<i>ENHANCED GRAVITROPISM 2</i> encodes a STERILE ALPHA MOTIF–containing protein that controls root growth angle in barley and wheat

Abstract

The root growth angle defines how roots grow toward the gravity vector and is among the most important determinants of root system architecture. It controls water uptake capacity, nutrient use efficiency, stress resilience, and, as a consequence, yield of crop plants. We demonstrated that the <i>egt2</i> (<i>enhanced gravitropism 2</i>) mutant of barley exhibits steeper root growth of seminal and lateral roots and an auxin-independent higher responsiveness to gravity compared to wild-type plants. We cloned the <i>EGT2</i> gene by a combination of bulked-segregant analysis and whole genome sequencing. Subsequent validation experiments by an independent CRISPR/Cas9 mutant allele demonstrated that <i>egt2</i> encodes a STERILE ALPHA MOTIF domain-containing protein. In situ hybridization experiments illustrated that <i>EGT2</i> is expressed from the root cap to the elongation zone. We demonstrated the evolutionary conserved role of <i>EGT2</i> in root growth angle control between barley and wheat by knocking out the <i>EGT2</i> orthologs in the A and B genomes of tetraploid durum wheat. By combining laser capture microdissection with RNA sequencing, we observed that seven expansin genes were transcriptionally down-regulated in the elongation zone. This is consistent with a role of <i>EGT2</i> in this region of the root where the effect of gravity sensing is executed by differential cell elongation. Our findings suggest that <i>EGT2</i> is an evolutionary conserved regulator of root growth angle in barley and wheat that could be a valuable target for root-based crop improvement strategies in cereals.