The role of barley cell wall polysaccharides in host plant defence mechanisms against powdery mildew

Abstract

• The cell wall is the first line of plant defence, presenting a barrier that protects cells from infectious pathogens in the surrounding environment. Plants respond dynamically to pathogen attack at the cell wall level by developing papillae at the infection site. In many plant/pathogen interactions, papillae formation is an important determinant of pre-invasion resistance by the host species. While many aspects of papillae are known, the cell wall components responsible for making papillae an effective barrier to fungal penetration are not fully understood. This project aimed to define the role of cell wall polysaccharides in the papillae-based penetration resistance mechanism of barley. Using the barley-powdery mildew host-pathogen system, papillae polysaccharide composition and the genetic factors responsible for their biosynthesis were examined. • Here it is demonstrated that the major polysaccharides found in barley papillae are callose, arabinoxylan and cellulose. The papillae are layered with an inner core consisting of callose and arabinoxylan and an outer layer containing arabinoxylan and cellulose. A higher level of polysaccharide staining at non-penetrated papillae compared to the penetrated papillae was observed and this suggested that the polysaccharides are necessary components of the papillae-based penetration resistance mechanism of host plants. • The members of the Glucan synthase-like (Gsl)gene family found in the barley genome have been characterised and identified and when HvGsl6 was silenced this resulted in a loss of callose in the papillae and an increased rate of successful fungal penetration. • A number of candidate genes from several glycosyltransferase families suspected to be associated with the biosynthesis of arabinoxylan in papillae have been identified. Transient down-regulation and up-regulation of the individual candidate genes using a biolistic DNA delivery system led to an altered level of susceptibility to powdery mildew. However, the highest levels of resistance were observed when GT43 (MLOC_54026) and the GT47 (MLOC_14407) genes were over-expressed together. These genes are putatively involved in arabinoxylan backbone biosynthesis. • Furthermore, this PhD study also contributed to the characterisation of the role of the HvCslD2 gene in non-host resistance, a project led by Dr. Patrick Schweizer, IPK, Germany. We showed that silencing of the HvCslD2 gene in barley results in reduced cellulose accumulation in the papillae during powdery mildew infection, suggesting that HvCslD2 is a key gene involved in cellulose biosynthesis in papillae. • The association of high levels of arabinoxylan and cellulose deposition in papillae with the penetration resistance mechanism of the host plant, provides new targets for the improvement of papilla composition. The identification of the genes involved in the biosynthesis of each papilla component will provide new targets for the generation of novel crop lines with greater disease resistance.