The Ambivalence of the Barley Mlo Locus: Mutations Conferring Resistance Against Powdery Mildew (Blumeria graminis f. sp. hordei) Enhance Susceptibility to the Rice Blast Fungus Magnaporthe grisea

Abstract

Recessive alleles of the barley Mlo locus confer non-race-specific resistance against the powdery mildew fungus Blumeria graminis f. sp. hordei (Bgh). Recently the Mlo gene has been isolated and it was suggested that the Mlo product is a negative regulator of cell death. Thus, loss of function can precondition cells to a higher responsiveness for the onset of multiple defense functions. Here, we document an enhanced susceptibility of barley mlo mutants to the rice blast fungus Magnaporthe grisea. The disease phenotype is independent of the barley cultivar in which the mlo allele has been introgressed and occurs in equal amounts in barley backcross lines of cv. Ingrid carrying the mlo-1, mlo-3, or mlo-5 allele. Ror genes, which are required for the full expression of mlo resistance in barley against Bgh, do not affect the specific mlo-mediated phenotype observed after M. grisea infection. Formation of an effective papilla restricts blast development in epidermal cells of Mlo plants. In contrast, papillae are mostly penetrated in mlo mutants and, as a consequence, the fungus spreads into adjacent mesophyll cells. Both wild-type plants and mlo mutants did not differ in perception of a purified elicitor derived from M. grisea. Thus, we hypothesize that a functional Mlo protein is a prerequisite for penetration resistance of barley to fungal pathogens like M. grisea. The benefit of mlo alleles for durable resistance in barley and a proposed role of mlo-type-mutations in rice are discussed.

A regulon conserved in monocot and dicot plants defines a functional module in antifungal plant immunity

“…Both Arabidopsis mlo2 and barley mlo plants exhibit enhanced disease symptoms toward necrotrophic and hemibiotrophic pathogens (7,16,17), and a number of the coexpressed genes also have a known role in defense against these classes of pathogens (Dataset S6). Therefore, we tested a subset of the insertion lines for their responses to the necrotrophic fungi Alternaria brassicicola and Botrytis cinerea.…”

Barley stripe mosaic virus-induced gene silencing (BSMV-IGS) as a tool for functional analysis of barley genes potentially involved in nonhost resistance

“…Barley plants carrying a loss of function mutation at the Mlo locus are completely resistant to all known isolates of Bgh but behave hypersusceptible to M. oryzae and Bipolaris sorokiniana. [14][15][16] It has already been shown that silencing of Mlo using transient-induced gene silencing based on biolistic transgene delivery phenocopied the resistance of mlo-mutant plants against Bgh. 17 But to our knowledge this approach hasn't been addressed for barley using VIGS so far.…”

Insights into Nonhost Disease Resistance: Can They Assist Disease Control in Agriculture?

“…Recessive mutants of Mlo in both barley and Arabidopsis are resistant to adapted mildew species (Panstruga, 2005), which indicates that this form of resistance cannot be negated by adapted pathogen effectors. However, whereas mlo mutations are ineffective against rusts (Jorgensen, 1992) and mlo mutant barley is more susceptible than the wild type to the rice blast pathogen M. grisea (Jarosch et al, 1999), the PEN1 pathway in Arabidopsis appears to be specific against mildew species (Shimada et al, 2006). A challenge for molecular genetics will be to identify genes analogous to Mlo for combating these pathogens through activation of prehaustorial defenses.…”