Plant Molecular Biology 39: 809–821, 1999.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
Characterization of a desiccation-responsive small GTP-binding protein
(Rab2) from the desiccation-tolerant grass Sporobolus stapﬁanus
Patrick J. O’Mahony and Melvin J. Oliver
Plant Stress and Water Conservation Laboratory, USDA ARS, Rte. 3, Box 215, Lubbock, TX79401, USA (
Received 24 August 1998; accepted in revised form 10 November 1998
Key words: desiccation tolerance, differential display, GTP-binding, Rab2, Sporobolus stapﬁanus
We have used differential display to detect altering mRNA levels in response to desiccation and rehydration in
leaves of the desiccation tolerant grass Sporobolus stapﬁanus. One of the RT-PCR products identiﬁed was used
to isolate a cDNA of 999 bp which encodes a protein of 210 amino acids (predicted size 23 kDa). This protein
displays considerable sequence similarity to mammalian and plant Rab2, a small GTP-binding protein, possessing
several conserved motifs common to these regulatory proteins. Sporobolus Rab2 was expressed in Escherichia coli
yielding a protein with an apparent molecular mass of ca. 30 kDa which was shown to have the ability to bind GTP.
Rab2 transcript accumulated early in response to a decrease in relative water content (RWC) and remained high
even in dried leaves. Rehydration of desiccated leaves resulted in a decrease in levels within3hofrewetting, with
a brief increase at ca. 12 h. Accumulation of Rab2 transcript was also evident during drying and rehydration of the
roots of S. stapﬁanus, as well as in leaves of the desiccation-sensitive grass Sporobolus pyramidalis. Earlier work
on S. stapﬁanus concluded that the plant hormone ABA has little effect on inducing desiccation tolerance, however
Rab2 transcript does exhibit a small increase in accumulation in response to exogenous ABA. A possible role for
Rab2 with respect to desiccation tolerance and damage repair is discussed.
Desiccation is lethal to most vegetative plant tis-
sues, however a small number of plants (desiccation-
tolerant or resurrection plants) have evolved mech-
anisms to tolerate this extreme water stress. Such
plants can withstand long-term extreme dehydration
(to air dryness) and regain full biological potential
upon rehydration [6, 32].
Plants that can survive desiccation of their vegeta-
tive tissues are broadly categorized into two groups:
fully desiccation-tolerant plants, which can withstand
complete loss of free protoplasmic water regardless
of the rate of water loss; and modiﬁed desiccation-
tolerant plants which survive desiccation only if water
loss is relatively slow. Fully desiccation-tolerant plants
The nucleotide sequence data reported will appear in the
EMBL, GenBank and DDJB Nucleotide Sequence Databases under
the accession number AF083256.
have only been found in the simpler plant groups of
bryophytes, lichens and algae [6, 28] while modiﬁed
desiccation-tolerant plants are represented in nearly
all major classes of vascular plants , the only ex-
ception being the gymnosperms (a taxonomic group
consisting of the cycads, conifers and gnetophytes).
In the angiosperms at least 60 species from a vari-
ety of different taxonomic groups exhibit modiﬁed
vegetative desiccation tolerance.
The proposed mechanisms by which desiccation
tolerant plants can survive desiccation are based on
three criteria: (1) limitation of damage incurred to
a repairable level, (2) maintenance of physiological
integrity in the dried state, and (3) mobilization of
repair mechanisms upon rehydration . These crite-
ria are encompassed by a two-component system in
desiccation-tolerant plants, protection of cellular in-
tegrity by either inducible or constitutive protective
mechanisms, and repair of desiccation- or rehydration-