot.

Hormonal regulation of phototropism in the light-grown sunflower seedling, Helianthus annuus L.: immobility of endogenous indoleacetic acid and inhibition of hypocotyl growth by illuminated cotyledons

J. exp. Bot.

The power of movement in plants

Abscisic acid, xanthoxin and violaxanthin in the caps of gravistimulated maize roots

Planta

Phototropism in seedlings of sunflower, Helianthus annuus L.

Meded. Landb-Hogesch. Wageningen

Effects of different wavelengths on phototropic sensitivity of sunflower Helianthus annuus seedlings

Phytomorphology

Relationships between xanthoxin, phototropism, and elongation growth in the sunflower seedling Helianthus annuus L.

Planta

Phototropism in hypocotyls of radish—I. Isolation and identification of growth inhibitors, cis- and trans-raphanusanins and raphanusamide, involved in phototropism of radish hypocotyls

Pl. Physiol.

Phototropism in hypocotyls of radish—IV. Flank growth and lateral distribution of cis- and trans-raphanusanins in the first positive phototropic curvature

Pl. Physiol.

Cited by (24)

Chapter 23 Phototropism in higher plants

Phototropic stimulation induces the conversion of glucosinolate to phototropism-regulating substances of radish hypocotyls

A new auxin-inhibiting substance, 4-Cl-6,7-dimethoxy-2-benzoxazolinone, from light-grown maize shoots

Structure-activity relationships of benzoxazolinones with respect to auxin-induced growth and auxin-binding protein

Even Distribution of Endogenous Indole-3-acetic Acid in Phototropism of Pea Epicotyls

Revision of the structure of raphanusanins, phototropism-regulating substances of radish hypocotyls