Stem cell niche ontogeny during lateral root developmentin Arabidopsis thaliana

Abstract

Stem cell niches are critical elements of plant organs, supporting their design, growth and maintenance. Typically, a stem cell niche (SCN) is composed of stem cells, able to asymmetrically divide to self-renew and generate a daughter which further proliferates before differentiating into a given tissue. Stem cells are maintained undifferentiated by an organizing center cells (OC). In the model plant Arabidopsis thaliana, three stem cell niches are driving the main axis of growth. The shoot apical meristem, at the apex of the shoot, generate aerial tissues. The root apical meristem, at the tip of roots, drive soil exploration for nutrients, water and anchorage, while the cambium is in charge of the radial expansion of plant organs. The root system of Arabidopsis thaliana is critical for the plant adaptability to its environment. It is composed of a primary root formed during embryogenesis, and lateral roots originating from cells deep inside the primary root tissues. Lateral root are formed post-embryonically and robustly reproduce the primary root organization, therefore being a convenient model to investigate de novo stem cell niche establishment. Early lateral root (LR) development has been well characterized. However, a comprehensive view on the ontogeny of lateral root stem cell niche establishment from the earliest stages of LR development up to post emergence is still to be proposed. The aim of this thesis is thus to characterize the process of stem cell niche establishment in Arabidopsis thaliana lateral roots from the beginning of lateral root development until it reaches a primary root-like organization. First, I defined a developmental frame of reference to follow LR development from stage I up to 2 mm post-emergence using cell wall staining and confocal microscopy of LR transverse section. The analysis of topological features in relation to the LR length indicate a progressive onset of a primary root like stem cell niche topology, post emergence. Transcriptional reporters of genes involved in the primary root stem cell niche patterning overlap to delineate the stem cell niche and its organizing center in short emerged LR, ranging from 80 to 250μm. Second, I investigated the behavior of the SCN and its OC. To this end I assessed cell proliferation in LR of different length. While the stem cell niche organizer is proliferative prior emergence, quiescence is acquired post-emergence in short lateral roots. Genotoxic stress sensitivity assay and lineage tracing suggest a progressive onset of the organizing center functions within the stem cell niche. Finally, I challenged the proliferative behavior of the stem cell niche organizer using mutants and ectopic expression of OC quiescence regulators. However those approaches did not strongly affected the stem cell niche organization. The originality of this work is the investigation of the functional aspect of SCN establishment with a focus at the cellular resolution spanning lateral root development from initiation up to 2mm post-emergence. The observations collected in this study may contribute to broaden the current view on stem cell niche establishment during lateral root development. Possibly by expanding the focus of further investigations up to post-emergence. I hope that the findings presented here will be helpful for deepening our understanding of plant organogenesis.