Taking Development to Three Dimensions

Phyllotaxis from a Single Apical Cell

Phyllotaxis, the geometry of leaf arrangement around stems, determines plant architecture. Molecular interactions coordinating the formation of phyllotactic patterns have mainly been studied in multicellular shoot apical meristems of flowering plants. Phyllotaxis evolved independently in the major land plant lineages. In mosses, it arises from a single apical cell, raising the question of how asymmetric divisions of a single-celled meristem create phyllotactic patterns and whether associated genetic processes are shared across lineages. We present an overview of the mechanisms governing shoot apical cell specification and activity in the model moss, Physcomitrium patens, and argue that similar molecular regulatory modules have been deployed repeatedly across evolution to operate at different scales and drive apical function in convergent shoot forms.

Design Principles of Branching Morphogenesis in Filamentous Organisms

The radiation of life on Earth was accompanied by the diversification of multicellular body plans in the eukaryotic kingdoms Animalia, Plantae, Fungi and Chromista. Branching forms are ubiquitous in nature and evolved repeatedly in the above lineages. The developmental and genetic basis of branch formation is well studied in the three-dimensional shoot and root systems of land plants, and in animal organs such as the lung, kidney, mammary gland, vasculature, etc. Notably, recent thought-provoking studies combining experimental analysis and computational modeling of branching patterns in whole animal organs have identified global patterning rules and proposed unifying principles of branching morphogenesis. Filamentous branching forms represent one of the simplest expressions of the multicellular body plan and constitute a key step in the evolution of morphological complexity. Similarities between simple and complex branching forms distantly related in evolution are compelling, raising the question whether shared mechanisms underlie their development. Here, we focus on filamentous branching organisms that represent major study models from three distinct eukaryotic kingdoms, including the moss Physcomitrella patens (Plantae), the brown alga Ectocarpus sp. (Chromista), and the ascomycetes Neurospora crassa and Aspergillus nidulans (Fungi), and bring to light developmental regulatory mechanisms and design principles common to these lineages. Throughout the review we explore how the regulatory mechanisms of branching morphogenesis identified in other models, and in particular animal organs, may inform our thinking on filamentous systems and thereby advance our understanding of the diverse strategies deployed across the eukaryotic tree of life to evolve similar forms.