The PIN2 ortholog in barley modifies root gravitropism and architecture

Roots provide the critical interface where plants acquire nutrients and water, but our limited understanding of the genetic controls modulating root system architecture (RSA) in crop species constrains opportunities to develop future cultivars with improved root systems. However, there is vast knowledge of root developmental genes in model plant species, which has the potential to accelerate progress in crops with more complex genomes, particularly given that genome editing protocols are now available for most species. PIN-FORMED2 (PIN2) encodes a root-specific polar auxin transporter, where its absence resulted in roots being unable to orient themselves using gravity, producing a significantly wider root system. To explore the role of PIN2 in a cereal crop, we used CRISPR/Cas9 (where CRISPR is clustered regularly interspaced short palindromic repeats) editing to knockout of PIN2 in barley (Hordeum vulgare). Like Arabidopsis, the roots of barley pin2 loss-of-function mutants displayed an agravitropic response at seedling growth stages, resulting in a significantly shallower and wider root system at later growth stages. Notably, despite the significant change in RSA, there was no change in shoot architecture or total shoot biomass, with an insensitivity to the effects of higher planting density. We discuss the future challenges and opportunities to harness the PIN2 pathway to optimize RSA in crops for a range of production scenarios without a shoot trade-off.