Wheat <i>Ym2</i> originated from <i>Aegilops sharonensis</i> and confers resistance to soil-borne <i>Wheat yellow mosaic virus</i> infection to the roots.

Abstract

<i>Wheat yellow mosaic virus</i> (WYMV) is a pathogen transmitted into its host's roots by the soil-borne vector <i>Polymyxa graminis</i>. <i>Ym1</i> and <i>Ym2</i> genes protect the host from the significant yield losses caused by the virus, but the mechanistic basis of these resistance genes remains poorly understood. Here, it has been shown that <i>Ym1</i> and <i>Ym2</i> act within the root either by hindering the initial movement of WYMV from the vector into the root and/or by suppressing viral multiplication. A mechanical inoculation experiment on the leaf revealed that the presence of <i>Ym1</i> reduced viral infection incidence, rather than viral titer, while that of <i>Ym2</i> was ineffective in the leaf. To understand the basis of the root specificity of the <i>Ym2</i> product, the gene was isolated from bread wheat using a positional cloning approach. The candidate gene encodes a CC-NBS-LRR protein and it correlated allelic variation with respect to its sequence with the host's disease response. <i>Ym2</i> (B37500) and its paralog (B35800) are found in the near-relatives, respectively, <i>Aegilops sharonensi</i>s and <i>Aegilops speltoides</i> (a close relative of the donor of bread wheat's B genome), while both sequences, in a concatenated state, are present in several accessions of the latter species<i>.</i> Structural diversity in <i>Ym2</i> has been generated via translocation and recombination between the two genes and enhanced by the formation of a chimeric gene resulting from an intralocus recombination event. The analysis has revealed how the <i>Ym2</i> region has evolved during the polyploidization events leading to the creation of cultivated wheat.