The document summarizes key aspects of cell cycle regulation in plants. It discusses the cell cycle phases of interphase (G1, S, G2) and mitosis. Key regulators of the plant cell cycle include cyclin-dependent kinases (CDKs) and cyclins. CDKs activate during specific cell cycle phases upon binding to cyclins. Plant hormones also influence the cell cycle through effects on CDKs, cyclins and other regulators. Precise control of the cell cycle is important for plant growth and development.

2. K. Suresh naidu
M.Sc, 2nd sem.,
Depart. of Plant Physiology,
C.P.C.A,S.D.A.U.,
Sardarkrushinagar-385 506.
Course-: GP – 508 : CELL BIOLOGY & MOLECULAR GENETICS
An
Assignment On
PHYSIOLOGY & REGULATION OF CELL CYCLE & CELL
DIVISION

3. Cell : Structural and functional unit in all living organisms.
Cell division : The process of formation of new cells from the pre-
existing cells is referred to as Cell division.
 In bacteria cell division takes place by fission of pre-existing cells.
In higher organisms there are two types of cell divisions-
Mitosis
Meiosis

4. Cell cycle : The period in which one cycle of cell division is completed
is called cell cycle.
It has two phases : 1.Interphase (DNA synthesis)
2.Mitotic phase (Nuclear division)
INTERPHASE :
It consists of -
 G1(RNA & protein synthesis),
 S(DNA synthesis),
 G2(RNA & protein synthesis) phases.

6. MODELS USED TO STUDY CELL CYCLE :
1.YEAST -
a).Saccharomyces cerevisiae
(Budding Yeast)
 Unusually short G2 Phase.
 Model for Cellbiology problems.
b).Schizosaccharomyces pombe
(Fission yeast)
 Typical cell cycle with long G2.
 Good model for studies of Growth
Control.

7. 3. CULTURED MAMMALIAN CELLS (IMMORTAL
LINES) were employed to examine the molecular
mechanism that control cell cycle progression.
4. In case of plants Arabdiopsis thaliana is the
most widely studied plant model.

8. REGULATION OF CELL CYCLE
 Cell cycle control plays a fundamental role in plants by supporting their
largely Post embryonic growth and Development.
CORE REGULATORS OF THE PLANT CELL CYCLE -
 Four distinct cell cycle phases are generally recognized: G1, S, G2 and M.
 In G1 and G2 phases , during which the cell cycle machinery is prepared
to enter the next phase. These “gap” phases function also as checkpoints.
 Among the bulk of cell cycle players, Cyclin- -dependent kinases coupled
with cyclins play a crucial role in the regulation of cell cycle progression

9. Cyclin-Dependent Kinases :
 A specific class of serine-threonine protein kinases, termed cyclin-
dependent kinases (CDKs) due to their activation by association with
cyclins, are central regulators of the cell cycle.
 Plant CDKs can be classified into 7 different groups, from A to G, on the
basis of characteristic amino acid cyclin-binding motifs:
Sr.no Name of CDK Aminoacid cyclin binding
motif
1. CDKA PSTAIRE hallmark
2. CDKB PPT(A/T)LRE
3. CDKF & CDKD N(I/F)TALRE
4. CDKC PITAIRE
5. CDKE SPTAIRE
6. CDKG PLTSLRE

10.  Among the CDKs, CDKA plays a pivotal role during the whole cell cycle,
being especially important for the transition from G1 to S-phase and G2
to M-phase.
 The later transition, G2/M-phase, is further regulated by another type
of CDKs that is exclusive to plants, the CDKB type.
 CDKD and the plant-specific CDKF function as CDK-activating
kinases (CAKs).
 CDKG is possibly involved in coupling cell cycle progression with
splicing of transcripts .
 CDKs C and E no direct participation in the cell cycle has been shown.

11. Cyclins :
 The name cyclins (CYCs) stems from the transient and cyclical
appearance of these proteins during the cell cycle.
 CDKs need to be activated by complex formation with cyclins to become
functional.
 Most cyclins are characterized by a 100 amino acid sequence called
cyclin box that is required for proper binding to CDKs .
 plant specific features like postembryonic development, meristematic
growth or even developmental alterations in response to environmental
fluctuations, plants should have evolved a tightly regulated, yet flexible cell
cycle machinery, which could explain this high number of cyclins.

12. Sr.no Name of Cyclin Function
1. CYC A S-phase progression
2. CYC B G2/M transition
3. CYC C lack of evidence for a role
4. CYC D G2/M transition & G1/S transition
5. CYC H regulates CDKD in its function as CAK
6. CYC L controls CDKG
7. CYC P interact with CDKA
8. CYC T lack of evidence for a role

13.  Monomeric CDKs need to be complexed with CYCs and activated
via phosphorylation by CDK-activating kinases (CAKs) to be
fully functional.
CDK Activators
 CDKD complexed with CYCH or CDKF alone can act as CDK
activating kinases in both G1/S and G2/M transitions by
phosphorylating CDKA and CDKA/B, respectively.

14. CDK Inhibitors :
 Plants contain cyclin-dependent kinase inhibitors (CKIs)
and they are often designated as Kip-Related Proteins
(KRPs).
 CKIs/KRPs can inactivate both CDKs and cyclins by direct
binding as the conserved domain is essential for the interaction
between CDKs and cyclins.
 CDKs’ activity can also be repressed by WEE1 protein kinases.
These kinases have the ability to inhibit the activity of
assembled CDK/cyclin complexes through phosphorylation.

15. The Retinoblastoma Related Protein (RBR Proteins) :
 RB-related (RBR) proteins were identified in several plant species
monocots seem to have two copies of the RBR gene with
complementary functions.
 Members of the E2F family of transcription factors are as well present in
plants
 To become functional, E2Fa, E2Fb and E2Fc require heterodimerization
with either DPa or DPb while E2Fd, E2Fe and E2F heterodimerize with
members of the DP-E2F-like (DEL) protein family.

16. CELL CYCLE PROGRESSION AND CYTOKINESIS
G1 and S-phase
 In the beginning of the cell cycle, cells are committed to acquire all the
machinery necessary to replicate their genome and ensure that this
duplication will be performed correctly.
 The first phase of the cell cycle, the G1 phase, is therefore a crucial
period of enormous changes for the cell.
 In the presence of hormones and growth factors, the levels of CYCD
increase in the cell and, without repression through KRPs, CYCD is able
to associate with CDKA forming an inactive CDKA/CYCD complex that
characterizes the beginning of the G1 phase.

18.  The complex only becomes active after phosphorylation by CDK-
activating kinases involving CDKF or CDKD coupled with CYCH.
 Although in its active state, a CDKA/CYCD complex can still be
inactivated by the WEE1 protein kinase inhibitor in certain conditions.
 The RBR protein associated with the E2F/ DP heterodimeric complex is
the target of active CDKA/CYCD.
 The CDKA/CYCD complex phosphorylates RBR allowing its dissociation
from E2F/DP.
 Once the E2F/ DP complex is released from repression by RBR it turns
into its fully active state and triggers G1 to S-phase transition by
initiating the transcription of genes necessary for DNA replication,
unless DEL proteins are present and compete with E2F/DP for binding
sites.

19.  E2F targets include genes involved in DNA repair and chromatin
dynamics like CDC6, CDT1, MCM3, ORC1 and ORC3, RNR1b, PCNA, and
others involved in the later G2 to M progression, such as CDKB1, MYB
and APC.
 CDC6 and CDT1 genes along with the origin recognition complex (ORC)
enable the loading of MCM proteins to the replication origins, thus
licensing them for activation in S-phase.
 ORC phosphorylation by the DBFCDC7 complex is imperative, so that
ORC can move and the replication initiation site can be exposed to the
replisome complex, allowing replication to start.

20. G2 and Mitosis
 Once DNA replication in S-phase is complete, cell cycle progresses into
G2 phase preparing the cell to enter mitosis.
 Increased expression of CYCB but also CYCA and CYCD are observed in the
beginning of G2.
 These cyclins can associate with either CDKA or CDKB establishing CDK/
CYC complexes. Negative regulation of theses complexes can either occur
through the specific binding of KPRs to CDKs and cyclins, preventing their
association to form the complex, or after the complex formation by its
direct phosphorylation through WEE1.
 The progression into M-phase may then require the participation of
CDC25-like phosphatase, possibly activating the CDK/CYC complexes by
releasing the CDK inhibitory phosphate groups.

21.  CAKs have a central role in CDK/CYC activation. Once active, CDK/CYC are
able to phosphorylate KPRs and prevent further inactivation through these
kinase inhibitors prior to CDK/CYC complex formation.
 The key substrates of activated CDK/CYC complexes are three MYB
repeat (MYB3R) transcription factors that bind to the M-phase-Specific
Activator (MSA) elements in the promoter region of their target genes.
 MYB3R phosphorylation by CDK/CYC triggers the expression of M-phase
specific genes such as KNOLLE, CDC20, CYCA,CYCB and NACK1.

22. Cytokinesis
 The moment when cytokinesis takes place is somehow regulated by
the degradation of cyclins through ubiquitination by the anaphase-
promoting complex (APC) to target them to the 26S proteasome.
 Cytokinesis in plants depends on a structure called phragmoplast
that is formed at the center of the cell division plane just before
cytokinesis and consists of microtubules, actin filaments and vesicles.
 SNARE proteins, a core complex of membrane anchored proteins
involved in membrane fusion, localize to the phragmoplast and
interact with the syntaxin KNOLLE and the N-methylmaleide-sensitive
fusion (NSF) protein.
 These proteins are the machinery that allow the
transport, accumulation and fusion of the vesicles to form the cell
plate.

24. HORMONAL REGULATION OF THE CELL CYCLE
 Plant hormones have an important role in the control of the cell cycle as
they can directly regulate key players of the cell cycle.
 The main decisions concerning cell division and proliferation are
controlled by auxin and cytokinins, two hormones that have partially
overlapping and at times complementary functions, and whose crosstalk
is crucial for proper development.
 Auxin (in most cases, indole-3-acetic acid) is the plant hormone that
has been most studied and is decisively involved in cell division, cell
polarity and also cell fate determination.
 Cytokinins are important regulators of shoot and root development by
controlling their balance.They also have important functions
delaying senescence and, together with auxin, controlling cell
differentiation and proliferation.

25.  Auxins and Cytokinins have an important role in G2 to M transition as
they activate the CDC25-like phosphatase that is potentially involved in
this cell cycle transition and, therefore, possibly inducing CDK’s
activation.
 Brassinosteroids and Gibberellins are also known to increase the
expression of CYCD, prompting the G1 to S-phase transition.
 Abscisic acid was also found to play a role in the plant cell cycle. It is not
only involved in the repression of CDKA and DNA replication genes but
also in the activation of KRPs during low temperature.
 Jasmonic acid appears to repress both S-phase and M-phase
transitions.
 Ethylene seems to constrain G2/M cell cycle transition

26. Two ways to influence Cell cycle :
1.Increasing Hormones like : Auxin,Cytokinins.
2.RBR pathway : Certain Viruses influence Cyclin D and the E2F-
DP complex directly by producing a viral protein AL1 that binds
RBR and relieves RBRs capacity to suppress E2Fa. This leads to
activation of S-phase genes
why pathogens almost exclusively target the hormonal pathway ?
Because the RBR pathway is less susceptible to
manipulation. However, it poses a contradiction: Altering the
hormonal levels is a common and simple pathogen strategy
leading to effective manipulation of cell proliferation.

27. Plant Tumors as an Opportunity :
 Nowadays tumors are considered more than simply unordered
proliferating tissues.
 Instead, they are seen as proliferating masses of cells with
complex interactions that are mostly not understood, although
great progress has been made in the last decade.
Agrobacterium tumefaciens (Crown Gall Tumor)
 After detecting phenolic molecules resulting from an exposed
wound, Agrobacterium proceeds to transfer its own T-DNA to the
plant genome and subsequently manipulate key hormone levels
in a way it can promote uncontrolled tissue proliferation and
tumor formation

28. Agrobacterium
Transformed
Plant cell
Transformed
Plant

29.  The heart of plant cell cycle regulation are auxin, cytokinins and
the RBR pathway. Auxin manipulation is a key element for
Agrobacterium success in crown gall formation.

30. CONCLUSION :
 There has been significant progress in our understanding of the
plant cell cycle and its impact on plant growth and development
during recent years.
 The internal signals and external cues influence the plant cell
cycle is the focus of many studies, with large-scale approaches
like transcriptomics, interactomics and mutant screens
supplying candidate genes.
 An integral understanding of all players and their network
connections should have an impact on plant breeding and
engineering programs with the promise to increase agricultural
yields in the future.

31. Reference :
 J. A. Bryant and D. Francis (2011). The plant cell cycle. Annals of
Botany 107: 1063.
 Kristiina Himanen(2003).CELL CYCLE REGULATION DURING
PLANT GROWTH AND DEVELOPMENT, GENE EXPRESSION STUDIES
IN ARABIDOPSIS THALIANA (L.) Heynh.Department of Plant
Systems Biology Flanders Interuniversity Institute for
Biotechnology, VIB University of Ghent Belgium.
 Patrícia A. Pereira, Francisco V. Sousa, Jörg D. Becker (2012) Decision-
Making in the Plant Cell Cycle.Canal BQ-n.9.

32.  Peter W. Doerner (1994) Cell Cycle Regulation in Plants. Plant
Physiol.106: 823-827.
 Walter Dewitte and James A.H. Murray (2003) THE PLANT CELL CYCLE
Annu. Rev. Plant Biol.54:235–64.

33. THANK YOU