Figure 6. Alternative models for the evolutionary history of SAR in plants.

Given the previous reports of SAR in Angiosperms and Gymnosperms, and the current report of an SAR-like mechanism in Bryophytes, three alternate models for the evolutionary history of SAR exist. In one model, SAR predates the divergence of vascular and non-vascular plants (point A in the figure). In this model, the SAR mechanism in all plants should be highly conserved unless SAR mechanisms became divergent, and more specialized for vascular tissue, subsequent to the evolution of vascular plants. Alternately, SAR arose independently in non-vascular plants (point ‘B’ on figure) and in vascular plants (points ‘C’ or ‘D’). In these models, the SAR mechanisms should be less similar between the two groups. Without documentation of SAR in the pteridophytes, it is unknown if SAR in vascular plants arose before pteridophytes diverged from other vascular plants (point ‘C’ on figure) or after this divergence (point ‘D’ on figure).