Abstract
The MLO (Mildew Locus O) gene family encodes plant-specific proteins containing seven transmembrane domains and likely acting in signal transduction in a calcium and calmodulin dependent manner. Some members of the MLO family are susceptibility factors toward fungi causing the powdery mildew disease. In tomato, for example, the loss-of-function of the MLO gene SlMLO1 leads to a particular form of powdery mildew resistance, called ol-2, which arrests almost completely fungal penetration. This type of penetration resistance is characterized by the apposition of papillae at the sites of plant-pathogen interaction. Other MLO homologs in Arabidopsis regulate root response to mechanical stimuli (AtMLO4 and AtMLO11) and pollen tube reception by the female gametophyte (AtMLO7). However, the role of most MLO genes remains unknown. In this work, we provide a genome-wide study of the tomato SlMLO gene family. Besides SlMLO1, other 15 SlMLO homologs were identified and characterized with respect to their structure, genomic organization, phylogenetic relationship, and expression profile. In addition, by analysis of transgenic plants, we demonstrated that simultaneous silencing of SlMLO1 and two of its closely related homologs, SlMLO5 and SlMLO8, confer higher level of resistance than the one associated with the ol-2 mutation. The outcome of this study provides evidence for functional redundancy among tomato homolog genes involved in powdery mildew susceptibility. Moreover, we developed a series of transgenic lines silenced for individual SlMLO homologs, which lay the foundation for further investigations aimed at assigning new biological functions to the MLO gene family.
Highlights
Many important crop species can be affected by the powdery mildew (PM) disease, resulting in great yield losses in agricultural settings
Characterization of SlMLO Gene Family or induced mlo-mutant alleles are in use for about seven decades to introduce resistance in spring barley breeding programs (Jørgensen, 1992; Büschges et al, 1997; Reinstädler et al, 2010)
A total of 17 tomato MLO-like loci were identified through BLAST interrogation of the tomato genomic sequence database (SGN), using AtMLO protein sequences as query
Summary
Many important crop species can be affected by the powdery mildew (PM) disease, resulting in great yield losses in agricultural settings. Biochemical analysis showed that the barley HvMLO protein contains seven transmembrane domains integral to the plasma membrane, with an extracellular amino-terminus and an intracellular carboxy-terminus. The latter harbors a calmodulinbinding domain likely involved in sensing calcium influxes into cells (Devoto et al, 1999). HvMLO might be exploited by the fungus to impair vesicle-associated defense mechanism at plant-pathogen interaction sites, facilitating its penetration (Panstruga and Schulze-Lefert, 2003; Opalski et al, 2005; Miklis et al, 2007). This feature makes HvMLO a typical representative of susceptibility genes (S-genes) (Miklis et al, 2007; van Schie and Takken, 2014)
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