Control of plant architecture by distinctive TALE homeobox gene interactions

In eukaryotes, transcription factor (TF)-based network is a widely used mechanism to regulate fundamental developmental processes. Both animals and plants utilize three-amino-acid-loop-extension (TALE) homeodomain (HD) transcription factors to subdivide their body plan. In animals, MEIS/PBC TF heterodimers are involved in the specification of the anterior-posterior axis of animals, whereas in plants their KNOX/BELL orthologs play an important role in plant architecture in general, and more specifically, in shoot meristem function. BELL-KNOX interactions are selective, and essential for site-specific DNA binding. We aim to understand the functional relevance of BELL-KNOX interactions. We focused on functional characterization of the BELL protein ATH1, and to a lesser extent, also two related BELL proteins PNY and PNF, in relation to their interacting partners, the KNOX proteins STM, KNAT1/BP, KNAT2 and KNAT6. Our finding that combined mutations in three genes that encode STM-interacting BELL proteins, ATH1, PNY and PNF, phenocoy stm mutants, not only highlights the role of these BELL proteins in SAM function, but also provides a strong indication that in plants, TALE HD proteins operate as heterodimeric complexes. Since ATH1 was further found to be required for proper establishment of meristm-organ boundaries and for control of stem growth in conjunction with a different set of KNOX protein, KNAT2 and KNAT6, this indicates that combinatorial control of developmental processes by TALE HD proteins might also be a common feature in plants. Interestingly, a more complicated network seems to underlie the function of KNOX-BELL heterodimeric complex as our data suggested they can interact at multiple levels (Chapter 3). Additional complicacy of this network comes from the overlapping and opposite roles of BELL proteins, ATH1 and PNY, depending on different developmental stages (Chapter 4). In conclusion, the research presented in this thesis suggests that BELL and KNOX proteins act as functional dimeric unit to control different aspects of plant architecture. And their interactions form a complex network throughout development, which is most likely to fine-tune TALE HD heterodimer activity.