RACE1, a Japanese Blumeria graminis f. sp. hordei isolate, is capable of overcoming partially mlo-mediated penetration resistance in barley in an allele-specific manner

Abstract

Loss-of-function mutation of the MILDEW RESISTANCE LOCUS O (Mlo) gene confers durable and broad-spectrum resistance to powdery mildew fungi in various plants, including barley. In combination with the intracellular nucleotide-binding domain and leucine-rich repeat receptor (NLR) genes, which confer the race-specific resistance, the mlo alleles have long been used in barley breeding as genetic resources that confer robust non-race-specific resistance. However, a Japanese Blumeria graminis f. sp. hordei isolate, RACE1, has been reported to have the potential to overcome partially the mlo-mediated penetration resistance, although this is yet uncertain because the putative effects of NLR genes in the tested accessions have not been ruled out. In this study, we examined the reproducibility of the earlier report and found that the infectious ability of RACE1, which partially overcomes the mlo-mediated resistance, is only exerted in the absence of NLR genes recognizing RACE1. Furthermore, using the transient-induced gene silencing technique, we demonstrated that RACE1 can partially overcome the resistance in the host cells with suppressed MLO expression but not in plants possessing the null mutant allele mlo-5.

Long‐term and rapid evolution in powdery mildew fungi

“…The isolate B . hordei RACE1 emerged naturally in Japan and exhibits reproducible, albeit weak, virulence on barley mlo plants (Lyngkjær et al, 1995). RACE1 overcomes mlo ‐based resistance in cultivars carrying alleles with reduced Mlo expression levels but not in null alleles of Mlo (Yaeno et al, 2021).…”

“…This conundrum may be explained by the fact that farmers only deploy mlo ‐resistant spring barley but not winter barley varieties (Czembor, 2016; Jørgensen, 1992). Further, so far, all mlo ‐virulent isolates are only capable of partial virulence on mlo ‐resistant barley (Kusch et al, 2021; Schwarzbach, 1979; Yaeno et al, 2021), and this capacity may be an adaptation with a trade‐off affecting overall fitness even on susceptible Mlo wild‐type barley cultivars (Kusch et al, 2021).…”

A fungal plant pathogen overcomesmlo-mediated broad-spectrum disease resistance by rapid gene loss

“…Ingrid (BCI) mutant plants (Figure 1A and 1B). B. hordei SK1 exhibited a similar level of virulence on a range of near-isogenic BCI mlo lines with various mutational defects in Mlo [75], indicating that in contrast to B. hordei RACE1 [95] the enhanced virulence of this strain is independent of the host mlo allele (Figure 1C and Supplementary Figure 1). We found comparable host cell entry levels on two mlo mutants in different barley lines, Pallas mlo -5 (approx.…”