Fig. 5.

Origin of fast root gravitropism facilitated seed plant adaption to the dry land. A schematic diagram representing an evolution of fast root gravitropism that can be detected only in seed plants. It depicts (i) the root anatomical innovation with evolution of the apex-specific gravity perception that is spatially separated from the elongation zone where differential growth occurs and (ii) the functional innovations of the auxin transporter PIN2 that occurred in two disparate plant evolution stages and in different parts of the PIN2 protein sequence ultimately leading to the shootward subcellular localization of PIN2. This novel PIN2 property endowed roots with the ability to transport auxin shootwards enabling the auxin-based signaling from the place of gravity perception to the zone of growth regulation. The fast root gravitropism has evolved as a result of these concomitant anatomical and functional innovations exclusively in seed plants as one of the important adaptions to dry land