%0 Generic
%A Tonn, Nina
%C Heidelberg
%D 2021
%F heidok:28599
%R 10.11588/heidok.00028599
%T SUPPRESSOR OF MAX2 1-LIKE3 (SMXL3), SMXL4 and SMXL5 establish post-embryonic phloem development in Arabidopsis thaliana
%U https://archiv.ub.uni-heidelberg.de/volltextserver/28599/
%X During post-embryonic development, plants rely on the integrity of phloem within their root systems. The phloem is part of the vasculature and transports energy metabolites from leaves into mitotically active regions such as the root apical meristem (RAM). Loss of function of genes regulating phloem development can result in severe changes in root growth and plant body architecture. The redundantly active genes SUPPRESSOR OF MAX2 1-LIKE3 (SMXL3), SMXL4 and SMXL5 are central regulators of early phloem formation. However, molecular mechanisms underlying SMXL3/4/5 gene activities during early phloem development are mostly unknown. The functional relevance of SMXL3/4/5 protein domains including the EAR motif is also unclear. The aim of my dissertation was to characterise the mode of action of SMXL3/4/5 during early events of phloem development in detail using Arabidopsis thaliana roots as model organ to investigate spatiotemporal tissue formation.    First, I investigated at which developmental steps SMXL3/4/5 genes are required to promote phloem development in the RAM, how they interact genetically with positive regulators (OPS, BRX) and how their function is affected by negative regulators (CLE26, CLE45). I found that SMXL4/5 function is required to initiate and promote the activities other of of genes regulating phloem development (OPS, BRX, BAM3, CVP2 and APL), and that SMXL4/5 protein functions are possibly required to attenuate CLE-mediated suppression of phloem differentiation. Furthermore, I examined whether the highly conserved EAR motif of SMXL5 is functionally relevant to promote early phloem development. Here, I tested whether protein accumulation was altered for EAR motif-mutated SMXL5 proteins (SMXL5mEAR) in planta, and if phloem formation could be restored in smxl4;smxl5 double mutants complemented with SMXL5mEAR proteins. My data suggest that SMXL5 protein function is independent from the EAR motif indicating that SMXL5 proteins do not act as canonical EAR repressors in the context of phloem development. Last, I aimed at identifying new genes that are functionally related to SMXL3/4/5 during early phloem development. Therefore, I performed an ethyl methanesulfonate (EMS)-based mutagenesis of smxl4;smxl5 double mutants to screen for genetic suppressors that alleviate the phloem defects characteristic for smxl4;smxl5 mutants. I found that mutagenesis of yet unknown suppressor genes in the smxl4;smxl5 background could indeed restore phloem development. Further analysis including genome mapping is required to identify candidate genes that result in the suppression of the smxl4;smxl5 mutant phenotype.    In conclusion, I postulate that SMXL3, SMXL4 and SMXL5 genes are required to establish the post-embryonic phloem lineage and regulate the phloem-specific developmental program in the RAM. Together, a complex, tightly balanced network of molecular players depending on SMXL3/4/5 activities ensures the formation of phloem within the root system.