Abstract

Speaker 1: Sarah Robinson (Group leader, Sainsbury Laboratory) Title: Visualising hypocotyl elongation in response to stress-induced microtubule reorientation Abstract: Morphogenesis in plants is controlled by the spatial pattern of the mechanical properties across the tissue. During growth, internal mechanical stresses can develop and also serve as an important determinant of plant development. To investigate the mechanical properties and responses to mechanical stress in the developing tissues of the model plant Arabidopsis we developed an automated confocal micro-extensometer (ACME). ACME enables forces to be applied to tissues, while they are imaged with a confocal microscope. These images were analysed to extract 3D cellular strain measurements; revealing spatial gradients in mechanical properties that correlate with the pattern of growth. We also used ACME to investigate responses to mechanical stress. We imaged the cytoskeleton in the different layers of the hypocotyl while mechanical stress was applied. The results were analysed by building a finite element model. We saw that when a relative compressive force was applied the pattern of tissue stress changed, leading to a reorientation of the microtubules in the epidermis but not the inner layers. As the epidermis usually restricts growth, this reorientation led to growth increasing in these samples. This may mimic the response of a seedling whose growth is restricted as it pushes through the soil and responds with an increase in growth.

Speaker 2: Arnaud Ambrosini (postdoc soon joining Roepjer lab) Title: Mechanical role of the nucleus during orthogonal force production within epithelium Abstract: Mechanical forces are critical regulators of cell shape changes and developmental morphogenetic processes. Forces generated along the epithelium apico-basal cell axis have recently emerged as essential for tissue remodeling in three dimensions. Yet the cellular machinery underlying those orthogonal forces remains poorly described. We found that during Drosophila leg folding cells eventually committed to die produce apico-basal forces through the formation of a dynamic actomyosin contractile tether connecting the apical surface to a basally relocalized nucleus. We show that the nucleus is anchored to basal adhesions by a basal F-actin network and constitutes an essential component of the force-producing machinery. Finally, we demonstrate force transmission to the apical surface and the basal nucleus by laser ablation. Thus, this work reveals that the nucleus, in addition to its role in genome protection, actively participates in mechanical force production and connects the contractile actomyosin cytoskeleton to basal adhesions.