A Post-Haustorial Defense Mechanism is Mediated by the Powdery Mildew Resistance Gene, PmG3M, Derived from Wild Emmer Wheat

Abstract

The destructive wheat powdery mildew disease is caused by the fungal pathogen Blumeria graminis f. sp. tritici (Bgt). PmG3M, derived from wild emmer wheat Triticum dicoccoides accession G305-3M, is a major gene providing a wide-spectrum resistance against Bgt. PmG3M was previously mapped to wheat chromosome 6B using an F6 recombinant inbred line (RIL) mapping population generated by crossing G305-3M with the susceptible T. durum wheat cultivar Langdon (LDN). In the current study, we aimed to explore the defense mechanisms conferred by PmG3M against Bgt. Histopathology of fungal development was characterized in artificially inoculated leaves of G305-3M, LDN, and homozygous RILs using fluorescence and light microscopy. G305-3M exhibited H2O2 accumulation typical of a hypersensitive response, which resulted in programmed cell death (PCD) in Bgt-penetrated epidermal cells, while LDN showed well-developed colonies without PCD. In addition, we observed a post-haustorial resistance mechanism that arrested the development of fungal feeding structures and pathogen growth in both G305-3M and resistant RIL, while LDN and a susceptible RIL displayed fully developed digitated haustoria and massive accumulation of fungal biomass. In contrast, both G305-3M and LDN exhibited callose deposition in attempt to prevent fungal invasion, supporting this as a mechanism of a basal defense response not associated with PmG3M resistance mechanism per se. The presented results shed light on the resistance mechanisms conferred by PmG3M against wheat powdery mildew.

Mining for New Sources of Resistance to Powdery Mildew in Genetic Resources of Winter Wheat

“…However, it has been reported that PmG3M is derived from an T. dicoccoides introgression (Xie et al, 2012 ). It was also recently investigated functionally and shown to provide a post-haustorial defense mechanism (Wei et al, 2020 ). The other reported genes for Chr6B tend to map to the short arm of Chr6B ( Pm11, Pm12, Pm20 , and Pm07j126 ) or in the case of Pm27 143,716,930 bp downstream of MTA37.…”

The Wild Emmer Wheat Gene PmG16, Conferring Resistance to Powdery Mildew, is an Orthologue of Pm60 from Triticum Urartu

“…1a-d). The same post-haustorium resistance mechanism has also been found in WEW accession G305-3M which contains PmG3M (Wei et al 2020). Most Pm genes cloned so far are induced by Bgt at different time points from 6 to 48 hpi, such as Pm5e , Pm41 (Li et al 2020), Pm21 (He et al 2018), Pm2 (Chen et al 2019), and Pm24 (Lu et al 2020.…”

“…Moreover, since WEW is an excellent source for other disease resistance genes/alleles, in future breeding program we propose to pyramid several such genes from WEW and transfer them into durum and bread wheat varieties to obtain more durable resistance to different diseases. These genes can include powdery mildew resistance genes PmG16 and PmG3M (Xie et al 2012;Wei et al 2020), and the broad spectrum yellow rust resistance genes from WEW, such as Yr15 (Klymiuk et al 2018) andYr36 (Fu et al 2009). Altogether, the results of our study demonstrate that WEW is a valuable genetic resource for improving disease resistance in wheat cultivars.…”

“…Bgt #15 structures were observed in the inoculated leaves under a uorescence microscope as described by Klymiuk et al (2018) and Wei et al (2020). Leaf segments were incubated at 37°C for 24 hours in 15 ml of 1 M KOH, neutralized with 50 mM Tris at pH 7.5, and stained with 1.0 ml of 20 µg/ml wheat germ agglutinin (WGA) in 50 mM Tris for 24 hours at 4°C.…”

Long-read genome sequencing accelerated the cloning ofPm69by resolving the complexity of a rapidly evolving resistance gene cluster in wheat

“…Pm69 (PmG3M ), identified from WEW accession G305-3M, is a dominant gene conferring a wide-spectrum resistance to Bgt isolates from around the globe 38 . Pm69 is the only gene derived from WEW that was mapped to the telomeric region of chromosome arm 6BL 36,38,39 .…”