Abstract
Strawberry powdery mildew, caused by Podosphaeraaphanis is a major fungal disease that affects strawberry yield and quality. In the model plant species Arabidopsis and the crop plants barley, tomato and pea, the Mildew resistance locus O (MLO) proteins have been found to be required for powdery mildew susceptibility. The present study, based on the sequence of a wild plum (Prunus americana) MLO protein, identified 16 MLO genes within the genome of woodland strawberry, Fragaria vesca and examined their expression pattern in response to powdery mildew infection in three diploid strawberry cultivars. Phylogenetic analysis showed that the FvMLO genes can be classified into six clades. Four FvMLO genes were grouped into clade III, which comprises MLO genes from Arabidopsis, tomato and grapevine that mediate powdery mildew susceptibility. A RNA-seq analysis of two diploid strawberry cultivars, F. vescassp. vesca accession Hawaii 4 (HW) and F. vesca f. semperflorens line “Yellow Wonder 5AF7” (YW) at 1 d (1 DAI) and 8 d (8 DAI) after infection showed the expression of 12 out of the 16 FvMLO genes. The comparison of Fragments Per Kilobase of transcript per Million mapped reads (FPKM values) detected by RNA-seq and expression values of qRT-PCR for FvMLO genes showed substantial agreement. The FvMLO3 gene, which was grouped in clade III and orthologous to the Arabidopsis,tomato and grapevine genes, was highly expressed in YW compared to other FvMLO genes across varieties. The results showed that FvMLO genes can be used as potential candidates to engineer powdery mildew resistance in strawberry based on MLO suppression or genome editing.
Highlights
The mildew resistance locus o (MLO) genes encode a specific class of plant-specific proteins that possess seven transmembrane helices and a C-terminal calmodulin-binding domain [1, 2]
Based on the chromosomal location of FvMLO genes, FvMLO4 was found to be an isoform of the FvMLO3 gene and the former was excluded in the phylogenetic analysis
A phylogenetic analysis was performed with the amino acid sequences encoded by the FvMLO genes together with a set of VvMLO protein sequences from grapevine [9, 46] and selected MLO proteins from barley, rice, wheat, maize, tomato, A. thaliana, P. americana and M. toringoides
Summary
The mildew resistance locus o (MLO) genes encode a specific class of plant-specific proteins that possess seven transmembrane helices and a C-terminal calmodulin-binding domain [1, 2]. The mutant MLO genes confer durable and broad spectrum powdery mildew resistance in monocots and dicots and the functions of these genes are highly conserved in the plant powdery mildew interactions. The presence of HvMLO is an absolute requirement for the powdery mildew fungi to penetrate the host cell wall successfully [19], whereas the recessive loss-of-function alleles of MLO gene showed durable and broad spectrum resistance to these pathogens by forming cell-wall appositions [20,21,22,23]. The loss-of-function mutations in PsMLO1 showed durable and broad-spectrum resistance to powdery mildew in pea plants [32], and a transposable element insertion in the susceptibility gene CsMLO8 results in hypocotyl resistance to powdery mildew in cucumber [33]. The MLO genes serve as good candidate genes for genetic engineering strategies to impart resistance against powdery mildew pathogens
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