School of BioSciences - Theses
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ItemThe LEUNIG regulatory complex: How does it control shoot apical meristem formation and its post-embryonic activity?( 2021)Patterning along the apical-basal (A-B) axis is a crucial step during the early stages of plant embryogenesis and leads to the establishment of two poles of which each will develop their own stem cell niches. The activity of these meristems is responsible for post-embryonic growth, with the shoot apical meristem (SAM) generating the above-ground organs and the root apical meristem (RAM) producing the subterranean structures of the plant. While several transcriptional regulators governing A-B patterning have been identified, precisely how their regulatory function is orchestrated remains elusive. This study focuses on transcriptional co-regulators LEUNIG (LUG) and closely related LEUNIG_HOMOLOG (LUH) and their role in the formation of A-B patterning during embryogenesis as well as their post-embryonic maintenance. A link between the LUG regulatory complex and SAM formation and maintenance comes from the observation that lug mutants heterozygous for the luh allele (lug luh+/-) often have enlarged SAMs resulting from misregulated cell divisions. A more severe phenotype is observed in lug luh double mutants which are embryonically lethal. In this study, a detailed characterisation of lug luh embryo phenotype reveals that these mutants display aberrant cell divisions along the A-B axis, which correlates with defects in auxin distribution, complete loss of apical identity, and altered expression of transcription factors determining basal fate. Like other co-regulators, LUG and LUH lack intrinsic DNA-binding domains and instead must interact with DNA-binding cofactors to ensure recruitment to regulatory elements of target genes. This either involves direct contact between the co-regulators and transcription factors (TFs) or the formation of higher-order complexes with adaptor proteins such as SEUSS (SEU) or related SEUSS-LIKEs (SLKs), which facilitate binding to specific TFs. Results presented in this study provide insight into the molecular framework for the LUG regulatory complex activity during embryogenesis. Both yeast and in planta assays showed that LUG/LUH and SEU/SLKs physically associate with a variety of WUSCHEL-RELATED HOMEOBOX (WOX) TFs including members of the WOX2-module. Furthermore, genetic interactions between members of the WOX2-module and the LUG regulatory complex, support their mutual action during embryogenesis. Based on the reduced activity of HOMEODOMAIN LEUCINE-ZIPPER CLASS III (HD-ZIPIII) promoters in lug luh embryos, a model is proposed in which the LUG regulatory complex functions together with WOX2-module to promote apical identity and subsequent SAM initiation through regulation of the HD-ZIPIIIs. The activity of the LUG complex in promoting basal embryo identity through positive regulation of microRNA165/166 suggests that this complex also has functions that are independent of the WOX2-module. Preliminary work reported in this study further uncovered the role of the LUG regulatory complex in post-embryonic development. While the fasciated inflorescence meristems of lug luh+/- plants displayed defects in auxin transport and altered activity of stem cell markers, embryonically rescued lug luh mutants formed flat and differentiated SAMs. In addition, rescued lug luh mutants exhibited severely disorganised RAM and defects in quiescent center (QC) specification, supporting the involvement of the LUG complex in post-embryonic RAM maintenance.