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9/10/

Plant Molecular and Cellular Biology

Lecture 10: Plant Cell Cycle

Gary Peter

9/10/ PMCB Lecture 9: G. Peter Learning Objectives z Explain similarities anddifferences between fungal,mammalian and plant cell cycles z Explain the role of the cell cyclemachinery in plant development

Plant Cell Cycle is Conserved 9/10/ PMCB Lecture 9: G. Peter De Veylder et al., 2007 Nature Rev. MCB 8: 655-

9/10/ PMCB Lecture 9: G. Peter Plant CDK, Cyclin Regulatorsare Conserved: All PredictedArabidopsis CDKs and Cyclins The Plant Cell, Vol. 14, 903–916, April 2002 Arabidopsis thaliana has 12 distinct bona fide CDKs and more than 49 different cyclins

CDKs 9/10/ PMCB Lecture 9: G. Peter z CDK classifications based on conserved sequencescorresponds well with their functional diversity. z The major drivers of the plant cell cycle are the A- and B-type CDKs. z A-type CDKs (CDKA) are most similar to mammalian CDK and CDK , they are constitutively present during the cell cycle, and they control the G1–S and G2–M transitions. z B-type CDKs (CDKB) are specific to plants and their accumulation depends on the phase of the cell cycle, they reacha maximum level at the G2–M transition, the phase at which theiractivity is required.

Cyclins 9/10/ PMCB Lecture 9: G. Peter z Cell-cycle progression requires thesequential association of the CDKs withdifferent types of cyclin. z D-type cyclins (CYCD) regulate the G1–Stransition, although there is accumulatingevidence that they also might have a role inthe G2–M transition. z A-type cyclins (CYCA) are mainly present fromS phase to M phase. z B-type cyclin (CYCB) levels peak at the G2–Mtransition and during M phase1. z To date, the exact composition ofdifferent CDK–cyclin complexes in plantsis largely unknown. **CyclinType

infamily

A 10 B 11 D 10 H 1 Arabidopsis**

Cyclin D z Functional redundancy high- inferred from thelack of strong phenotypes in single geneknockouts z Expression is modulated by cytokinins, auxins,brassinosteroids, sucrose, gibberellins z CYCD3;1- is rate limiting for calli starved ofexogenous cytokinin and overexpression issufficient to compensate for exogenous cytokinin z CYCD2;1- overexpression stimulates fastergrowth leading to larger plants with similar sizedcells 9/10/ PMCB Lecture 9: G. Peter

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Dry Weight Increases3-4.5X after 35 days

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Height Increases~36% after 55 days

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Cell Division RateIncreased

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Meristem organizationunaffected

Ectopic Expression of AtCyclin D2 Stimulates Growth

Cockcroft et.al. ’ Nature 405: 575-

Cyclin A z

Single gene knockouts show limitedphenotypes

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Multigene knockouts with RNAi have embryodefects

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Lines overexpressing CYCA’s have ectopiccell division and delayed differentiation – highlevels of S phase specific genes and CDK-CYCA activity and impaired shoot and rootregeneration

9/10/ PMCB Lecture 9: G. Peter

Ectopic Expression of Cyclin A inRoots Stimulates Cell Division z

After 6 days, dryweight increased 6-8X

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Root apical growthrate increased 30-40%

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Cell # increased

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Root morphologywas normal

Doerner et al. ‘96 Nature 380: 520-

Cyclin B z

Overexpression in roots stimulates growthdramatically – overall root

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They trigger G2-M transition even in Xenopus

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CYCG;1 binds to and activates both A- andB- CDKs in vitro

9/10/ PMCB Lecture 9: G. Peter

Annual Reviews Control of G1/S Transition z Expression of cyclin Ds areregulated by hormones andsucrose z Similar players andfeedback mechanisms z Rb (single gene) and E2F(three genes) z CKI (Krps) z Are different meristemsregulated differently withdifferent gene familymembers? z Lateral roots z Shoot apical meristems

Transcriptional Control of G1/S z Conservedtranscriptional activatorsE2Fa/b and DP andretinoblastoma relatedprotein is inactivated byCDKA-cycD z Plant innovation - E2Fcis a transcriptionalrepressor thatsuppresses cell division 9/10/ PMCB Lecture 9: G. Peter De Veylder et al., 2007 Nature Rev. MCB 8: 655-

Potential Role of RBR and E2F-DP in Chromatin Remodeling z Histone deacetylases are targeted to promoters viainteraction with retinoblastoma like proteins which bindto E2F-DP transactivators z Inactivation of RBR releases HDACs to remodelchromatin around important genes 9/10/ PMCB Lecture 9: G. Peter De Veylder et al., 2007 Nature Rev. MCB 8: 655-

G2/M Transition z Cyclin B triggers thistransition; however z Cyclin D alsoexpressed at thisboundary andCYCD3;3 kinaseswere active at G2/Mtransition as well asG1/S 9/10/ PMCB Lecture 9: G. Peter

Transcriptional Control ofG2/M z A model for theinduction of M-phasespecific genes z CDK phosphorylatesthe COOH tail ofMYB3RA 9/10/ PMCB Lecture 9: G. Peter De Veylder et al., 2007 Nature Rev. MCB 8: 655-

Mitosis to Endocycle z RB-E2F pathway is crucial for endocycle for continued DNAsynthesis z CDKB1;1 regulation of G2-M transition critical for endocycle z Elimination of mitotic CDK activity involves activation of CKIs andrepression and proteolysis of G2-M-specific cyclins 9/10/ PMCB Lecture 9: G. Peter

The Interaction ofDevelopmental Programs withthe Cell Cycle is Critical z Plant meristem formation, maintenance andfunction depend on proper regulation of the cellcycle machinery z A whole host of genes have been identified thatare critical for meristem formation, identity, andmaintenance z Shoot and root meristems have different genesinvolved in meristem formation andmaintenance 9/10/ PMCB Lecture 9: G. Peter

9/10/ PMCB Lecture 9: G. Peter Plant Shoot Apical Meristem z Example of where celldivision control occursin plants z External signals mustinteract with cell cyclecontrol elements Biochem. Molec Bio. of Plants, 2000

Control of Asymmetric Division withinthe Pericycle Cells for Lateral RootInduction

z Auxin induces lateralroots to form frompericycle cells at thexylem poles z Early upregulation ofE2Fa and CYCDgenes z Early repression ofICK/KRP genes z Auxin regulation ofCYCA and CYCB? 9/10/ PMCB Lecture 9: G. Peter De Veylder et al., 2007 Nature Rev. MCB 8: 655-

Model for Control of Cell Differentiation: CellCycle for Cells Flanks of the Meristem that donot Divide z Threshold of CDK z Activity above aspecific amount cellsstay in a divisionpattern z Below a specificamount of activityallows them todifferentiate 9/10/2008 PMCB Lecture 9: G. Peter De Veylder et al., 2007 Nature Rev. MCB 8: 655-665

9/10/2008 PMCB Lecture 9: G. Peter Summary z

The basic mechanisms are conserved, basedon sequence similarity of the differentproteins

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Plants have more gene family memberswhich may be related to the sessile nature ofplants

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The endoreduplication cycle in dicot plantsoccurs commonly

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Integration of the cell cycle with developmentis just starting to be worked out