Bettina Kessel

The maize disease resistance gene Htn1 against northern corn leaf blight encodes a wall-associated receptor-like kinase

Significance Northern corn leaf blight (NCLB) is one of the most devastating fungal diseases of maize. The Htn1 disease resistance gene confers quantitative field resistance against most NCLB isolates. Here we show that Htn1 encodes a putative wall-associated receptor-like kinase (RLK). RLKs act as important components of the first tier of the plant innate immune system by perceiving pathogen- or host-derived elicitors on the cell surface. RLKs are often associated with resistance to nonadapted pathogens and are a component of nonhost resistance. Our work demonstrates that the Htn1-RLK plays an important role in host resistance against adapted fungal pathogens. Northern corn leaf blight (NCLB) caused by the hemibiotrophic fungus Exserohilum turcicum is an important foliar disease of maize that is mainly controlled by growing resistant maize cultivars. The Htn1 locus confers quantitative and partial NCLB resistance by delaying the onset of lesion formation. Htn1 represents an important source of genetic resistance that was originally introduced from a Mexican landrace into modern maize breeding lines in the 1970s. Using a high-resolution map-based cloning approach, we delimited Htn1 to a 131.7-kb physical interval on chromosome 8 that contained three candidate genes encoding two wall-associated receptor-like kinases (ZmWAK-RLK1 and ZmWAK-RLK2) and one wall-associated receptor-like protein (ZmWAK-RLP1). TILLING (targeting induced local lesions in genomes) mutants in ZmWAK-RLK1 were more susceptible to NCLB than wild-type plants, both in greenhouse experiments and in the field. ZmWAK-RLK1 contains a nonarginine-aspartate (non-RD) kinase domain, typically found in plant innate immune receptors. Sequence comparison showed that the extracellular domain of ZmWAK-RLK1 is highly diverse between different maize genotypes. Furthermore, an alternative splice variant resulting in a truncated protein was present at higher frequency in the susceptible parents of the mapping populations compared with in the resistant parents. Hence, the quantitative Htn1 disease resistance in maize is encoded by an unusual innate immune receptor with an extracellular wall-associated kinase domain. These results further highlight the importance of this protein family in resistance to adapted pathogens.

Improving resistance to the European corn borer: a comprehensive study in elite maize using QTL mapping and genome-wide prediction

Key messageThe efficiency of marker-assisted selection for native resistance to European corn borer stalk damage can be increased when progressing from a QTL-based towards a genome-wide approach.AbstractMarker-assisted selection (MAS) has been shown to be effective in improving resistance to the European corn borer (ECB) in maize. In this study, we investigated the performance of whole-genome-based selection, relative to selection based on individual quantitative trait loci (QTL), for resistance to ECB stalk damage in European elite maize. Three connected biparental populations, comprising 590 doubled haploid (DH) lines, were genotyped with high-density single nucleotide polymorphism markers and phenotyped under artificial and natural infestation in 2011. A subset of 195 DH lines was evaluated in the following year as lines per se and as testcrosses. Resistance was evaluated based on stalk damage ratings, the number of feeding tunnels in the stalk and tunnel length. We performed individual- and joint-population QTL analyses and compared the cross-validated predictive abilities of the QTL models with genomic best linear unbiased prediction (GBLUP). For all traits, the GBLUP model consistently outperformed the QTL model despite the detection of QTL with sizeable effects. For stalk damage rating, GBLUP’s predictive ability exceeded at times 0.70. Model training based on DH line per se performance was efficient in predicting stalk breakage in testcrosses. We conclude that the efficiency of MAS for ECB stalk damage resistance can be increased considerably when progressing from a QTL-based towards a genome-wide approach. With the availability of native ECB resistance in elite European maize germplasm, our results open up avenues for the implementation of an integrated genome-based selection approach for the simultaneous improvement of yield, maturity and ECB resistance.

Variation and covariation for Gibberella ear rot resistance and agronomic traits in testcrosses of doubled haploid maize lines

Gibberella ear rot (GER) of maize caused by Fusarium graminearum reduces grain yield and leads to contamination of the grains with deoxynivalenol (DON), a mycotoxin that adversely affects the health of humans and animals. The objectives of this study were to (1) analyze means and genotypic variances for line per se performance (LP) and testcross performance (TP) of doubled haploid (DH) lines for GER severity and DON concentration as well as for some agronomic traits, (2) examine correlations among these traits, (3) validate QTL for resistance detected in previous studies for LP and their effect on TP and (4) investigate the relative efficiency of indirect selection (RE) for LP to improve TP. Testcross progenies of 94 DH lines originating from four flint populations were developed using a susceptible dent tester as pollinator. Artificial inoculations with F. graminearum led to appreciable disease development. Average TP for GER severity and DON concentration were lower than the mean mid-parent values of the tester and DH lines, indicating mid-parent heterosis for resistance. Genotypic variation for resistance was significant for LP and TP. Genotypic correlations between LP and TP were low and resistance QTL for LP had no significant effects on TP. Accordingly, RE for resistance was low, suggesting to allocate resources mostly to the evaluation of testcrosses. Correlations of resistance to GER and DON contamination with grain yield (measured under non-inoculated conditions) were not significant, indicating that selection for resistance and higher grain yield can be carried out simultaneously.

Fungal resistance mediated by maize wall-associated kinase ZmWAK-RLK1 correlates with reduced benzoxazinoid content

Wall-associated kinases (WAKs) have recently been identified as major components of fungal and bacterial disease resistance in several cereal crop species. However, the molecular mechanisms of WAK-mediated resistance remain largely unknown. Here, we investigated the function of the maize gene ZmWAK-RLK1 (Htn1) that confers quantitative resistance to northern corn leaf blight (NCLB) caused by the hemibiotrophic fungal pathogen Exserohilum turcicum. ZmWAK-RLK1 was found to localize to the plasma membrane and its presence resulted in a modification of the infection process by reducing pathogen penetration into host tissues. A large-scale transcriptome analysis of near-isogenic lines (NILs) differing for ZmWAK-RLK1 revealed that several differentially expressed genes are involved in the biosynthesis of the secondary metabolites benzoxazinoids (BXs). The contents of several BXs including DIM2 BOA-Glc were significantly lower when ZmWAK-RLK1 is present. DIM2 BOA-Glc concentration was significantly elevated in ZmWAK-RLK1 mutants with compromised NCLB resistance. Maize mutants that were affected in overall BXs biosynthesis or content of DIM2 BOA-Glc showed increased NCLB resistance. We conclude that Htn1-mediated NCLB resistance is associated with a reduction of BX secondary metabolites. These findings suggest a link between WAK-mediated quantitative disease resistance and changes in biochemical fluxes starting with indole-3-glycerol phosphate.