Plant Molecular and Cellular Biology Lecture 10: Plant Cell Cycle Gary Peter

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1 Plant Molecular and Cellular Biology Lecture 10: Plant Cell Cycle Gary Peter 9/10/2008 1

2 Learning Objectives Explain similarities and differences between fungal, mammalian and plant cell cycles Explain the role of the cell cycle machinery in plant development 9/10/2008 PMCB Lecture 9: G. Peter 2

3 Plant Cell Cycle is Conserved De Veylder et al., 2007 Nature Rev. MCB 8: /10/2008 PMCB Lecture 9: G. Peter 3

4 Plant CDK, Cyclin Regulators are Conserved: All Predicted Arabidopsis CDKs and Cyclins Arabidopsis thaliana has 12 distinct bona fide CDKs and more than 49 different cyclins 9/10/2008 PMCB Lecture 9: G. Peter 4 The Plant Cell, Vol. 14, , April 2002

5 CDKs CDK classifications based on conserved sequences corresponds well with their functional diversity. The major drivers of the plant cell cycle are the A- and B- type CDKs. A-type CDKs (CDKA) are most similar to mammalian CDK1 and CDK2, they are constitutively present during the cell cycle, and they control the G1 S and G2 M transitions. B-type CDKs (CDKB) are specific to plants and their accumulation depends on the phase of the cell cycle, they reach a maximum level at the G2 M transition, the phase at which their activity is required. 9/10/2008 PMCB Lecture 9: G. Peter 5

6 Cyclins Cell-cycle progression requires the sequential association of the CDKs with different types of cyclin. D-type cyclins (CYCD) regulate the G1 S transition, although there is accumulating evidence that they also might have a role in the G2 M transition. A-type cyclins (CYCA) are mainly present from S phase to M phase. B-type cyclin (CYCB) levels peak at the G2 M transition and during M phase1. To date, the exact composition of different CDK cyclin complexes in plants is largely unknown. Arabidopsis Cyclin Type # in family A 10 B 11 D 10 H 1 9/10/2008 PMCB Lecture 9: G. Peter 6

7 Cyclin D Functional redundancy high- inferred from the lack of strong phenotypes in single gene knockouts Expression is modulated by cytokinins, auxins, brassinosteroids, sucrose, gibberellins CYCD3;1- is rate limiting for calli starved of exogenous cytokinin and overexpression is sufficient to compensate for exogenous cytokinin CYCD2;1- overexpression stimulates faster growth leading to larger plants with similar sized cells 9/10/2008 PMCB Lecture 9: G. Peter 7

8 Ectopic Expression of At Cyclin D2 Stimulates Growth Cockcroft et.al. 00 Nature 405: Dry Weight Increases 3-4.5X after 35 days Height Increases ~36% after 55 days Cell Division Rate Increased Meristem organization unaffected

9 Cyclin A Single gene knockouts show limited phenotypes Multigene knockouts with RNAi have embryo defects Lines overexpressing CYCA s have ectopic cell division and delayed differentiation high levels of S phase specific genes and CDK- CYCA activity and impaired shoot and root regeneration 9/10/2008 PMCB Lecture 9: G. Peter 9

10 Ectopic Expression of Cyclin A in Roots Stimulates Cell Division After 6 days, dry weight increased 6-8X Root apical growth rate increased 30-40% Cell # increased Root morphology was normal Doerner et al. 96 Nature 380:

11 Cyclin B Overexpression in roots stimulates growth dramatically overall root They trigger G2-M transition even in Xenopus CYCG;1 binds to and activates both A- and B- CDKs in vitro 9/10/2008 PMCB Lecture 9: G. Peter 11

12 Control of G1/S Transition Expression of cyclin Ds are regulated by hormones and sucrose Similar players and feedback mechanisms Rb (single gene) and E2F (three genes) CKI (Krps) Are different meristems regulated differently with different gene family members? Lateral roots Shoot apical meristems Annual Reviews

13 Transcriptional Control of G1/S Conserved transcriptional activators E2Fa/b and DP and retinoblastoma related protein is inactivated by CDKA-cycD Plant innovation - E2Fc is a transcriptional repressor that suppresses cell division 9/10/2008 PMCB Lecture 9: G. Peter 13 De Veylder et al., 2007 Nature Rev. MCB 8:

14 Potential Role of RBR and E2F- DP in Chromatin Remodeling Histone deacetylases are targeted to promoters via interaction with retinoblastoma like proteins which bind to E2F-DP transactivators Inactivation of RBR releases HDACs to remodel chromatin 9/10/2008 around important PMCB Lecture 9: G. genes Peter 14 De Veylder et al., 2007 Nature Rev. MCB 8:

15 G2/M Transition Cyclin B triggers this transition; however Cyclin D also expressed at this boundary and CYCD3;3 kinases were active at G2/M transition as well as G1/S 9/10/2008 PMCB Lecture 9: G. Peter 15

16 Transcriptional Control of G2/M A model for the induction of M-phase specific genes CDK phosphorylates the COOH tail of MYB3RA2 9/10/2008 PMCB Lecture 9: G. Peter 16 De Veylder et al., 2007 Nature Rev. MCB 8:

17 Mitosis to Endocycle RB-E2F pathway is crucial for endocycle for continued DNA synthesis CDKB1;1 regulation of G2-M transition critical for endocycle Elimination of mitotic CDK activity involves activation of CKIs and 9/10/2008 repression and proteolysis PMCB of G2-M-specific Lecture 9: G. Peter cyclins 17

18 The Interaction of Developmental Programs with the Cell Cycle is Critical Plant meristem formation, maintenance and function depend on proper regulation of the cell cycle machinery A whole host of genes have been identified that are critical for meristem formation, identity, and maintenance Shoot and root meristems have different genes involved in meristem formation and maintenance 9/10/2008 PMCB Lecture 9: G. Peter 18

19 Plant Shoot Apical Meristem Example of where cell division control occurs in plants External signals must interact with cell cycle control elements 9/10/2008 PMCB Lecture 9: G. Peter 19 Biochem. Molec Bio. of Plants, 2000

20 Control of Asymmetric Division within the Pericycle Cells for Lateral Root Induction Auxin induces lateral roots to form from pericycle cells at the xylem poles Early upregulation of E2Fa and CYCD genes Early repression of ICK/KRP genes Auxin regulation of De Veylder et al., 2007 Nature Rev. MCB 8: /10/2008 CYCA and CYCB? PMCB Lecture 9: G. Peter 20

21 Model for Control of Cell Differentiation: Cell Cycle for Cells Flanks of the Meristem that do not Divide Threshold of CDK Activity above a specific amount cells stay in a division pattern Below a specific amount of activity allows them to differentiate De Veylder et al., 2007 Nature Rev. MCB 8: /10/2008 PMCB Lecture 9: G. Peter 21

22 Summary The basic mechanisms are conserved, based on sequence similarity of the different proteins Plants have more gene family members which may be related to the sessile nature of plants The endoreduplication cycle in dicot plants occurs commonly Integration of the cell cycle with development is just starting to be worked out 9/10/2008 PMCB Lecture 9: G. Peter 22