Plant Molecular Biology 49: 305–317, 2002.
Perrot-Rechenmann and Hagen (Eds.), Auxin Molecular Biology.
© 2002 Kluwer Academic Publishers. Printed in the Netherlands.
Complex physiological and molecular processes underlying root
Rujin Chen, Changhui Guan, Kanokporn Boonsirichai and Patrick H. Masson
Laboratory of Genetics, University of Wisconsin-Madison, 445 Henry Mall, Madison, WI 53706, USA (
correspondence; e-mail email@example.com)
Received 8 May 2001; accepted in revised form 11 July 2001
Key words: gravitropism, gravity sensing, gravity signal transduction, roots, statolith
Gravitropism allows plant organs to guide their growth in relation to the gravity vector. For most roots, this
response to gravity allows downward growth into soil where water and nutrients are available for plant growth
and development. The primary site for gravity sensing in roots includes the root cap and appears to involve the
sedimentation of amyloplasts within the columella cells. This process triggers a signal transduction pathway that
promotes both an acidiﬁcation of the wall around the columella cells, an alkalinization of the columella cytoplasm,
and the development of a lateral polarity across the root cap that allows for the establishment of a lateral auxin
gradient. This gradient is then transmitted to the elongation zones where it triggers a differential cellular elongation
on opposite ﬂanks of the central elongation zone, responsible for part of the gravitropic curvature. Recent ﬁndings
also suggest the involvement of a secondary site/mechanism of gravity sensing for gravitropism in roots, and
the possibility that the early phases of graviresponse, which involve differential elongation on opposite ﬂanks
of the distal elongation zone, might be independent of this auxin gradient. This review discusses our current
understanding of the molecular and physiological mechanisms underlying these various phases of the gravitropic
response in roots.
Abbreviations: CEZ, central elongation zone; DEZ, distal elongation zone; ER, endoplasmic reticulum; EZ, elon-
gation zone; IAA, indole-3-acetic acid; InsP
, inositol 1,4,5-trisphosphate; NAA, 1-naphthaleneacetic acid; NPA,
naphthylphthalamic acid; 2,4-D, 2,4-dichlorophenoxyacetic acid
Plasticity in growth behavior allows plants to sur-
vive dramatic changes in their environments, despite
sessility. Early in their life cycles, seeds are often
dropped off on the ground in a random orientation.
Upon germination, roots, as well as shoots of young
seedlings may be oriented upward, laterally or down-
ward. Consequently, these organs will have to reorient
themselves in order to assume a position that properly
suits their functions, i.e. roots orienting downward into
the soil for anchorage as well as water and nutrient
uptake, and shoots orienting upward into the sunlight
Throughout their life cycles, plants use environ-
mental parameters to guide their organs’ growth so
that they can adapt to and take full advantage of the
changing environment. For instance, shoots can di-
rect their growth in relation to the vectors of light
(phototropism) or gravity (gravitropism). These tropic
responses allow shoots to resume vertical upward
growth after prostration by high winds or heavy rains.
In agriculture, this important process allows salvaging
of signiﬁcant quantities of crop products.
Roots guide their growth in relation to gravity,
light, gradients of temperature (thermotropism), hu-
midity (hydrotropism),ions, chemicals (chemotropism),
and oxygen (oxytropism). Quite amazingly, the infor-
mation provided by these environmental parameters is