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CURATOR
A pinboard by
Ricky Milne

Postdoctoral Fellow, CSIRO, Australia

PINBOARD SUMMARY

Broad spectrum disease resistance in wheat

Wheat is an important crop worldwide, but many wheat cultivars are susceptible to disease-causing fungal pathogens that are capable of inflicting significant yield losses. Recent epidemics of the fungal disease, stem rust, have overcome resistance genes present in commercial cultivars resulting in major crop losses. Stripe rust disease is also looming as a similar threat with the potential to cause worldwide losses if uncontrolled.

Numerous pathogen resistance genes have been identified in wheat, the majority of which confer resistance to a single type of pathogen, for example stem rust, and fall into a specific class of genes containing conserved domains known as nucleotide binding-leucine rich repeats (NB-LRR). These genes confer strong disease resistance but are prone to being overcome by pathogens, potentially leading to epidemics. A smaller number of resistance genes provide partial resistance to multiple pathogens. Examples of multi-pathogen resistance genes are the ABC transporter, Lr34, and the hexose transporter Lr67, which confer resistance to multiple pathogens in wheat – stem rust, stripe rust, leaf rust and powdery mildew. These genes provide durable resistance which is yet to be overcome. Broad spectrum resistance genes may be used as safeguards to minimise the impact of future pathogen outbreaks.

The broad aim of my work is to understand the underlying mechanisms behind these broad spectrum resistance genes in wheat. It is not well understood how these genes are able to confer resistance to multiple pathogens. However the pathways that they function in are likely to be conserved across many species, since Lr34 has been transferred to rice, maize, sorghum and barley and it is able to confer resistance to pathogens which only infect these species and not wheat. Therefore understanding how these genes function could benefit not only the wheat industry but many other crops which are impacted by fungal diseases.

10 ITEMS PINNED

The wheat Lr34 gene provides resistance against multiple fungal pathogens in barley.

Abstract: The Lr34 gene encodes an ABC transporter and has provided wheat with durable, broad-spectrum resistance against multiple fungal pathogens for over 100 years. Because barley does not have an Lr34 ortholog, we expressed Lr34 in barley to investigate its potential as a broad-spectrum resistance resource in another grass species. We found that introduction of the genomic Lr34 sequence confers resistance against barley leaf rust and barley powdery mildew, two pathogens specific for barley but not virulent on wheat. In addition, the barley lines showed enhanced resistance against wheat stem rust. Transformation with the Lr34 cDNA or the genomic susceptible Lr34 allele did not result in increased resistance. Unlike wheat, where Lr34-conferred resistance is associated with adult plants, the genomic Lr34 transgenic barley lines exhibited multipathogen resistance in seedlings. These transgenic barley lines also developed leaf tip necrosis (LTN) in young seedlings, which correlated with an up-regulation of senescence marker genes and several pathogenesis-related (PR) genes. In wheat, transcriptional expression of Lr34 is highest in adult plants and correlates with increased resistance and LTN affecting the last emerging leaf. The severe phenotype of transgenic Lr34 barley resulted in reduced plant growth and total grain weight. These results demonstrate that Lr34 provides enhanced multipathogen resistance early in barley plant development and implies the conservation of the substrate and mechanism of the LR34 transporter and its molecular action between wheat and barley. With controlled gene expression, the use of Lr34 may be valuable for many cereal breeding programmes, particularly given its proven durability.

Pub.: 29 May '13, Pinned: 19 Apr '18

The wheat durable, multipathogen resistance gene Lr34 confers partial blast resistance in rice

Abstract: The wheat gene Lr34 confers durable and partial field resistance against the obligate biotrophic, pathogenic rust fungi and powdery mildew in adult wheat plants. The resistant Lr34 allele evolved after wheat domestication through two gain‐of‐function mutations in an ATP‐binding cassette transporter gene. An Lr34‐like fungal disease resistance with a similar broad‐spectrum specificity and durability has not been described in other cereals. Here, we transformed the resistant Lr34 allele into the japonica rice cultivar Nipponbare. Transgenic rice plants expressing Lr34 showed increased resistance against multiple isolates of the hemibiotrophic pathogen Magnaporthe oryzae, the causal agent of rice blast disease. Host cell invasion during the biotrophic growth phase of rice blast was delayed in Lr34‐expressing rice plants, resulting in smaller necrotic lesions on leaves. Lines with Lr34 also developed a typical, senescence‐based leaf tip necrosis (LTN) phenotype. Development of LTN during early seedling growth had a negative impact on formation of axillary shoots and spikelets in some transgenic lines. One transgenic line developed LTN only at adult plant stage which was correlated with lower Lr34 expression levels at seedling stage. This line showed normal tiller formation and more importantly, disease resistance in this particular line was not compromised. Interestingly, Lr34 in rice is effective against a hemibiotrophic pathogen with a lifestyle and infection strategy that is different from obligate biotrophic rusts and mildew fungi. Lr34 might therefore be used as a source in rice breeding to improve broad‐spectrum disease resistance against the most devastating fungal disease of rice.

Pub.: 15 Oct '15, Pinned: 19 Apr '18

The durable wheat disease resistance gene Lr34 confers common rust and northern corn leaf blight resistance in maize.

Abstract: Maize (corn) is one of the most widely grown cereal crops globally. Fungal diseases of maize cause significant economic damage by reducing maize yields and by increasing input costs for disease management. The most sustainable control of maize diseases is through the release and planting of maize cultivars with durable disease resistance. The wheat gene Lr34 provides durable and partial field resistance against multiple fungal diseases of wheat, including three wheat rust pathogens and wheat powdery mildew. Because of its unique qualities, Lr34 became a cornerstone in many wheat disease resistance programs. The Lr34 - resistance is encoded by a rare variant of an ATP-binding cassette (ABC) transporter that evolved after wheat domestication. An Lr34 - like disease resistance phenotype has not been reported in other cereal species, including maize. Here, we transformed the Lr34 resistance gene into the maize hybrid Hi-II. Lr34 - expressing maize plants showed increased resistance against the biotrophic fungal disease common rust and the hemi-biotrophic disease northern corn leaf blight. Furthermore, the Lr34 - expressing maize plants developed a late leaf tip necrosis phenotype, without negative impact on plant growth. With this and previous reports it could be shown that Lr34 is effective against various biotrophic and hemi-biotrophic diseases that collectively paracitise all major cereal crop species. This article is protected by copyright. All rights reserved.

Pub.: 14 Oct '16, Pinned: 19 Apr '18

The Lr34 adult plant rust resistance gene provides seedling resistance in durum wheat without senescence.

Abstract: The hexaploid wheat (Triticum aestivum) adult plant resistance gene, Lr34/Yr18/Sr57/Pm38/Ltn1, provides broad spectrum resistance to wheat leaf rust (Lr34), stripe rust (Yr18), stem rust (Sr57) and powdery mildew (Pm38) pathogens, and has remained effective in wheat crops for many decades. The partial resistance provided by this gene is only apparent in adult plants and not effective in field grown seedlings. Lr34 also causes leaf tip necrosis (Ltn1) in mature adult plant leaves when grown under field conditions. This D genome encoded bread wheat gene was transferred to tetraploid durum wheat (T. turgidum) cultivar Stewart by transformation. Transgenic durum lines were produced with elevated gene expression levels when compared with the endogenous hexaploid gene. Unlike nontransgenic hexaploid and durum control lines, these transgenic plants showed robust seedling resistance to pathogens causing wheat leaf rust, stripe rust and powdery mildew disease. The effectiveness of seedling resistance against each pathogen correlated with the level of transgene expression. No evidence of accelerated leaf necrosis or upregulation of senescence gene markers was apparent in these seedlings suggesting senescence is not required for Lr34 resistance, although leaf tip necrosis occurred in mature plant flag leaves. Several abiotic stress response genes were upregulated in these seedling in the absence of rust infection as previously observed in adult plant flag leaves of hexaploid wheat. Increasing day length significantly increased Lr34 seedling resistance. These data demonstrate that expression of a highly durable, broad spectrum adult plant resistance gene can be modified to provide seedling resistance in durum wheat. This article is protected by copyright. All rights reserved.

Pub.: 23 Dec '16, Pinned: 19 Apr '18

The wheat Lr34 multi-pathogen resistance gene confers resistance to anthracnose and rust in sorghum.

Abstract: The ability of the wheat Lr34 multi-pathogen resistance gene (Lr34res) to function across a wide taxonomic boundary was investigated in transgenic Sorghum bicolor. Increased resistance to sorghum rust and anthracnose disease symptoms following infection with the biotrophic pathogen Puccinia purpurea and the hemibiotroph Colletotrichum sublineolum respectively occurred in transgenic plants expressing the Lr34res ABC transporter. Transgenic sorghum lines that highly expressed the wheat Lr34res gene exhibited immunity to sorghum rust compared to the low expressing single copy Lr34res genotype that conferred partial resistance. Pathogen induced pigmentation mediated by flavonoid phytoalexins was evident on transgenic sorghum leaves following P. purpurea infection within 24-72 hours, which paralleled Lr34res gene expression. Elevated expression of flavone synthase II, flavanone 4-reductase and dihydroflavonol reductase genes which control the biosynthesis of flavonoid phytoalexins characterised the highly expressing Lr34res transgenic lines 24 h post inoculation with P. purpurea. Metabolite analysis of mesocotyls infected with C. sublineolum showed increased levels of 3-deoxyanthocyanidin metabolites was associated with Lr34res expression, concomitant with reduced symptoms of anthracnose. This article is protected by copyright. All rights reserved.

Pub.: 17 Mar '17, Pinned: 19 Apr '18