Discovery and Characterization of a Novel Tomato mlo Mutant from an EMS Mutagenized Micro-Tom Population.

Zhe Yan,Richard G. F. Visser,Robin Huibers,Danny Schipper,Anne-Marie A. Wolters,Michela Appiano,Ageeth van Tuinen,Fien Meijer-Dekens,Yuling Bai,Dongli Gao

doi:10.3390/genes12050719

Abstract

In tomato (Solanum lycopersicum), there are at least three SlMLO (Mildew resistance Locus O) genes acting as susceptibility genes for the powdery mildew disease caused by Oidium neolycopersici, namely SlMLO1, SlMLO5 and SlMLO8. Of the three homologs, the SlMLO1 gene plays a major role since a natural mutant allele called ol-2 can almost completely prevent fungal penetration by formation of papillae. The ol-2 allele contains a 19-bp deletion in the coding sequence of the SlMLO1 gene, resulting in a premature stop codon within the second cytoplasmic loop of the predicted protein. In this study, we have developed a new genetic resource (M200) in the tomato cv. Micro-Tom genetic background by means of ethyl methane sulfonate (EMS) mutagenesis. The mutant M200 containing a novel allele (the m200 allele) of the tomato SlMLO1 gene showed profound resistance against powdery mildew with no fungal sporulation. Compared to the coding sequence of the SlMLO1 gene, the m200 allele carries a point mutation at T65A. The SNP results in a premature stop codon L22* located in the first transmembrane domain of the complete SlMLO1 protein. The length of the predicted protein is 21 amino acids, while the SlMLO1 full-length protein is 513 amino acids. A high-resolution melting (HRM) marker was developed to distinguish the mutated m200 allele from the SlMLO1 allele in backcross populations. The mutant allele conferred recessive resistance that was associated with papillae formation at fungal penetration sites of plant epidermal cells. A comprehensive list of known mlo mutations found in natural and artificial mutants is presented, which serves as a particularly valuable resource for powdery mildew resistance breeding.

Highlights

Tomato (Solanum lycopersicum L.) is a model crop species of high economic value with interesting developmental features such as compound leaves, fleshy fruits, and sympodial shoot branching
During the ethyl methane sulfonate (EMS) treatment, the 1% v/v EMS concentration was mostly used to maximize the genomic variation with a minimum decrease in viability
There are no predicted natural mutations at the T65 position where the m200 SNP occurs. These results suggest that the point mutation in the SlMLO1 gene in the M200 mutant is a new and unique mutation

Summary

Introduction

Tomato (Solanum lycopersicum L.) is a model crop species of high economic value with interesting developmental features such as compound leaves, fleshy fruits, and sympodial shoot branching. An important aspect of the domesticated tomato is its lack of genetic diversity because of years of selection for a limited set of traits, such as fruit shape and size [9]. One way to achieve this goal is to use the diversity present in wild relatives It has been a general practice in tomato breeding to use wild relatives as a donor for introgression of valuable traits present in tomato varieties. Another way to increase genetic diversity is to introduce new mutations artificially. Chemical and physical mutagenesis are frequently used for this purpose in most of the economically important crop species [11].

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Conclusion