The barley Uniculme4 gene encodes a BLADE-ON-PETIOLE-like protein that controls tillering and leaf patterning

Tillers are vegetative branches that develop from axillary buds located in the leaf axils at the base of many grasses. Genetic manipulation of tillering is a major objective in breeding for improved cereal yields and competition with weeds. Despite this, very little is known about the molecular genetic bases of tiller development in important Triticeae crops such as barley (Hordeum vulgare) and wheat (Triticum aestivum). Recessive mutations at the barley Uniculme4 (Cul4) locus cause reduced tillering, deregulation of the number of axillary buds in an axil, and alterations in leaf proximal-distal patterning. We isolated the Cul4 gene by positional cloning and showed it encodes a BTB-ankyrin protein closely related to Arabidopsis BLADE-ON-PETIOLE1 (BOP1) and BOP2. Morphological, histological and in situ RNA expression analyses indicate that Cul4 acts at axil and leaf boundary regions to control axillary bud differentiation, as well as development of the ligule, which separates the distal blade and proximal sheath of the leaf. As the first functionally characterized BOP gene in monocots, Cul4 suggests partial conservation of BOP gene function between dicots and monocots, while phylogenetic analyses highlight distinct evolutionary patterns in the two lineages.