ENHANCED GRAVITROPISM 2encodes a STERILE ALPHA MOTIVE containing protein that controls root growth angle in barley and wheat

Abstract

The root growth angle defines how roots grow towards 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 egt2 (enhanced gravitropism 2) 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 EGT2 gene by a combination of bulked segregant analysis and whole genome sequencing. Subsequent validation experiments by an independent CRISPR/Cas9 mutant allele demonstrated that egt2 encodes a STERILE ALPHA MOTIF domain containing protein. In situ hybridization experiments illustrated that EGT2 is expressed from the root cap to the elongation zone. Subcellular localization experiments revealed that EGT2 localizes to the nucleus and cytoplasm. We demonstrated the evolutionary conserved role of EGT2 in root growth angle control between barley and wheat by knocking out the EGT2 orthologs in the A and B genomes of tetraploid durum wheat. By combining laser capture microdissection with RNA-seq, we observed that seven expansin genes were transcriptionally downregulated in the elongation zone. This is consistent with a role of EGT2 in this region of the root where the effect of gravity sensing is executed by differential cell elongation. Our findings suggest that EGT2 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.Significance StatementTo date the potential of utilizing root traits in plant breeding remains largely untapped. In this study we cloned and characterized the ENHANCED GRAVITROPISM2 (EGT2) gene of barley that encodes a STERILE ALPHA MOTIF domain containing protein. We demonstrated that EGT2 is a key gene of root growth angle regulation in response to gravity which is conserved in barley and wheat and could be a promising target for crop improvement in cereals.

Wheat root systems as a breeding target for climate resilience

“…That such a well-studied gene has only recently been shown to be involved in root system architecture perhaps highlights the comparative lack of research resources wheat root traits have previously been afforded. More recently, the barley root angle mutant enhanced gravitropism 2 (egt2) encoding a STERILE ALPHA MOTIF domain-containing protein has been cloned, which results in narrower and lateral root growth angle compared to wild-type, combined with higher response to gravity in an auxin-independent manner (Kirschner et al 2021). Combining mutations in the A and B genome copies of the orthologous gene in tetraploid wheat resulted in narrower seminal root growth angle, indicating EGT2 could be a target for root-based cereal crop improvement (Kirschner et al 2021).…”

QTL mapping reveals malt barley quality improvement in two dryland environments associated with extended grain fill and seminal root traits

“…Of course, the one-leaf stage is distant from reproductive and maturity stages, and connections drawn between seedling assay and field traits are speculative. However, correlations and colocations between seminal root traits and agronomic traits have been presented before (Ali et al, 2015;Robinson et al, 2018) and have laid the groundwork for subsequent research that has identified causal genes behind the connections (Feng et al, 2022;Kirschner et al, 2021). More research is required to discover how seminal root trait loci might associate with root phenotypes throughout development in the field and how such phenotypes could impact crop performance and environmental adaptation.…”

Genetic analyses of root traits: Implications for environmental adaptation and new variety development: A review

“…In wheat and barley, the gene conditioning enhanced gravitropism 2 ( egt2 ) was identified. The egt2 gene conditions root angle growth in response to gravity (Kirschner et al, 2021). Root elongation in soybean was controlled by the root‐specific expansin gene ( GmEXP1 ), which was located on chromosome 17 (Lee et al, 2003; Lo et al, 2015).…”