Evolution of fast root gravitropism in seed plants (Nature Comms)

Plants’ invasion of land was accompanied by the evolution of a root system which in seed plants includes a fast gravitropic response. Gravitropism occurs through three distinct steps – (1) gravity perception by dense starch-filled amyloplasts, (2) transmission of gravitropic signal by auxin through auxin influx (AUX1/LAX) and efflux (PIN) carriers, (3) gravity-stimulated root growth response, which includes root bending through growth promotion and inhibition. In this study about the origin of fast root gravitropism, Zhang et al. demonstrated that gravity sensing amyloplasts are not present in basal land plants, but only in the root apex in the seed plants. Furthermore, only PIN2 can mediate fast root gravitropism in Arabidopsis, and PIN2 proteins are exclusively present in the flowering plants. Additionally, the authors found that efficient shootward auxin transport mediated by functional PIN for fast root gravitropism appeared first in the seed plants. These findings demonstrate that alongside new reproductive strategies, anatomical innovations in roots helped seed plants become the dominant group of vascular plants in most habitats. (Summary by Arif Ashraf) Nature Comms 10.1038/s41467-019-11471-8