Branching under pressure: Influences of cell wall architecture and biomechanics on lateral root morphogenesis

Plants carry out a unique type of organogenesis in which cells do not move relative to each other but instead maintain their relative positions and grow in concert. The coordinated regulation of cell shape and size is thus essential for organ morphogenesis, but in a few developmental processes, most notably in invasive growth and the establishment of branched tissue architectures, cell and tissue growth in plants involves the displacement of surrounding or overlying tissues. Plant cells accomplish patterned developmental morphogenesis in part due to the mechanically complex architectures of their cell walls, which can anisotropically constrain the expansion that is facilitated in many cases by the cellular uptake of water that results in cell pressurization. Here, we focus on one example of patterned tissue growth and cell displacement, the formation and emergence of lateral roots, as a paradigm for understanding how cell wall architecture and cellular biomechanics influence the differentiation and growth of new organs in plants. We highlight recent advances in our knowledge of how hormone signaling, transcriptional regulation, cytoskeletal dynamics, and cell wall synthesis and remodeling influence lateral root initiation and emergence, and propose hypotheses and potential research directions for future studies of these complex but essential developmental processes.