Specification and evolution of lateral roots

Plants have evolved a remarkable capacity to develop new organs post-embryonically throughout their lifespan. A prime example of this is root branching. Root branching occurs in two ways: dichotomous and lateral branching. The dichotomous branching is the result of the division of the root apical meristem into two daughter meristems, likely through symmetric cell divisions of the root apical cell, as has recently been illustrated in the extant lycophyte Selaginella moellendorffii (Figure 1). Lateral root branching relies on the de novo specification of a subset of founder cells (hereinafter referred to as lateral root stem cells) located in the internal tissues of an existing root. This step is followed by initiation, in which the specified cells re-enter the cell cycle, and subsequently by primordium formation and emergence. In this primer, we summarize recent advances describing the molecular bases underlying lateral root stem cell specification in angiosperms and highlight the important positional signals that fine tune this process. By delving into the evolutionary origins of root branching, we point out that positional control of lateral root stem cell specification has not been the prevailing mechanism among all plants and discuss the process in ferns (i.e., a sister group of seed plants), where it seems to be under the control of lineage-dependent mechanisms.