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Final Report Summary - EVOBRANCHINGCONTROL2 (Characterisation of the integrative and conserved role of BRC1 in lateral branching control.)

The project “EvoBranchingControl2” aimed to better understand the integrative role of BRANCHED1 (BRC1) in the control of lateral branching in the model species Arabidopsis thaliana by investigating what are the downstream genetic pathways controlled by this transcription factor. In a second time, this project sought to check if in Solanaceae species , specially the tomato species (Solanum lycopersicum) and the potato species (S. tuberosum), which are species are of agronomical interest, the pathways controlled by this gene are conserved.

After submitting the project but before getting the results of the FP7 Marie Curie Action we already initiated the first experiments. Because of the highly promising preliminary results we obtained in the potato species we decided to mainly focus the work on this species. In Solanaceae, as a consequence of a whole genome duplication that occurred early after the rise of this family, there are two copies of the ortholog (corresponding gene) AtBRC1 gene called StBRC1a and StBRC1b.

The transgenic lines independently inactivating the function of the two genes (RNAinterfering lines) indeed show very interesting phenotype. The StBRC1a- RNAi lines show similar branching phenotype (plants having more banches due to the loss of apical dominance) in the aerial part that its orthologs AtBRC1. Interestingly, these lines also show a branching phenotype at the stolon and tuber level, stolons being also more branched and producing more potatoes than the control lines. The total weight stays however at similar value. At the tuber level, there also is a significant loss of apical dominance of the tube eyes. Most of them are indeed growing in a similar way to the apical eye.

Concerning the StBRC1b gene, the corresponding RNAi lines only show weak branching phenotype in either aerial or subterranean level. But they show thicker plant sometimes having aerial tubers. Theses lines produce the similar number of tubers compared to the wild type and control lines but significantly slightly thicker. This could mean this gene is following a neo-functionalization process thus acquiring a new function related to the control of the tuberization process. In a consistent manner, the transgenic lines overexpressing one of these two genes show the opposite phenotype. We also generated transgenic plants inactivating the two genes in the same time to check out if we can get more and bigger potatoes in this way.

During these two years we thus further characterized these two genes, investigating what are their role and their regulation. We specially focused on the role of the gene StBRC1b that has a new function in a phenomenon (tuberization) of first interest in the agricultural point of view. We could show the two genes are highly connected, the expression of the two being inter-dependent. These two genes also respond to different factors like endogenous ones like hormones (Auxin, Cytokinin, Gibberellin) or environmental like shadow and darkness. We also showed these two genes are interacting with the gene called SP6A that is considered as one of the key gene involved in the tuberization process (Navarro C. et al., Nature 2011).

At the protein level, by using different technics like two hybrid assay or Bimolecular Fluorescence Complementation (BiFC) method in confocal microscopy we showed the twoBRC1 genes are also highly connected as well as with the gene SP6A. We also showed that the protein BRC1a as particular system of regulation of its function at the cellular level. Indeed, an alternative splicing of this gene generate an second protein that has a cytoplasmic localization and preventing the normal version of the protein to go to the nucleus to accomplish its function of transcription factor.

In summary, this project allowed doing first characterization at the genetic an protein levels of two copies of the transcription factor BRC1 involved in two important processes of the potato species development that are the control of the plant architecture as well the control of tuberization. More than its importance at the scientific and academic level, because potatoe plants are also important for human and animal feeding, this work may have an important economical role achieving in this way the goal of the Marie Curie Action.

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