Plant Molecular Biology 41: 295–299, 1999.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
Retinoblastoma proteins in plants
Sarah M. de Jager and James A.H. Murray
Institute of Biotechnology, University of Cambridge, Tennis Court Road, Cambridge, CB2 1QT, UK (
Received 23 August 1999; accepted 2 September 1999
Key words: cell cycle, pocket proteins, Rb, retinoblastoma
The retinoblastomaproteinRb is part of a conservedpathwaythat controlsthe activation of cell division in animals.
Rb represses cell cycle transcription factors of the E2F family, and thereby prevents uncontrolled cell proliferation.
Rb itself is inactivated when phosphorylated by cyclin-dependent kinases, and the D-type cyclin kinases are
particularly important in this process during the reactivation of cell division in quiescent cells. In addition, Rb
has important developmental roles in controlling the onset of cellular differentiation in a number of cell types.
The recent discovery in plants of both Rb proteins and other components of the Rb pathway suggests that, far
from being restricted to the animal kingdom, Rb may have a conserved role in allowing multicellular organisms to
develop complex body plans consisting of many different cell types. This review assesses the potential roles of Rb
proteins in plant cell cycle control and development.
Cell division and differentiation in plants are highly
organised and localised processes that are intimately
connected with growth and development. How this
interconnection is realised at the molecular level is
largely unclear, but is an area of active research be-
ing tackled both by labs working on meristematic
organisation and maintenance and by those studying
conserved mechanisms of cell division control and
Elucidating the link between cell division and dif-
ferentiation may be facilitated by the recent surprising
discovery of homologues of the human retinoblas-
toma (Rb) protein in plants (Gutiérrez, 1998). Until
a few years ago, the intimate involvement of Rb at
the interface of mammalian cell cycle control, cellu-
lar differentiation and development, together with its
key role in cellular transformation and cancer, was
thought to suggest that Rb and its pathway of action
were unique to the vertebrates (Weinberg, 1995). This
view has been overturned by the discovery in a num-
ber of plant species of both Rb and other molecular
players in the pathway of Rb action. Functional inter-
actions between these proteins have been maintained
between plants and animals, as shown by ‘mix and
match’ experiments using in vitro systems (Huntley
et al., 1998). Interestingly, the components of this
pathway are not present in yeast or fungi, which use
unrelated proteins to control the same process, sug-
gesting that a common pathway relating cell division
and differentiation may have arisen only once in the
evolution of all higher eukaryotes. Was the invention
of the Rb pathway the factor that allowed elaboration
of complex three-dimensionalbody plans?
The human Rb protein plays a pivotal role in the
regulation of the cell division cycle, and particularly
in the switches between the quiescent and dividing
state (Weinberg, 1995). Rb acts by binding to and
negatively regulating a family of transcription factors
called E2Fs. Rb recruits histone deacetylase activity
to the promoter-bound E2Fs resulting in chromatin
modiﬁcations, which exclude the transcriptional ma-
chineryfrom the DNA (Brehm and Kouzarides,1999).
E2F-binding sites have been identiﬁed in the promot-
ers of a host of genes involved in DNA replication
and progression of the cell cycle. Thus by binding