Plant Molecular Biology 35: 1045–1051, 1997.
1997 Kluwer Academic Publishers. Printed in Belgium.
Cloning of genes whose expression is correlated with mitosis and localized in
dividing cells in root caps of Pisum sativum L.
and Martha C. Hawes
Departments of Plant Pathology and Molecular & Cellular Biology, University of Arizona, Tucson, AZ 85721,
author for correspondence; Present address: Department of Molecular, Cell and Developmental Biology,
University of California, Los Angeles, CA 90024–7009, USA)
Received 15 April 1997; accepted in revised form 25 August 1997
Key words: root cap cell differentiation, hydroxyproline-rich glycoprotein, callus protein P23, ribosomal protein
L41, whole mount in situ hybridization
Removal of border cells from pea roots synchronizes and induces root cap cell division, wall biogenesis and
differentiation. Three messages which are expressed differentially in such induced root caps have been cloned.
Sequence analyses showed that the PsHRGP1-encoded protein has high homology with a hydroxyproline-rich
glycoprotein. The PsCaP23-encoded protein has high homology with an alfalfa callus protein or translationally
controlled human or mouse tumor protein P23. The PsRbL41-encoded protein has high homology with a highly
basic 60S ribosomalproteinL41. In situ hybridizationshowed that PsHRGP1, PsCaP23and PsRbL41 messages are
localized within dividing cells of the root cap. PsHRGP1 is highlyexpressed in uninducedroot caps, but its message
is repressed by 10–11 times as soon as cell division and differentiation begin. Expression of PsHRGP1 recovers to
higher than (180%) its initial level in 30 min. PsHRGP1 is root-speciﬁc. PsCaP23 and PsRbL41 messages increase
ca. 3-fold within 15 min after root cap induction. All three genes represent small families of 3–5 closely related
genes in the pea genome.
Root caps have long been recognized as an excellent
system to study cell structure and function, because
their ﬁnal differentiated state is well deﬁned and easily
accessible . Root caps, attached to the apices of
roots, protect the apical meristem, serve as gravisens-
ing tissue, and have been proposed to open a passage
for the growing roots by producing root cap mucilage
. Gravity sensing statocytes renewed by meristem-
atic cell divisions develop and are transformed into
secretory cells, which produce mucilage before they
ﬁnally reach the periphery of root cap. The terminal
step in root cap development is the complete separa-
tion of peripheral root cap cells as they differentiate
The GenBank, EMBL and DDBJ accession numbers for PsHR-
GP1, PsCaP23 and PsRbL41 messages and deduced amino acid
sequences are U78952, L47968 and L47967, respectively.
into border cells [3, 4]. The time required for a single
cell to proceed through the root cap depends on the
size of the root cap, the species, and undeﬁned envir-
onmental variables [2, 4, 12].
Border cell production is elicited by an unknown
signals(s), which sequentially induces transcription of
genes involved in root cap cell differentiation . The
process of border cell production is tightly regulated,
such that when a species speciﬁc maximum number
of cells accumulates outside the root cap periphery,
cell production ceases . In Pisum sativum,thisset
number of border cells is ca. 3500 [4, 12]. The process
can be experimentally induced and synchronized from
plant to plant by removing the existing border cells,
either by immersing the root tip in water or by gently
wiping the cells from the root tip surface [3, 12]. This
results in a ca. 400% increase in mitosis in cells of the
root cap meristem within 30 min; after 2 h, presum-