1062-3604/03/3406- $25.00 © 2003
Russian Journal of Developmental Biology, Vol. 34, No. 6, 2003, pp. 360–364. Translated from Ontogenez, Vol. 34, No. 6, 2003, pp. 432–437.
Original Russian Text Copyright © 2003 by V. Polevoi, Tarakhovskaya, Maslov, A. Polevoi.
Zygotes of brown algae of the order Fucales (genera
) are known as a unique model sys-
tem for studying the induction of axial polarity in
plants. As maturation proceeds, the gametes of these
algae are released from conceptacles into water, where
eggs are fertilized and zygotes germinate. Thus at all
developmental stages, zygotes are not connected with
the maternal tissues and easily accessible for experi-
mental studies. The eggs of fucoid algae are apolar and
devoid of cell wall. The ﬁrst morphological manifesta-
tion of polarity can be observed only within ca. 14 h
after fertilization, when a rhizoid protuberance appears
on the zygote’s surface (Quatrano, 1978; Kropf, 1997).
The ﬁrst division of the zygote is asymmetrical and pro-
ceeds in the plane perpendicular to the formed polarity
axis. Daughter cells differ distinctly in structure and
fate: one of them usually gives rise to the thallus and
another, to rhizoid (Quatrano, 1974, 1978). The subcel-
lular signs of asymmetrical development of the zygote
arise long before the rhizoid appearance (table).
Fertilization appears to be the ﬁrst signal to the for-
mation of polarity axis and, in the absence of other
inductive inﬂuences, the rhizoid forms at the site of
sperm entry. Fertilization is followed by a period, when
the forming polarity axis still remains labile and can be
easily changed by various external inﬂuences, such as
unilateral illumination, gravitation, external electrical
ﬁeld, concentration gradients of auxin,
(Jaffe, 1968; Quatrano, 1974, 1978).
Which factors are essential for the formation of
polarity axis? Torrey and Galun (1970) demonstrated
that the appearance of a rhizoid protuberance in zygotes
was inhibited by 2,3,5-triiodobenzoic acid (TIBA):
within 20 h after fertilization, the zygotes still remained
apolar and, if a division took place by that time, apolar
spherical embryos formed. In higher plants, TIBA is a
speciﬁc inhibitor of polar transport of indole-3-acetic
acid (IAA) from the cell (Goldsmith, 1977; Morris,
2000), while the presence of the latter is necessary for
the induction of root formation (Polevoi, 1986). Based
on this, it was proposed that (Davidson, 1950; Polevoi,
1984, 1986) that IAA can play the key role in the induc-
tion of polarity and rhizoid formation in fucoid algae.
Polevoi (1984, 1986) also proposed that: (1) fucoid
zygotes secrete IAA in the ambient medium; IAA con-
centration is higher near a certain region of the zygote
(with reference to its position on the substrate, etc.);
and a rhizoid protuberance forms in this region.
The aim of this work was to elucidate whether the
zygotes do secrete IAA in the ambient
medium at any developmental stage and estimate the
signiﬁcance of this phenomenon for the formation of
polarity axis and rhizoid formation in zygotes with the
help of treatment with exogenous IAA and TIBA.
MATERIALS AND METHODS
Studies were carried out on the eggs and zygotes of
L. collected in the region of the
Marine Biological Station, St. Petersburg State Univer-
sity (White Sea).
Role of Auxin in Induction of Polarity
V. V. Polevoi
, E. R. Tarakhovskaya, Yu. I. Maslov, and A. V. Polevoi
St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg, 199034 Russia
E-mail: email@example.com; firstname.lastname@example.org
Received April 4, 2002; in ﬁnal form, June 18, 2003
—We studied the effects of auxin (indole-3-acetic acid) on formation of the primary polarity axis in
zygotes of the brown algae
L. Within the ﬁrst 2.5 h after fertilization, the zygotes release this
phytohormone in the ambient medium. The treatment of developing zygotes with the inhibitor of indole-3-ace-
tic acid transport from the cell 2,3,5-triiodobenzoic acid at 5 mg/l arrests the auxin secretion and leads to its
accumulation in the cells. This causes a signiﬁcant delay in zygote polarization. The treatment of zygotes with
the exogenous indole-3-acetic acid at 1 mg/l stimulates cell polarization and formation of a rhizoid protuber-
ance. When auxin was added to the medium with triiodobenzoic acid, the inhibitory effect of the latter was elim-
inated. It has been proposed that the content of indole-3-acetic acid in the ambient medium is a key factor in
the induction of polarity of the
, zygote, auxin, polarization.