In plants, the cell walls are one of the first lines of defence: they protect the cell from successful invasion and resist fungal pathogens. To defend themselves, plants reinforce the cell wall near the site of fungal penetration by producing a dome-shaped thickening of the cell wall, called a papilla, between the epidermal wall and the plasma membrane. If the fungus is successful in penetrating the papillae then it forms a feeding structure (called a haustorium), through which it can extract the necessary nutrients for further growth.
The image on the cover of Issue 212:2 of New Phytologist is a direct view of a barley leaf epidermal cell, infected with the powdery mildew causal agent, Blumeria f. sp. graminis (false coloured: barley cell wall, blue; papillae, red; fungal haustorium, purple).
Papillae that have been effective in preventing penetration by Blumeria graminis f. sp. hordei (Bgh) are traditionally believed to contain callose as the main polysaccharide component. However, recent evidence presented by Alan Little’s group demonstrated that papillae that are successful in preventing the penetration attempts of Bgh contain significantly higher concentrations of callose, arabinoxylan and cellulose.
In two companion papers published in Issue 212:2, Alan Little and colleagues identified the key genes responsible for the biosynthesis of callose and cellulose in the barley papillae. By modulating these key genes, the researchers were able to modify the composition of the papillae, and affect their ability to block fungal penetration attempts. Knowing which genes are involved in the biosynthesis of each papilla component will help to create novel crop lines with greater disease resistance.
To capture the image, the researchers used a scanning electron microscope. In order to view the fungal structures on the inside surface of the epidermal cell, they needed to split the cell layer in half. This was achieved by critical point drying the plant tissue and placing the single cell epidermal layer between two adhesive tabs. The critical point drying of the tissue made the cell wall fragile enough to allow the cell to be split in two when the adhesive tabs were quickly pulled apart.
Read the two companion papers below:
Chowdhury, J. et al. (2016) Down-regulation of the glucan synthase-like 6 gene (HvGsl6) in barley leads to decreased callose accumulation and increased cell wall penetration by Blumeria graminis f. sp. hordei. New Phytologist 212:2, 434-443
Douchkov, D. et al. (2016) The barley (Hordeum vulgare) cellulose synthase-like D2 gene (HvCslD2) mediates penetration resistance to host-adapted and nonhost isolates of the powdery mildew fungus. New Phytologist 212:2, 421-433
Mike Whitfield (@mgwhitfield)