Three-dimensional study of spur morphogenesis in the flower of Staphisagria picta (Ranunculaceae) – from cellular level to organ scale

Floral spurs are invaginations borne by perianth organs (petals and/or sepals) that have evolved repeatedly in various angiosperm clades. They typically store nectar and can limit the access of flower visitors (potential pollinators) to this reward, resulting in pollination specialization possibly leading to speciation in both pollinator and plant lineages. Despite the ecological and evolutionary importance of nectar spurs, the cellular mechanisms involved during their development have been described in detail in a handful of species only, and primarily with respect to epidermal cells. In an attempt to fill in this gap, we designed a new automated method to analyse in a three-dimensional space image generated using confocal microscopy, describing the cellular characteristics (volume, sphericity, orientation in space) of the developing spur. We applied this method to the study of spur morphogenesis in Staphisagria picta (Ranunculaceae), all tissues (i.e. epidermis, parenchyma, vascular system) taken as a whole. We found that spur development is marked by an early phase of dominant cell proliferation, followed by a phase of dominant anisotropic (directional) cell expansion. The acquisition of the spur’s curvature seems to be achieved by a complex interplay of anisotropic cell elongation and cell division, both mechanisms being unevenly distributed among the different parts of the spur.