MIF/IMAGENE · MIF genes: key regulators of floral meristem termination at the cross talk of multihormonal pathways

MIF genes: Key regulators of floral meristem termination at the cross talk of multihormonal pathways

Plant growth is an important process, providing food and natural resources for industry, chemicals, pharmaceuticals and renewable energy. In the present worldwide socio-economical context, plant biologists are facing challenges related to food and human nutrition supply, plant adaptation to occurring climate change, exhaustion of fossil resources and are thus expected to produce fundamental knowledge to be transferred to the societal and economical demands. Plant growth and development involve fundamental cellular processes such as cell division, cell expansion and cell differentiation, impacting plant yield and consequently the quality of plant products. As far as fruit organogenesis is concerned, it results from the coordination between two complementary processes: cell proliferation and cell differentiation, but the physiological mechanisms involved in are still poorly understood. The objectives of the MIF/IMAGENE project were to acquire a deeper knowledge on the fundamental biological processes involved in fruit organogenesis and development as to contribute in enabling the agriculture professionals to master fruit production both from a quantitative and a qualitative point of view. Our project aimed at focusing on the mechanisms by which a new regulator belonging to the MIni Zinc Finger (MIF) family, the inhibitor of meristem activity (IMA/MIF2) gene, participates in the integration of multi-hormonal signalling pathways and in the regulation of meristem activity. IMA/MIF2 acts at the floral meristem (FM) level to regulate its activity and at early fruit development as to coordinate cell division and differentiation respectively. Previously we discovered that the IMA/MIF2 protein interacts with a single protein which was identified as the CSN5 subunit of the COP9-signalosome complex. The COP9-signalosome plays an essential role in the response to light and phytohormones. Interestingly the csn5 mutant in Arabidopsis thaliana displays pleiotropic developmental defects that are strikingly similar to what were described for IMA/MIF2 overexpressors suggesting that IMA acts as an inhibitor of CSN function through its physical interaction with CSN5. The interaction was confirmed in cellulo by performing in vivo subcellular localisation experiments using transient expressions of IMA/MIF2-YFP and CSN5-YFP in onion epidermal cells followed by confocal microscopic observations, as well as using the Bimolecular Fluorescence Complementation (BiFC) approach to confirm the interaction in onion cells and mustard hypocotyls. We then demonstrated that the interaction of IMA/MIF2 with the CSN5 subunit of the COP9-signalosome complex affects the activity of the whole complex, implying that IMA/MIF2 is able to inhibit the CSN5-associated protein activity in planta. The main conclusion drawn from our data indicates that IMA/MIF2 acts as an inhibitor of the COP9-signalosome function through its physical interaction with CSN5. As a consequence IMA/MIF2 is involved in the modulation of the signalosome activity as to regulate the response of plant cells to multihormonal signalisation pathways driving important physiological and developmental processes in plant growth. Finally this work led to several unforeseen developments of putative socio-economical applications. Since the COP9-signalosome complex is a key regulator in integrating exogenous signals such as light or endogenous hormones, via the ubiquitin/ 26S proteasome degradation machinery, its essential function is to regulate the degradation of target proteins (transcriptional regulators, cell cycle regulators, etc.) in response to various stimuli. Thus we decided to determine whether the COP9-signalosome complex and its regulatory protein IMA/MIF2 are involved in the regulation of plant-pathogen interactions. With pathologist colleagues belonging to our laboratory, we were able to demonstrate that plants mutated for the CSN5 gene encoding the subunit five (CSN5) of the COP9-signalosome complex exhibit an increase in resistance to plum pox virus (PPV) and turnip mosaic virus (TuMV) compared to wild plants. In addition plants over-expressing the IMA/MIF2 protein, which partially mimic CSN5 mutants exhibit also a higher resistance to these viruses than wild type plants but to a lower extent than CSN5 mutants. These data indicate that the COP9-signalosome or a component of the signalling pathway controlled by the signalosome involving CSN5A protein is essential for viral infection. As a result we deposited an application for a Patent aimed at describing a new method for improving plant resistance to viruses taking advantage of the CSN5 loss-of-function. In addition, a newly designed project has been initiated to investigate the putative inhibitory effect of IMA/MIF2 proteins on the proliferation of cancer cells. IMA/MIF2 proteins are only present in plant cells but the physical interaction with the ubiquitously found CSN5 subunit of the signalosome could be exploited with a tremendous advantage in terms of inhibitory effect on cell proliferation. Indeed, it has been described that the up-regulation of CSN5 is found to be implicated in cancer cell proliferation. Thus we started a new collaboration with a medical research group within the University of Bordeaux to investigate the use of IMA/MIF2 proteins as a new plant anti-cancer molecule.

Data: CORDIS, © European Union