Barley guanine nucleotide exchange factorHvGEF14 is an activator of the susceptibility factorHvRACB and supports host cell entry byBlumeria graminisf.sp.hordei

AbstractIn 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.HvGEF14is expressed in leaf epidermal tissue and downregulated after inoculation withBlumeria graminisf.sp.hordei. Protein-protein interaction assays indicate thatHvGEF14 and the type I barley ROP proteinHvRACB can interact in yeast andin planta. Overexpression ofHvGEF14 further recruited the ROP-GTP downstream interactorHvRIC171 to the cell periphery and let to interaction ofHvRACB with an extended CRIB (Cdc42/Rac Interactive Binding motif) peptide ofHvRIC171 in a similar manner as constitutively activatedHvRACB. Finally, the over expression ofHvGEF14led to enhanced susceptibility to fungal entry whileHvGEF14RNAi provoked a trend to more penetration resistance.HvGEF14 might therefore play a role in the activation ofHvRACB in barley epidermal cells, which is crucial for fungal penetration success.HighlightThe activated GTPaseHvRACB is a susceptibility factor of barley in response toBlumeria graminisf.sp.hordei. The newly discovered guanine nucleotide exchange factorHvGEF14 is aHvRACB activator.