Analysis of allelic variants of RhMLO genes in rose and functional studies on susceptibility to powdery mildew related to clade V homologs

Peihong Fang,Yuling Bai,Thomas Debener,Xin Zhang,Zhao Zhang,Xintong Liu,Jérémy Clotault,Fabrice Foucher,Paul Arens,Marinus J M Smulders,Juliane Geike,Deepika Lakwani,Helgard Kaufmann

doi:10.1007/s00122-021-03838-7

Abstract

Key messageRose has 19 MLO genes. Of these, RhMLO1 and RhMLO2 were shown to be required for powdery mildew infection, which suggests their potential as susceptibility targets towards disease resistance.Powdery mildew, caused by Podosphaera pannosa, is one of the most serious and widespread fungal diseases for roses, especially in greenhouse-grown cut roses. It has been shown that certain MLO genes are involved in powdery mildew susceptibility and that loss of function in these genes in various crops leads to broad-spectrum, long-lasting resistance against this fungal disease. For this reason, these MLO genes are called susceptibility genes. We carried out a genome-wide identification of the MLO gene family in the Rosa chinensis genome, and screened for allelic variants among 22 accessions from seven different Rosa species using re-sequencing and transcriptome data. We identified 19 MLO genes in rose, of which four are candidate genes for functional homologs in clade V, which is the clade containing all dicot MLO susceptibility genes. We detected a total of 198 different allelic variants in the set of Rosa species and accessions, corresponding to 5–15 different alleles for each of the genes. Some diploid Rosa species shared alleles with tetraploid rose cultivars, consistent with the notion that diploid species have contributed to the formation of tetraploid roses. Among the four RhMLO genes in clade V, we demonstrated using expression study, virus-induced gene silencing as well as transient RNAi silencing that two of them, RhMLO1 and RhMLO2, are required for infection by P. pannosa and suggest their potential as susceptibility targets for powdery mildew resistance breeding in rose.

Highlights

Rose is one of the most important ornamental plants worldwide, holding great economic and cultural value
One of the matched sequences from R. chinensis, RC4G0104600, encoded a protein of 321 amino acids, which is much shorter than all MLO homologs reported in, for example, the genomes of Arabidopsis and grapevine (Devoto et al 2003; Feechan et al 2008)
We have identified 19 MLO homologs in rose using the recently published rose genome of the diploid garden rose Old Blush

Summary

Introduction

Rose is one of the most important ornamental plants worldwide, holding great economic and cultural value. Extended author information available on the last page of the article for this wide variation is the crossing of different rose species by a large number of breeders for a long period of time. Because of this activity, wild species with varying ploidy levels have contributed to the genome of tetraploid cut and garden roses (Koopman et al 2008; Dubois and Sakr 2012; Zhang et al 2013; Vukosavljev et al 2013; Smulders et al 2019)

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