Blumeria graminis interactions with barley conditioned by different single R genes demonstrate a temporal and spatial relationship between stomatal dysfunction and cell death.

Hypersensitive response (HR) against Blumeria graminis f. sp. hordei infection in barley (Hordeum vulgare) was associated with stomata "lock-up" leading to increased leaf water conductance (g(l)). Unique spatio-temporal patterns of HR formation occurred in barley with Mla1, Mla3, or MlLa R genes challenged with B. graminis f. sp. hordei. With Mla1, a rapid HR, limited to epidermal cells, arrested fungal growth before colonies initiated secondary attacks. With Mla3, mesophyll HR preceded that in epidermal cells whose initial survival supported secondary infections. With MlLa, mesophyll survived and not all attacked epidermal cells died immediately, allowing colony growth and secondary infection until arrested. Isolines with Mla1, Mla3, or MlLa genes inoculated with B. graminis f. sp. hordei ranging from 1 to 100 conidia mm(2) showed abnormally high g(l) during dark periods whose timing and extent correlated with those of each HR. Each isoline showed increased dark g(l) with the nonpathogen B. graminis f. sp. avenae which caused a single epidermal cell HR. Guard cell autofluorescence was seen only after drying of epidermal strips and closure of stomata suggesting that locked open stomata were viable. The data link stomatal lock-up to HR associated cell death and has implications for strategies for selecting disease resistant genotypes.

Pathogen-derived nitric oxide influences formation of the appressorium infection structure in the phytopathogenic fungus Blumeria graminis.

Nitric oxide (NO) is an important signal in plant resistance to pathogens. Here we report that NO is also generated by Blumeria graminis f.sp. hordei as a pathogenesis determinant on barley. Infection by B. graminis f.sp. hordei is dependent on appressorium formation in order to penetrate the host. Using fluorescent dye diaminofluorescein-2 diacetate (DAF-2DA) and confocal laser scanning microscopy, transient NO generation was detected within the B. graminis f.sp. hordei appressorium during its maturation. To confirm that NO was indeed being measured, DAF-2DA fluorescence was suppressed using a NO scavenger and a mammalian NO synthase inhibitor. Both chemicals affected the number of appressorial lobes produced by the fungus. These data indicate that NO plays a key role in formation of B. graminis f.sp. hordei appressoria.

NO way to live; the various roles of nitric oxide in plant-pathogen interactions.

Nitric oxide has attracted considerable interest from plant pathologists due its established role in regulating mammalian anti-microbial defences, particularly via programmed cell death (PCD). Although NO plays a major role in plant PCD elicited in response to certain types of pathogenic challenge, the race-specific hypersensitive response (HR), it is now evident that NO also acts in the regulation of non-specific, papilla-based resistance to penetration by plant cells that survive attack and, possibly, in systemic acquired resistance. Equally, the potential roles of NO signalling/scavenging within the pathogen are being recognized. This review will consider key defensive roles played by NO in living cells during plant-pathogen interactions, as well as in those undergoing PCD.

What do microbes encounter at the plant surface? Chemical composition of pea leaf cuticular waxes.

In the cuticular wax mixtures from leaves of pea (Pisum sativum) cv Avanta, cv Lincoln, and cv Maiperle, more than 70 individual compounds were identified. The adaxial wax was characterized by very high amounts of primary alcohols (71%), while the abaxial wax consisted mainly of alkanes (73%). An aqueous adhesive of gum arabic was employed to selectively sample the epicuticular wax layer on pea leaves and hence to analyze the composition of epicuticular crystals exposed at the outermost surface of leaves. The epicuticular layer was found to contain 74% and 83% of the total wax on adaxial and abaxial surfaces, respectively. The platelet-shaped crystals on the adaxial leaf surface consisted of a mixture dominated by hexacosanol, accompanied by substantial amounts of octacosanol and hentriacontane. In contrast, the ribbon-shaped wax crystals on the abaxial surface consisted mainly of hentriacontane (63%), with approximately 5% each of hexacosanol and octacosanol being present. Based on this detailed chemical analysis of the wax exposed at the leaf surface, their importance for early events in the interaction with host-specific pathogenic fungi can now be evaluated. On adaxial surfaces, approximately 80% of Erysiphe pisi spores germinated and 70% differentiated appressoria. In contrast, significantly lower germination efficiencies (57%) and appressoria formation rates (49%) were found for abaxial surfaces. In conclusion, the influence of the physical structure and the chemical composition of the host surface, and especially of epicuticular leaf waxes, on the prepenetration processes of biotrophic fungi is discussed.