Barley guanine nucleotide exchange factor Hv GEF14 is an activator of the susceptibility factor Hv RACB and supports host cell entry by Blumeria graminis

Abstract In barley ( Hordeum vulgare ), the function of ROPs appears central to polar cell development and the interaction outcome with parasitic fungi but little is known about ROP activation. Guanine nucleotide exchange factors (GEFs) facilitate the exchange of GDP with GTP and thereby turn ROPs into a signalling-activated ROP-GTP state. Plants possess a unique class of GEFs harbouring a plant specific ROP nucleotide exchanger domain (PRONE). We performed phylogenetic analyses and annotated barley PRONE GEFs. HvGEF14 is expressed in leaf epidermal tissue and downregulated after inoculation with Blumeria graminis f.sp. hordei . Protein-protein interaction assays indicate that Hv GEF14 and the type I barley ROP protein Hv RACB can interact in yeast and in planta . Overexpression of Hv GEF14 further recruited the ROP-GTP downstream interactor Hv RIC171 to the cell periphery and let to interaction of Hv RACB with an extended CRIB (Cdc42/Rac Interactive Binding motif) peptide of Hv RIC171 in a similar manner as constitutively activated Hv RACB. Finally, the over expression of HvGEF14 led to enhanced susceptibility to fungal entry while HvGEF14 RNAi provoked a trend to more penetration resistance. Hv GEF14 might therefore play a role in the activation of Hv RACB in barley epidermal cells, which is crucial for fungal penetration success. Highlight The activated GTPase Hv RACB is a susceptibility factor of barley in response to Blumeria graminis f.sp. hordei . The newly discovered guanine nucleotide exchange factor Hv GEF14 is a Hv RACB activator.