The Role of Two Homeodomain Genes PENNYWISE(PNY) and POUNDFOOLISH(PNF) in Regulating Meristem Maintenance of Shoot Apical Meristem
All aerial organs are produced from shoot meristems, which are dynamic self-sustaining structures located at the growing tips of shoots. Shoot meristems are subdivided into discrete domains with distinct functions. A population of stem cells is maintained in the central zone (CZ), while organogenesis occurs in the peripheral zone (PZ). In the core of the meristem, the rib meristem produces the pith of the stem. On the flanks of the shoot meristem a small strip of lateral organ boundary cells mediates the separation of the lateral organ from the meristem. Proper communication between these zones and the lateral organ boundary is essential for meristem integrity and function. Understanding the mechanisms that control shoot meristem fate and integrity will shed light on the mechanisms that control the patterning of the shoot. The KNOTTED1-Like HOMEOBOX (KNOX) transcription factor (TF) SHOOT MERISTEMLESS (STM) physically interacts with two related BELL1-like homeodomain (BLH) proteins called PENNYWISE (PNY) and POUND-FOOLISH (PNF) to regulate processes that maintain meristem cell fate. Genetic studies indicate that STM-PNY/PNF also regulates boundary function and flower meristem identity during inflorescence development. Studies from the second chapter of this thesis indicate that communication between PNY/PNF and the lateral organ boundary is essential for maintaining meristem integrity as well as controlling inflorescence patterning events. Based on genetic studies, it is unclear if STM and PNY/PNF regulate developmental processes, such as stem cell homeostasis, boundary function and flower meristem identity, directly or indirectly. Experimental evidence from chapter two suggests that STM but not PNY/PNF directly regulates a gene, which encodes a protein that controls stem cell homeostasis. In the CZ and PZ of the shoot meristem, PNY-STM act to negatively regulate lateral organ boundary genes. However, in the absence of PNY/PNF function, STM functions to specify lateral organ boundary identity on the flanks of the shoot meristem. Although PNY/PNF indirectly regulates flower specification via the lateral organ boundary, studies in chapter two suggest that these BLH proteins and at least one KNOX protein directly regulates an early flower meristem identity gene. Results from this thesis, indicate that the spatial expression patterns and formation of specific KNOX-BLH complexes is essential for maintaining meristem cell fate, integrity and flower meristem identity. Furthermore, PNY/PNF and STM act to regulate these processes in an indirect and direct manner to ensure meristem integrity and function is maintained.