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.
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ItemFrom nucleotide sugars to polysaccharides: How do plants control the delivery of substrates for cell wall biosynthesis and protein glycosylation?( 2021)Plant cell walls constitute one of the most abundant raw biomaterials on Earth. The synthesis of long-chain olysaccharides, the main components of plant cell walls starts ab ovo in the cytoplasm where most of the building blocks for polysaccharide synthesis, so-called nucleotide sugars, are produced. The monosaccharide moieties of nucleotide sugars are incorporated into growing polysaccharide chains either directly at the plasma membrane by lycosyltransferases (GTs) that form cellulose synthase complexes or by those residing in the Golgi apparatus. In the latter case, nucleotide sugars have to pass the Golgi membranes with the help of nucleotide sugar transporters (NSTs). Once inside, they are used by Golgi GTs which assemble polysaccharide chains and decorate proteins and lipids with sugar residues. Recent evidence suggests that in plants nucleotide sugars can be guided to specific polysaccharides and/or glycan decorations, yet the molecular mechanisms of these processes are not fully understood. This PhD research attempts to explore the phenomenon of the targeted substrate delivery by investigating two possible hypotheses: the spatiotemporal distribution of proteins within the Golgi apparatus and the occurrence of direct interactions between NSTs and GTs. The author of this dissertation has tested both of those hypotheses by investigating protein-protein interactions, localising the individual components to their specific sub-compartments within the cell and tracking changes in mutant plants where these processes are modulated. The bifunctional UDP-RHAMNOSE/UDP-GALACTOSE TRANSPORTER (URGT) family from the model plant Arabidopsis thaliana was selected for this study. While this family has six members which in vitro are capable of transporting the same substrates, plant mutant studies indicate substrate preference and targeted delivery to specific cell wall components in vivo. This thesis presents the first evidence that members of the URGT family localise to specific sub-Golgi compartments. The colocalisation studies undertaken as part of this thesis place URGT1 and URGT5 in the cis-Golgi, URGT2, URGT4 and URGT6 – in the medial Golgi, while URGT3 seems to localise to trans-Golgi stacks. Protein-protein interactions studies have identified multiple interaction partners for the six URGTs. Notably, many of those are known galactosyltransferases, which aligns with the transport function of the URGTs. It is therefore highly likely that the identified candidates are true interactors, which use the proximity of the transporter to increase the process efficiency by substrate channelling. This finding is further supported by the fact that observed interactions between URGT family members and GTs often localise to the same sub-Golgi compartment. The study identified new potential galactosyl- and rhamnosyltransferases, including two putative arabinogalactan protein galactosyltransferases. The data obtained during this project suggest, that URGTs may determine the flow of substrate through both spatial separation within the sub-Golgi stacks and direct interactions with GTs. The study presents new insight into the sugar substrate delivery processes in plants, suggests that similar processes may take place in other NST families and that their specificity may be similarly tuned by localisation and interactions.