Diversifying the menu for crop powdery mildew resistance

Powdery mildew, a potentially severe crop disease, can be controlled by mlo mutations, which suppress fungal proliferation but typically also reduce yield. Li et al., 2022Li S. Lin D. Zhang Y. Deng M. Chen Y. Lv B. Li B. Lei Y. Wang Y. Zhao L. et al.Genome-edited powdery mildew resistance in wheat without growth penalties.Nature. 2022; (Published online February 9, 2022)https://doi.org/10.1038/s41586-022-04395-9Crossref Scopus (125) Google Scholar demonstrate that productivity can be restored by overexpressing a host sugar transporter, thus offering a new option for economically and environmentally benign disease control. Powdery mildew, a potentially severe crop disease, can be controlled by mlo mutations, which suppress fungal proliferation but typically also reduce yield. Li et al., 2022Li S. Lin D. Zhang Y. Deng M. Chen Y. Lv B. Li B. Lei Y. Wang Y. Zhao L. et al.Genome-edited powdery mildew resistance in wheat without growth penalties.Nature. 2022; (Published online February 9, 2022)https://doi.org/10.1038/s41586-022-04395-9Crossref Scopus (125) Google Scholar demonstrate that productivity can be restored by overexpressing a host sugar transporter, thus offering a new option for economically and environmentally benign disease control. The fungal disease powdery mildew is a threat to many crops in temperate climates. In barley, mutations of Mildew resistance locus O (Mlo), discovered in 1942, give near-total resistance. This gene is unusual because resistance is conferred by recessive loss-of-function (mlo) alleles and because it has been durable, remaining effective against all pathogen races for 8 decades. By contrast, most other major genes controlling disease resistance in plants are dominant and are not durable because they are ineffective against specifically virulent pathogen genotypes. MLO is a plasma membrane-localized protein with seven transmembrane domains (Büschges et al., 1997Büschges R. Hollricher K. Panstruga R. Simons G. Wolter M. Frijters A. van Daelen R. van der Lee T. Diergaarde P. Groenendijk J. et al.The barley Mlo gene: a novel control element of plant pathogen resistance.Cell. 1997; 88: 695-705https://doi.org/10.1016/S0092-8674(00)81912-1Abstract Full Text Full Text PDF PubMed Scopus (901) Google Scholar). Perhaps remarkably, the mechanism of mlo-mediated resistance is still unknown, but the finding that Mlo is largely conserved across the plant kingdom has paved the way for inducing resistance by knocking it out in diverse crops including tomato, pea, cucumber, and wheat as well as the model plant Arabidopsis (Kusch and Panstruga, 2017Kusch S. Panstruga R. mlo-based resistance: an apparently universal “weapon” to defeat powdery mildew disease.Mol. Plant Microbe Interact. 2017; 30: 179-189https://doi.org/10.1094/MPMI-12-16-0255-CRCrossref PubMed Scopus (167) Google Scholar). Most artificial mlo mutations are associated with reduced yield, but barley breeders can mitigate the yield penalty through reassortment of the genetic background (Kjær et al., 1990Kjær B. Jensen H.P. Jensen J. Jørgensen J.H. Associations between three ml-o powdery mildew resistance genes and agronomic traits in barley.Euphytica. 1990; 46: 185-193https://doi.org/10.1007/BF00027217Crossref Scopus (60) Google Scholar). Moreover, although the natural allele mlo11 has a slightly weaker effect than artificial mutations, it nonetheless suppresses mildew almost completely and has a smaller yield penalty (Piffanelli et al., 2004Piffanelli P. Ramsay L. Waugh R. Benabdelmouna A. D’Hont A. Hollricher K. Jørgensen J.H. Schulze-Lefert P. Panstruga R. A barley cultivation-associated polymorphism conveys resistance to powdery mildew.Nature. 2004; 430: 887-891https://doi.org/10.1038/nature02781Crossref PubMed Scopus (157) Google Scholar). Li et al., 2022Li S. Lin D. Zhang Y. Deng M. Chen Y. Lv B. Li B. Lei Y. Wang Y. Zhao L. et al.Genome-edited powdery mildew resistance in wheat without growth penalties.Nature. 2022; (Published online February 9, 2022)https://doi.org/10.1038/s41586-022-04395-9Crossref Scopus (125) Google Scholar describe a new mlo allele, Tamlo-R32, which restores plant growth and yield in hexaploid bread wheat while maintaining mildew resistance. This allele, generated by CRISPR-Cas9 mutagenesis, is associated with a 304 kb in-frame deletion incorporating part of the Mlo B-genome homeolog and a paralog of it. An altered local chromatin landscape leads to upregulated expression of the adjacent Tonoplast Monosaccharide Transporter 3 (TMT3) gene. Li et al. demonstrate that ectopic expression of TMT3 restores plant vigor in the Tamlo-R32 line. The trait can be reconstituted by inducing the same deletion in elite wheat backgrounds, while in the phylogenetically distant species Arabidopsis, overexpression of TMT3 in an mlo background complemented growth defects. This fortuitous discovery offers an exciting option for achieving disease resistance apparently without a yield penalty, but its application to plant breeding faces several challenges. The first is that it is notoriously difficult to measure small changes in fitness experimentally, including yield penalties in crops. Moreover, varieties’ yield is greatly affected by environmental factors such as weather, soil, and crop management. Li et al., 2022Li S. Lin D. Zhang Y. Deng M. Chen Y. Lv B. Li B. Lei Y. Wang Y. Zhao L. et al.Genome-edited powdery mildew resistance in wheat without growth penalties.Nature. 2022; (Published online February 9, 2022)https://doi.org/10.1038/s41586-022-04395-9Crossref Scopus (125) Google Scholar found no yield penalty at two sites near Beijing, but breeders will want to test whether this holds elsewhere. In addition, there are “unknown unknowns”: undesirable pleiotropic effects that are not detected until the gene is trialed in diverse genetic backgrounds and a range of environments. mlo alleles in barley are associated with reduced mycorrhization (Jacott et al., 2020Jacott C.N. Charpentier M. Murray J.D. Ridout C.J. Mildew Locus O facilitates colonization by arbuscular mycorrhizal fungi in angiosperms.New Phytol. 2020; 227: 343-351https://doi.org/10.1111/nph.16465Crossref PubMed Scopus (13) Google Scholar) and greater susceptibility to necrotrophic pathogens (Kusch and Panstruga, 2017Kusch S. Panstruga R. mlo-based resistance: an apparently universal “weapon” to defeat powdery mildew disease.Mol. Plant Microbe Interact. 2017; 30: 179-189https://doi.org/10.1094/MPMI-12-16-0255-CRCrossref PubMed Scopus (167) Google Scholar). The possibility that the mlo allele in Tamlo-R32 might have similar costs cannot be excluded, but the widespread use of mlo11 in barley breeding shows that the benefits of strong mlo-based resistance outweigh any small penalties (Brown, 2021Brown J.K.M. Achievements in breeding cereals with durable disease resistance in Northwest Europe.in: Oliver R. Achieving Durable Disease Resistance in Cereals. Burleigh Dodds Science Publishing, 2021: 825-871https://doi.org/10.19103/AS.2021.0092.39Crossref Google Scholar). Successful plant breeding must advance all commercially important traits. Although Li et al.’s method of achieving resistance without a significant yield penalty is novel, breeders have taken a different approach to combining these traits in popular cereal varieties. In contrast to mlo11 in spring barley, effective, durable mildew resistance in many UK winter wheat and spring oat varieties is polygenic and quantitative, with few varieties being completely resistant (Figure 1). This resistance has no detectable yield penalty, which has most likely been achieved because, by selection to improve both these traits (and many others), breeders have accumulated minor genes that have a benefit in terms of disease resistance that outweigh any yield cost (Brown, 2021Brown J.K.M. Achievements in breeding cereals with durable disease resistance in Northwest Europe.in: Oliver R. Achieving Durable Disease Resistance in Cereals. Burleigh Dodds Science Publishing, 2021: 825-871https://doi.org/10.19103/AS.2021.0092.39Crossref Google Scholar). This happens by the accelerated form of Darwinian evolution on which plant breeding is based, even when the breeder does not know which genes are involved. The use of durable resistance, both mlo11 in barley and quantitative resistance in wheat and oats, has greatly reduced the significance of mildew in UK cereals over the past 40 years. Although there is significant quantitative resistance in local varieties (Yu et al., 2001Yu D.Z. Yang X.J. Yang L.J. Jeger M.J. Brown J.K.M. Assessment of partial resistance to powdery mildew in Chinese wheat varieties.Plant Breed. 2001; 120: 279-284https://doi.org/10.1046/j.1439-0523.2001.00592.xCrossref Scopus (29) Google Scholar), wheat mildew remains problematic in China. The combination of mlo mutations with upregulated TMT3 as in the Tamlo-R32 line (Li et al., 2022Li S. Lin D. Zhang Y. Deng M. Chen Y. Lv B. Li B. Lei Y. Wang Y. Zhao L. et al.Genome-edited powdery mildew resistance in wheat without growth penalties.Nature. 2022; (Published online February 9, 2022)https://doi.org/10.1038/s41586-022-04395-9Crossref Scopus (125) Google Scholar) may be a more rapid solution than breeding for quantitative resistance, depending in part on whether or not it does indeed have a fitness cost. Even if it does have a yield penalty, it might be mitigated by reassorting the genetic background through the normal process of plant breeding, as with mlo in barley (Kjær et al., 1990Kjær B. Jensen H.P. Jensen J. Jørgensen J.H. Associations between three ml-o powdery mildew resistance genes and agronomic traits in barley.Euphytica. 1990; 46: 185-193https://doi.org/10.1007/BF00027217Crossref Scopus (60) Google Scholar). It would be interesting to investigate whether mitigation of the yield penalty in barley involves modification of sugar transport, as suggested by TMT3 overexpression in wheat (Li et al., 2022Li S. Lin D. Zhang Y. Deng M. Chen Y. Lv B. Li B. Lei Y. Wang Y. Zhao L. et al.Genome-edited powdery mildew resistance in wheat without growth penalties.Nature. 2022; (Published online February 9, 2022)https://doi.org/10.1038/s41586-022-04395-9Crossref Scopus (125) Google Scholar). While there are other routes to achieving mildew resistance without a yield penalty in cereals, combining mlo with TMT3 overexpression may be particularly useful in crops where specific varieties are exceptionally important or breeding is especially complex (Myles, 2013Myles S. Improving fruit and wine: what does genomics have to offer?.Trends Genet. 2013; 29: 190-196https://doi.org/10.1016/j.tig.2013.01.006Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar). Many classic wine-making grapevines are very susceptible to mildew and thus require extensive fungicide treatments, but as they have been propagated clonally for hundreds of years, breeding to improve resistance while retaining their complex of other desirable properties is near impossible. Here, genome engineering offers the prospect of durable mildew control with less fungicide use. Another potential target is mildew resistance in top fruit. This is desirable in apples, pears, plums, cherries, and related species, but fruit quality for storage, transport, and eating are the leading requirements. As breeding fruit trees is slower than cereals, it takes longer to advance traits by accumulating minor genes. Gene editing may provide a shortcut to mildew resistance, allowing breeders to focus on other important traits. Relaxation of regulations in countries such as the United States, Canada, Brazil, Argentina, China, and Australia offers scope for innovative applications of gene editing for crop improvement, including disease control. J.K.M.B. is funded by the BBSRC Plant Health Strategic Programme. B.B.H.W. is funded by the King Abdullah University of Science and Technology. The authors declare no competing interests.