Abstract
Powdery mildew (PM) caused by Podosphaera aphanis is a major fungal disease of cultivated strawberry. Mildew Resistance Locus O (MLO) is a gene family described for having conserved seven-transmembrane domains. Induced loss-of-function in specific MLO genes can confer durable and broad resistance against PM pathogens. However, the genomic structure and potential role of MLO genes for PM resistance have not been characterized yet in the octoploid cultivated strawberry. In the present study, MLO gene families were characterized in four diploid progenitor species (Fragaria vesca, F. iinumae, F. viridis, and F. nipponica) and octoploid cultivated (Fragaria ×ananassa) strawberry, and potential sources of MLO-mediated susceptibility were identified. Twenty MLO sequences were identified in F. vesca and 68 identified in F. ×ananassa. Phylogenetic analysis divided diploid and octoploid strawberry MLO genes into eight different clades, in which three FveMLO (MLO10, MLO17, and MLO20) and their twelve orthologs of FaMLO were grouped together with functionally characterized MLO genes conferring PM susceptibility. Copy number variations revealed differences in MLO composition among homoeologous chromosomes, supporting the distinct origin of each subgenome during the evolution of octoploid strawberry. Dissecting genomic sequence and structural variations in candidate FaMLO genes revealed their potential role associated with genetic controls and functionality in strawberry against PM pathogen. Furthermore, the gene expression profiling and RNAi silencing of putative FaMLO genes in response to the pathogen indicate the function in PM resistance. These results are a critical first step in understanding the function of strawberry MLO genes and will facilitate further genetic studies of PM resistance in cultivated strawberry.
Highlights
Introduction The Mildew Resistance LocusO (MLO) gene family is present in several crop species and was described for having conserved seven-transmembrane (TM) and Cterminal calmodulin-binding (CaMB) domains that are functionally important for powdery mildew resistance[1,2].The genetic diversity among Mildew Resistance Locus O (MLO) gene members has been explored in a wide variety of plant species[3,4,5], but still little information is known about its origin and biochemical function associated with plant development and response to biotic and abiotic stresses
Three FveMLO genes and ten FaMLO genes were clustered with previously characterized MLO genes known to be in powdery mildew (PM) resistance/ susceptibility in other plant species
FaMLO10, FaMLO17 and FaMLO20 are the most identical to functionally characterized MLO genes associated with PM susceptibility
Summary
Introduction The Mildew Resistance LocusO (MLO) gene family is present in several crop species and was described for having conserved seven-transmembrane (TM) and Cterminal calmodulin-binding (CaMB) domains that are functionally important for powdery mildew resistance[1,2].The genetic diversity among MLO gene members has been explored in a wide variety of plant species[3,4,5], but still little information is known about its origin and biochemical function associated with plant development and response to biotic and abiotic stresses. O (MLO) gene family is present in several crop species and was described for having conserved seven-transmembrane (TM) and Cterminal calmodulin-binding (CaMB) domains that are functionally important for powdery mildew resistance[1,2]. A recent study revisited the biological origin of MLO gene family that traced it back from the ancestral species of unicellular photosynthetic eukaryote and, that in the course of evolution diversified to the present MLO proteins[6]. The MLO gene family has been the focus of attention in many crop species because transgenic downregulation or elimination of specific endogenous MLO genes has led to powdery mildew (PM) resistance[10]. The first MLObased resistance trait was first characterized in barley (HvMLO), where a loss-of-function mutation in an MLO gene conferred broad resistance against PM pathogens[12]
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