The auxin efflux carrier PIN1a regulates vascular patterning in cereal roots

Summary Barley ( Hordeum vulgare ) is an important global cereal crop and a model in genetic studies. Despite advances in characterising barley genomic resources, few mutant studies have identified genes controlling root architecture and anatomy, which plays a critical role in capturing soil resources. Our phenotypic screening of a TILLING mutant collection identified line TM5992 exhibiting a short‐root phenotype compared with wild‐type (WT) Morex background. Outcrossing TM5992 with barley variety Proctor and subsequent SNP array‐based bulk segregant analysis, fine mapped the mutation to a cM scale. Exome sequencing pinpointed a mutation in the candidate gene HvPIN1a , further confirming this by analysing independent mutant alleles. Detailed analysis of root growth and anatomy in Hvpin1a mutant alleles exhibited a slower growth rate, shorter apical meristem and striking vascular patterning defects compared to WT. Expression and mutant analyses of PIN1 members in the closely related cereal brachypodium ( Brachypodium distachyon ) revealed that BdPIN1a and BdPIN1b were redundantly expressed in root vascular tissues but only Bdpin1a mutant allele displayed root vascular defects similar to Hvpin1a . We conclude that barley PIN1 genes have sub‐functionalised in cereals, compared to Arabidopsis ( Arabidopsis thaliana ), where PIN1a sequences control root vascular patterning.