Differential effects of lesion mimic mutants in barley on disease development by facultative pathogens

Lesion mimic mutants display spontaneous necrotic spots and chlorotic leaves as a result of mis-regulated cell death programmes. Typically these mutants have increased resistance to biotrophic pathogens but their response to facultative fungi that cause necrotrophic diseases is less well studied. The effect of altered cell death regulation on the development of disease caused by Ramularia collo-cygni, Fusarium culmorum and Oculimacula yallundae was explored using a collection of barley necrotic (nec) lesion mimic mutants. nec8 mutants displayed lower levels of all three diseases compared to nec9 mutants, which had increased R. collo-cygni but decreased F. culmorum disease symptoms. nec1 mutants reduced disease development caused by both R. collo-cygni and F. culmorum. The severity of the nec1-induced lesion mimic phenotype and F. culmorum symptom development was reduced by mutation of the negative cell death regulator MLO. The significant reduction in R. collo-cygni symptoms caused by nec1 was completely abolished in the presence of the mlo-5 allele and both symptoms and fungal biomass were greater than in the wild-type. These results indicate that physiological pathways involved in regulation of cell death interact with one another in their effects on different fungal pathogens

B<i>rachypodium distachyon</i> exhibits compatible interactions with <i>Oculimacula </i>spp. and <i>Ramularia collo-cygni</i>, providing the first pathosystem model to study eyespot and ramularia leaf spot diseases

Brachypodium distachyon (Bd) has established itself as an essential tool for comparative genomic studies in cereals and increasing attention is being paid to its potential as a model pathosystem. Eyespot and ramularia leaf spot (RLS) are important diseases of wheat, barley and other small-grain cereals for which very little is known about the mechanisms of host resistance despite urgent requirements for plant breeders to develop resistant varieties. This work aimed to test the compatibility of interaction of two Bd accessions with the cereal pathogens Oculimacula spp. and Ramularia collocygni, the causal agents of eyespot and RLS diseases, respectively. Results showed that both Bd accessions developed symptoms similar to those on the natural host for all pathogen species tested. Microscopy images demonstrated that R. collo-cygni produced secondary conidia and both Oculimacula spp. formed characteristic infection structures on successive tissue layers. Visual disease assessment revealed that quantitative differences in disease severity exist between the two Bd accessions. The results presented here provide the first evidence that Bd is compatible with the main causal agents of eyespot and RLS diseases, and suggest that future functional genetic studies can be undertaken to investigate the mechanisms of eyespot and RLS disease resistance using Bd