A NEW HYPERGRAVITROPIC ROOT MUTANT FROM THE BARLEY TILLMORE COLLECTION

Root architectural traits play a critical role in crop adaptation; thus, they have started to be considered in breeding programs aimed at the release of new cultivars with improved soil exploration, water and nutrients absorption, lodging resistance, and yield. One of the most important root architectural traits is root growth angle (RGA), namely the direction of root growth in respect of the gravity vector. RGA potentially affects the volume of the soil explored and root depth. For a given growing root tip at a given time, competing gravitropic versus antigravitropic offset mechanisms act to set RGA. However, only a few genes involved in controlling RGA have been discovered so far. The aim of this work is the characterization of a new hypergravitropic mutant and the identification of the gene or genes that control the phenotype. The TILLMore mutant collection was developed by chemical mutagenesis of the barley cv. Morex and allowed us to identify the first two barley genes controlling RGA, namely Enhanced gravitropism1 (EGT1) and EGT2. By means of a new phenotypic screening of TILLMORE using minirhizotrones (based on CD cases), new root mutant lines were identified, including TM1354 that shows narrower (steeper) seminal and lateral root angle than the wild-type. The phenotypic data of F1 plants derived from cv. Morex (wt) × TM1354 showed a wild-type root angle suggesting that the mutated allele is recessive. Wole genome sequencing (WGS) based on short reads (ILLUMINA) of TM1354, revealed no SNPs in previously identified EGT genes, suggesting that a new gene controls the TM1354 phenotype. Mapping-by-sequencing located the new RGA-controlling locus (named Egt3) on chromosome 6H in a region encompassing 31.5 Mb. Four genes with altered protein sequence were identified, one showing a premature stop codon. Validation of candidate genes is in progress. RNAseq on the root tip and on leaves is also in progress in order to clarify the network of genic functions involved in determining the TM1354 hypergravitropic phenotype.