Factores que intervienen en la motivación de los estudiantes en el

Zoltán MAGYAR / Principal Investigator

Gyöngyi Angéla BATTANCS / technician

The key factor in growth is the duration of cell proliferation and the timing of the exit from prolifer- ation to cell expansion and differentiation (figure 1). In plants, cell proliferation is largely concentrated in specialised regions known as meristems, which con- tain 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 en- doreduplication, a process of continous dna syn- thesis without intervening mitosis. We are interested in the molecular mechanisms which maintain stem cell activity in the meristems; control the balance be- tween cell division and differentiation and regulate

cycle transitions, they also coordinate cell prolifera- tion with cell growth and differentiation. according to the current model, e2fs can work both as positive and negative regulators of transcription, depending on their structure and on the function of the retino- blastoma (rB) tumour suppressor protein.

auxin is a plant growth hormone that regulates cell division in a concentration dependent manner; elevated auxin levels activate cell division in the meristems, whereas reduced amounts repress mi- tosis as cells leave the meristematic regions, and in parallel it enhances cell growth. We discovered that auxin increases the stability of the e2fB protein, and co-expression of e2fB with its dimerization partner dPa in plant cells could maintain cell pro-

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Figure 1: Mechanisms for organ size control. (a) Organ formation, exem-

plified here by leaf development, consist of two stages. The first phase is underpinned by cell proliferation, characterized by intense macromo- lecular/cytoplasmic synthesis and rapid cell division. The second phase is characterized by cell expansion and differentiation. Differentiation takes place along a 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 summarizes proliferative inputs, and the black arrow indicates the arrest of proliferation and the 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) de- laying 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 dif- ferent (Bögre et al., Genome Biology 2008)

The maintenance of stem cells in the plant meristems is crucial for the growing plant. Previous studies demon- strated that the retinoblastoma-related protein 1 (rBr1) is a stem cell regulator in plants. recently we have found that ectopic co-expression of e2fB and dPa het- erodimeric 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 ar- row. 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 endoredu- plication 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 have antagonistic func-

tions when they form complexes with rBr1; e2fa- rBr1 keeps cells in the mitotic cycle, whereas binding of rBr1 to e2fB stimulates cell cycle exit during leaf development (figure 3). our major aim is to identify the downstream targets of these e2f transcriptional complexes. stress such as drought could change the activity of e2f complexes, which might lead to growth repression. We seek to understand how and why under stress conditions plants stop growing, and what hap- pens at the cellular level. molecular insights into this process and how it is signalled may lead to opportuni- ties to engineer crops with increased stress tolerance and, consequently, with higher yields.

Figure 3: Proposed working model for E2FA and E2FB function when

they form complexes with RBR1.

Contact: [email protected]

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Plants exhibit a remarkable developmental plasticity as a consequence of their sessile way of life. During plant ontogenesis, exogenous (environmental) and endogenous (developmental) signals converge on interlinked signal- ing pathways having dynamic impacts on plant form and function. We use molecular, biochemical and cellular approaches to reveal and characterize plant-specific pathways of cellular signaling that underlie this developmen- tal plasticity.

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(fUnCTIonal Cell bIoloGY GRoUp)

Attila FEHÉR / Principal Investigator, Group Leader

Krisztina ÖTvÖS / staff scientist Mónika DOMOKI / staff scientist Judit BÍRó / staff scientist

Csilla FODOR-DUNAI / staff scientist Manuela E. JURCA / staff scientist Róza NAGY / technician

Gyöngyvér KATONA / technician