Root angle is controlled by<i>EGT1</i>in cereal crops employing an antigravitropic mechanism

Abstract

Root angle in crops represents a key trait for efficient capture of soil resources. Root angle is determined by competing gravitropic versus antigravitropic offset (AGO) mechanisms. Here we report a root angle regulatory gene termed <i>ENHANCED GRAVITROPISM1</i> (<i>EGT1</i>) that encodes a putative AGO component, whose loss-of-function enhances root gravitropism. Mutations in barley and wheat <i>EGT1</i> genes confer a striking root phenotype, where every root class adopts a steeper growth angle. <i>EGT1</i> encodes an F-box and Tubby domain-containing protein that is highly conserved across plant species. Haplotype analysis found that natural allelic variation at the barley <i>EGT1</i> locus impacts root angle. Gravitropic assays indicated that <i>Hvegt1</i> roots bend more rapidly than wild-type. Transcript profiling revealed <i>Hvegt1</i> roots deregulate reactive oxygen species (ROS) homeostasis and cell wall-loosening enzymes and cofactors. ROS imaging shows that <i>Hvegt1</i> root basal meristem and elongation zone tissues have reduced levels. Atomic force microscopy measurements detected elongating <i>Hvegt1</i> root cortical cell walls are significantly less stiff than wild-type. In situ analysis identified <i>HvEGT1</i> is expressed in elongating cortical and stele tissues, which are distinct from known root gravitropic perception and response tissues in the columella and epidermis, respectively. We propose that EGT1 controls root angle by regulating cell wall stiffness in elongating root cortical tissue, counteracting the gravitropic machinery's known ability to bend the root via its outermost tissues. We conclude that root angle is controlled by <i>EGT1</i> in cereal crops employing an antigravitropic mechanism.