Research - Institute of Plant Biology - Laboratory of Molecular Regulators of Plant Growth

Zoltán MAGYAR
senior research associate

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Eszter MOLNÁR research associate
Anita DUDÁSNÉ KOVÁCS scientific administrator
Tünde VASKÓ-LEVICZKY junior research associate
Márta DELI Ph.D. student
Erika ŐSZI Ph.D. student

MOLECULAR REGULATORS OF PLANT GROWTH

Plants and animals have different developmental strategies for growth yet individuals of both attain characteristic species-specific sizes constrained by their developmental genetic programmes. Additionally, growth can be significantly influenced by environmental factors specifically so in plants. We are seeking to understand how plant growth is regulated at the molecular level and through which molecular mechanisms environmental signals are able to influence growth.

The key factor in growth is the duration of cell proliferation and the timing of the exit from proliferation to cell expansion and differentiation (Figure 1). In plants cell proliferation is largely concentrated in specialised regions known as meristems, which contain the stem cells. In meristems udifferentiated cells are produced by cell proliferation, and when these cells stop dividing, as they leave the meristematic region they differentiate into specific tissues. During differentiation, plant cells frequently increase their DNA content by a modified mitotic cycle called endoreduplication, a process of continous DNA synthesis without intervening mitosis. We are interested in the molecular mechanisms which maintain stem cell activity in the meristems; control the balance between cell division and differentiation and parallel regulate the switch from mitotic cell cycle to endoreduplication during organ development. Our main interest is in genes involved in the regulatory mechanisms which make the decision to enter or leave the division cycle. We cloned a family of genes from the model plants Arabidopsis thaliana called E2F transcription factors, which are related to genes that control the same process in animals. The cannonical role for E2F transcription factors is to regulate cell cycle entry, but it is becoming apparent in many sytems that E2Fs have broader functions and that, besides the regulation of cell cycle transitions, they coordinate cell proliferation with cell growth and differentiation. The current model is that E2Fs can work both as positive and negative regulator of transcription, depend on their structure and on the function of the retinobalstoma (RB) tumour supressor protein. Post-embryonic growth in plants depends on the continuous supply of undifferentiated cells within meristems.

Auxin is a plant growth hormone regulates cell division in a concentration dependent manner; elevated auxin levels activate cell division in the meristems, while reduced amounts repress mitosis as cells leave the meristematic regions, and in parallel it enhances cell growth. We discovered the auxin increases the stability of E2FB protein, and co-expression of E2FB with its dimerization partner DPA in plant cells could maintain cell proliferation in the absence of auxin (Magyar et al., Plant Cell, 2005). Cytokinin, another plant hormone works opposite to auxin and the antagonistic functions of these two hormones appears to be a key mechanism which regulates meristematic functions. Our recent work indicates that cytokinin can change the activity of E2FB from a transcriptional activator to a transcriptional repressor. Our aim is to understand the molecular mechanisms leading to this switch in the E2F activity during hormone signalling, and to identify the downstream targets of E2Fs by using chromatin immunoprecipitation (ChIP) method.



Figure 1. Mechanisms for organ size control. (a) Organ formation, exemplified here by leaf development, consist of two stages. The first phase is underpinned by cell proliferation, characterized by intense macromolecular/cytoplasmic synthesis and rapid cell division. The second phase is characterized by cell expansion and differentiation. Differentiation takes place along basipetal gradient (that is, from leaf tip to leaf base), as indicated here by the gradient in cell size and cell greening. The red arrow summerizes the proliferative inputs, and the black arrow the arrest of proliferation and initiation of differentiation. (b,c) The two principal mechanisms for controlling organ size. Enlargement of organs can be produced by either (b) increasing proliferation signals or (c) delaying the transition between proliferation and differentiation. In both cases the number of cells available for organ formation at the end of the proliferative phase is increased, but the underlying mechanisms are different (Bögre et al., Genome Biology 2008).


The maintenance of stem cells in the plant meristems is crucial for the growing plants. Previous studies demonstrated that retinoblastoma-related protein 1 (RBR1) is a stem cell regulator in Arabidopsis. Recently we have found that ectopic co-expression of E2FB and DPA heterodimeric transcription factors increases the amount of stem cells in Arabidopsis roots (Figure 2), which further supports the involvement of the plant RBR-E2F pathway in the regulation of stem cell maintenance.



Figure 2. Ectopic co-expression of E2FB with DPA increases the amount of stem cells in the root meristem of Arabidopsis as indicated by red arrow. Position of the quiescent centre (QC) is indicated.


To unravel the molecular pathway controlling the switch from proliferation to differentiation, we use the first developing leaf pair of Arabidopsis thaliana as a model system. In this model system, cells gradually exit the mitotic cell cycle and engage into an endoreduplication cycle as they start to differentiate (Figure 1). Our results indicate that different E2F proteins enter into complex with the single RBR1 protein at different developmental stages. We suggest that two E2Fs from Arabidopsis, E2FA and E2FB has antagonistic functions when they form complex with RBR1; E2FA-RBR1 keeps cells in the mitotic cycle, while binding of RBR1 to E2FB stimulates cell cycle exit during leaf development (Figure 3). Our major aim is to identify the downstream targets for these E2F transcriptional complexes. Stress such as drought could change the activity of E2F complexes which might lead to repress growth. We are seeking to understand how and why under stress conditions plants stop growing, and what is happening at the cellular level. Molecular insights into this process and how it is signalled may lead to opportunities to engineer crops with increased stress tolerance and so higher yields.






Figure 3. . E2FA and E2FB could regulate different processes by making complex with RBR1 in tisssue specific manner. (A) E2FA and E2FB proteins show opposite expression pattern in the root meristem: E2FA-GFP signal is the strongest in dividing cells, while E2FB-GFP becomes more abundant in post mitotic cells. RBR-GFP protein shows ubiquitous expression. Confocal microscopic images. Arrows show the distal end of the meristem, arrowheads indicate the position of the quiescent centre. (B) Proposed working model for E2FA and E2FB function when they form complexes with RBR1.


One of our main questions is: how we could translate Arabidopsis knowledge related to growth and development into crop plants to improve yield? Growth relies on the production of cells, which in plants is restricted to meristems. Both entering and leaving cell proliferation is controlled by an evolutionary conserved transcriptional regulatory switch, the E2F-RB pathway. Our published work shows that overexpression of E2FB, an E2F transcription factor in Arabidopsis, dramatically increase the proliferation rate of cells. In our recent work, we could influence plant growth and seed production by modulating E2FB expression level within its own expression domain, indicating that E2FB is a key growth regulator. Moreover we have discovered that E2FB could regulate the accumulation of storage reserves during seed development. This makes E2FB a pivotal gene for crop improvement to attain increased yield. We aim to prove this concept in the related Brassica species by using the same strategy we have employed in Arabidopsis to increase E2FB expression level in rapeseed within its own expression domain. To establish how E2FB can boost biomass and yield we will determine the target genes and interacting proteins. The compendium of these genes will provide further material for targeted rapeseed breeding programs.


Figure 4. Growth of transgenic Arabidopsis is regulated by E2FB. Growth of transgenic Arabidopsis lines depends on the level of E2FB-GFP expression.

Selected publications

Magyar Z, Horvath B, Khan S, Mohammed B, Henriques R, De Veylder L, Bako L, Scheres B, Bogre L. Arabidopsis E2FA stimulates proliferation and endocycle separately through RBR-bound and RBR-free complexes. EMBO J. 31: (6)1480-1493. (2012)

Berckmans B, Vassileva V, Schmid SP, Maes S, Parizot B, Naramoto S, Magyar Z, Kamei CL, Koncz C, Bogre L, Persiau G, De Jaeger G, Friml J, Simon R, Beeckman T, De Veylder L. Auxin-Dependent Cell Cycle Reactivation through Transcriptional Regulation of Arabidopsis E2Fa by Lateral Organ Boundary Proteins. The Plant Cell 23:(10) pp. 3671-3683. (2011).

Henriques R, Magyar Z, Monardes A, Khan S, Zalejski C, Orellana J, Szabados L, de la Torre C, Koncz C, Bogre L. Arabidopsis S6 kinase mutants display chromosome instability and altered RBR1-E2F pathway activity. EMBO J. 29: (17)2979-2993 (2010).

Bögre L., Magyar Z., Lopez-Juez E. New clues to organ size control in plants. Genome Biology 2008; 9(7): 226

Magyar Z. Keeping the balance between proliferation and differentiation by the E2F transcriptional regulatory network is central to plant growth and development. Plant Growth Signaling edited by László Bögre and Gerrit Beemster 2008; Springer; Plant Cell Monographs; Vol.10; pp 89.

Lopez E, Dillon E, Magyar Z, Khan S, Hazeldine S, de Jager S, Murray J, Beemster G, Bögre L, Shanahan, H. Distinct Light-Mediated Gene Expression and Cell Cycle Program in the Shoot Apex and Cotyledons. Plant Cell 2008; 20(4):947-68

Horváth BM, Magyar Z, Zhang Y, Hamburger AW, Bakó L, Visser RG, Bachem CW, Bögre L. EBP1 regulates organ size through cell growth and proliferation in plants. EMBO J. 2006 Oct 18; 25(40):4909-20

Mészáros T, Helfer A, Hatzimasura E, Magyar Z, Serazetdinova L, Rios G, Bardoczy V, Teige M, Koncz C, Peck S, Bögre L. The Arabidopsis MAP kinase kinase MKK1 participates in defence responses to the bacterial elicitor flagellin. Plant Journal. 2006 Nov, 48(4):485-98

Magyar Z, De Veylder L, Atanassova A, Bakó L, Inzé D, Bögre L. The Role of the Arabidopsis E2FB Transcription Factor in Regulating Auxin-Dependent Cell Division. The Plant Cell. 2005 Sep;17(9):2527-41

Vlieghe K, Boudolf V, Beemster GT, Maes S, Magyar Z, Atanassova A, de Almeida Engler J, De Groodt R, Inze D, De Veylder L The DP-E2F-like gene DEL1 controls the endocycle in Arabidopsis thaliana. Curr Biol. 2005 Jan 11;15(1):59-63

Magyar Z, Atanassova A, De Veylder L, Rombauts S, Inze D. Characterization of two distinct DP-related genes from Arabidopsis thaliana. FEBS Lett. 2000 Dec 1;486(1):79-87.