RESERACH SUMMARY

We are studying the genetic basis of the control of axillary bud development in the model system Arabidopsis, and in the crop species tomato and potato in which control of lateral shoot branching is of great agronomical interest. We have characterised the Arabidopsis BRANCHED1 (BRC1) gene, which acts as a central switch of axillary bud development and outgrowth. We are now expanding our knowledge of the genetic networks involving BRC1 in Arabidopsis.

First, we identified and characterised two types of motifs conserved in the BRC1-like promoters, which act as transcriptional silencers of BRC1. One of these elements confers qualitative (spatial) regulatory information essential for driving BRC1 expression in axillary buds; the others act as silencers, necessary to maintain low BRC1 expression levels. We have identified two transcription factors that could bind to one of these elements and might be involved in the negative transcriptional regulation of BRC1.

Second, we have begun to understand the protein-protein interactions involving BRC1 and have isolated two factors that could modulate BRC1 activity. In addition, we identified the BRC1 protein domains mediating those interactions.

Third, we compared the transcriptomic profiles of wild type and brc1 axillary buds and identified genes that could be controlled directly by BRC1. We are systematically analysing the genetic control of branch suppression in response to shade in Arabidopsis and testing the working hypothesis that during the SAS, BRC1 is upregulated in response to reduction in the R:FR ratio and its activity is responsible for the branch suppression response.

Solanaceae is a family that includes a large number of species in which the control of branch outgrowth is of great agronomical interest, and for which understanding the function of some of the key players will help optimise plant architecture and yield. Our work shows that in tomato and potato, species with branching patterns divergent from those of Arabidopsis, two BRC1-like paralogues, BRC1a and BRC1b, are coexpressed in axillary buds. Reverse genetic analyses confirmed that tomato SlBRC1b plays a role in the promotion of axillary bud arrest. In contrast, SlBRC1a, which encodes a divergent protein with a novel C-t domain, has a still unclear role in this process. Evolution rate studies indicate that whereas BRC1b has evolved under a strong purifying selection in the clade comprising S. Lycopersicum, other closely related wild tomato species and potato, BRC1a has evolved at a faster rate under positive selection.