Highlighted article: Dmitry Lapin, Rhonda C. Meyer, Hideki Takahashi, Ulrike Bechtold, Guido Van den Ackerveken (2012) Broad-spectrum resistance of Arabidopsis C24 to downy mildew is mediated by different combinations of isolate-specific loci. New Phytologist DOI: 10.1111/j.1469-8137.2012.04344.x
It is a mark of how effective plant immune systems are that most bacteria, fungi, and viruses do not affect plants at all either because plant tissues are not suitable for them to live in, or they are fended off. Of course there are pathogens that are compatible with plants – and within species that share compatibility, there are pockets of resistance. Some sub-groups are resistant to specific pathogen isolates, and this is caused by dominant resistant genes. A much broader, more complicated, and less common form of resistance occurs when a particular accession is resistant to a whole pathogen species, or several species. This is broad-spectrum resistance, and it can be caused by a simple dominant gene or multiple genes. Natural broad-spectrum resistance is not simple to transfer from its origin to a commercial crop because it can come from a complex set of genes which are not necessarily all dominant.
In this paper, Lapin et al. characterized the genetic cause of Arabidopsis accession C24 resistance to downy mildew Hyaloperonospora arabidopsidis using segregation analysis and QTL mapping of recombinant inbred lines. The degree of resistance to downy mildew was evaluated by counting spores 6-8 days after inoculation. They identified 7 loci necessary for resistance to the three strains of downy mildew tested – not one of which conferred resistance to all three. When C24 was crossed with the wildtype accession col-0, some hybrids were susceptible to one strain of downy mildew, but all were immune to two other strains, indicating difference in dominance in the loci.
This research is a good pilot study investigating the genetic basis of broad-spectrum resistance to a pathogen. However the downy mildew H. arabidopsidis has over 100 identified isolates, of which only a few infect each species. Different isolates are compatible with different plant species, and the loci that make plants resistant or compatible to downy mildew are not the same in each species. Identifying the loci that confer broad-spectrum resistance in commercial crops is a big challenge, but here Lapin et al. have demonstrated that it is possible and have taken the first steps toward understanding and manipulating broad spectrum resistance.
Teaching resources: Plant Cell have a set of resources for teaching on plants and pathogens, aimed at undergraduate students.
Image credit: Mary Coates