WELCOME

K. Suresh naiduM.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

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

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.

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.

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.

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

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

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.

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.

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

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.

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.

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.

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.

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.

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 thatORC can move and the replication initiation site can be exposed to the replisome complex, allowing replication to start.

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.

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.

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.

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.

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

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.

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

Agrobacterium

TransformedPlant cell

TransformedPlant

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.

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.

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.

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.

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