THE MADS-DOMAIN SEEDSTICK PLAYS FUNDAMENTAL ROLES DURING TRANSMITTING TRACT DEVELOPMENT AND FRUIT GROWTH IN ARABIDOPSIS THALIANA

The MADS-box gene SEEDSTICK (STK) controls several aspects of plant reproduction. STK is co-expressed with CESTA (CES), a basic Helix-Loop-Helix (bHLH) transcription factor-encoding gene. CES was reported to control redundantly with the brassinosteroid positive signaling factors BRASSINOSTEROID ENHANCED EXPRESSION1 (BEE1) and BEE3 the transmitting tract development. Through the characterization of the stk ces-2 double mutant, we observed that STK and CES act together in the regulation of transmitting tract development. Combining the stk with ces-2 bee1 bee3 we have obtained the quadruple mutant showed a clear increase of the unfertilized ovules and septum defects. In the quadruple mutant carpel fusion was compromised, causing the formation of holes at the center of the septum where transmitting tract differentiates. These phenotypes do not allow a proper pollen tube growth in the double mutant stk ces-2, and even more in the quadruple mutant bee1 bee3 stk ces-2 leading to high number of unfertilized ovules. The transcriptome profile of the quadruple mutant bee1 bee3 stk ces-2 compared to wild type revealed a small subset of misregulated genes, which probably act downstream of the transcription factor described above, mainly involved in cell death, the extracellular matrix of the transmitting tract, cell wall composition and auxin signaling. This specific subset of downstream target genes controlled directly or indirectly by STK-CES-BEE1-BEE3 protein complex, open doors to a new regulatory network controls transmitting tract development. Altogether our data reveal new insights in the regulation of transmitting tract development together by bHLH and MADS-box transcription factors.