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Cell Cycle Regulation in Plants
A. S. N. Reddy and Irene S. Day
Colorado State University, Fort Collins, Colorado
229I. INTRODUCTION
Cell division is one of the fundamental processes of growth and development of plants and animals. The
time and place of cell division in an organism play a critical role in many developmental processes. The
development of a complex organism with a defined form and structure requires tightly regulated cell
growth and proliferation as well as transitions from cycling state to quiescent state and vice versa. In or-
der to duplicate the genetic material and produce two daughter cells, the cell goes through a set of orderly
events generally referred to as the cell cycle. The cell cycle consists of four distinct phases called gap1
(G 1 ), synthetic phase (S), gap2 (G 2 ), and mitosis (M). In the G 1 phase cells prepare for S phase, during
which DNA synthesis takes place and the cell replicates its chromosomes [1,2]. The completion of S
phase leads into another gap phase (G 2 ). Upon completion of G 2 , cells enter mitosis (M phase), where du-
plicated chromosomes segregate into two daughter cells [3]. However, it should be pointed out that in
some rare instances cycling cells have only two phases (M and S) without intervening gap phases (G 1 and
G 2 ). For example, the first 13 nuclear division cycles during Drosophilaembryo development do not have
any gap phases [4]. Similarly, nuclear division cycles during early endosperm development in plants seem
to lack gap phases [5].
Normal proliferating cells in G 1 can continue to cycle or revert to quiescent (G 0 ) state. The decision
to undergo another round of DNA synthesis and continue to cycle or to exit cell cycle to enter into a qui-
escent state (G 0 ) is made during G 1 phase [1]. Cells in G 0 state either terminally differentiate or can be
activated to reenter the cell cycle. These switches in and out of G 1 are primarily controlled by extracellu-
lar factors such as hormones and other mitogens [1]. However, once the cells enter into S phase, the cell
cycle events become independent of extracellular factors, leading to mitosis and production of two daugh-
ter cells. These events are mostly regulated by internal controls. Stringent control of decision points in the
cell cycle is vital for normal growth and development of organisms [1,4,6,7]. Deregulation of the regula-
tory mechanisms that control decision points in the cell cycle results in uncontrolled cell division leading
to abnormal growth. The biochemical and molecular mechanisms that regulate the cell cycle are of great
interest not only to help us understand how cells divide during normal growth and development of or-
ganisms but also to get insights into abnormal growth processes such as cancer. Knowledge derived from
cell cycle regulation in plants should enhance the ability to manipulate growth and developmental pro-
cesses in plants and could have practical implications. For instance, regeneration of plants is very critical
for crop improvement through genetic engineering [8,9]. However, the ability to regenerate a whole plant