Abstract
Two classes of genes are used for breeding rust resistant wheat. The first class, called R (for resistance) genes, are pathogen race specific in their action, effective at all plant growth stages and probably mostly encode immune receptors of the nucleotide binding leucine rich repeat (NB-LRR) class. The second class is called adult plant resistance genes (APR) because resistance is usually functional only in adult plants, and, in contrast to most R genes, the levels of resistance conferred by single APR genes are only partial and allow considerable disease development. Some but not all APR genes provide resistance to all isolates of a rust pathogen species and a subclass of these provides resistance to several fungal pathogen species. Initial indications are that APR genes encode a more heterogeneous range of proteins than R proteins. Two APR genes, Lr34 and Yr36, have been cloned from wheat and their products are an ABC transporter and a protein kinase, respectively. Lr34 and Sr2 have provided long lasting and widely used (durable) partial resistance and are mainly used in conjunction with other R and APR genes to obtain adequate rust resistance. We caution that some APR genes indeed include race specific, weak R genes which may be of the NB-LRR class. A research priority to better inform rust resistance breeding is to characterize further APR genes in wheat and to understand how they function and how they interact when multiple APR and R genes are stacked in a single genotype by conventional and GM breeding. An important message is do not be complacent about the general durability of all APR genes.
Highlights
Three wheat diseases, stem, leaf, and stripe rust, caused by Puccinia graminis f. sp. tritici (Pgt), P. triticina (Ptr), and P. striiformis f. sp tritici (Pst), respectively, cause important losses of grain production (McIntosh et al, 1995)
Genetic control has advantages for environmental and economic reasons, for farmers in the developing world, and because of the possibility that rust pathogens develop resistance to fungicides (Oliver, 2014). When it comes to genetic resistance used by wheat breeders there are two general classes of genes based on their phenotypic effects, pathogen race- or strain-specific resistance (R genes) and adult plant resistance (APR) genes
In some regions of the developed world there is the view “yield is king” and high yielding rust susceptible varieties are being chosen by farmers with the view that the yield benefit over resistant varieties will more than cover the cost of fungicides in disease years
Summary
Stem, leaf, and stripe (or yellow) rust, caused by Puccinia graminis f. sp. tritici (Pgt), P. triticina (Ptr), and P. striiformis f. sp tritici (Pst), respectively, cause important losses of grain production (McIntosh et al, 1995). When it comes to genetic resistance used by wheat breeders there are two general classes of genes based on their phenotypic effects, pathogen race- or strain-specific resistance (R genes) and adult plant resistance (APR) genes.
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