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

Abstract

Key message Rose 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. Abstract 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.

Comparative RNA-seq analysis reveals a critical role for ethylene in rose (Rosa hybrida) susceptible response to Podosphera pannosa

“…Rosa hybrida ‘Samantha’ was propagated by tissue culture ( Fang et al., 2021 ). Rose shoots with at least two leaves were cultured on half-strength Murashige and Skoog (MS) medium supplemented with 0.1 mg L -1 α-naphthaleneacetic (NAA) for 30 days at 22°C under a 16-h/8-h light/dark photoperiod for rooting.…”

Haplotype-resolved genome assembly and resequencing provide insights into the origin and domestication of modern rose

“…Therefore, it is not surprising that compared to intermediate old cultivars, disease resistance in modern cultivars has been selected during the last two hundred years of modern rose breeding history. In the selective sweep regions, genes homologous to Arabidopsis NCA1 43 that play a crucial role in the clearance of hydrogen peroxide, Arabidopsis RPW8 44 involved in powdery mildew resistance, and rose mildew resistance locus O (MLO) gene 45 , also responsible for powdery mildew resistance, were identified. Plant NB-LRR immune receptors have evolved the ability to specifically recognize a wide range of effector proteins from different pathogens 46 .…”

Ornamental plant breeding: entering a new era?

“…Despite the increasing affordability of sequencing, the minor economic significance of most ornamental crops hinders development of reference genomes for these species. Despite the availability of the reference genome, optimization of the plant transformation system has been a limiting factor in several crops including rose (Fang et al, 2021). The plant transformation system protocol must be optimized per plant species.…”