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Although quantitative trait loci (QTL) underpin many desirable agronomic traits, their incorporation into crop plants through marker-assisted selection is limited by the low predictive value of markers on phenotypic performance. Here we used candidate defense response (DR) genes to dissect quantitative resistance in rice using recombinant inbred (RI) and advanced backcross (BC) populations derived from a blast-resistant cultivar, Sanhuangzhan 2 (SHZ-2). Based on DNA profiles of DR genes, RI lines were clustered into two groups corresponding to level of resistance. Five DR genes, encoding putative oxalate oxidase, dehydrin, PR-1, chitinase, and 14-3-3 protein, accounted for 30.0, 23.0, 15.8, 6.7, and 5.5% of diseased leaf area (DLA) variation, respectively. Together, they accounted for 60.3% of the DLA variation and co-localized with resistance QTL identified by interval mapping. Average phenotypic contributions of oxalate oxidase, dehydrin, PR-1, chitinase, and 14-3-3 protein in BC lines were 26.1, 19.0, 18.0, 11.5, and 10.6%, respectively, across environments. Advanced BC lines with four to five effective DR genes showed enhanced resistance under high disease pressure in field tests. Our results demonstrate that the use of natural variation in a few candidate genes can solve a long-standing problem in rice production and has the potential to address other problems involving complex traits.

Bacterial speck disease of tomato, caused by Pseudomonas syringae pv. tomato ( Pst) is one of the most devastating diseases of tomato in the world. To investigate plant responses activated during plant-pathogen interaction, we studied the expression analysis of selected defense-response genes and the microbe or pathogen associated molecular patterns (MAMP or PAMP) triggered immunity (PTI) marker genes in heirloom tomatoes challenged with virulent Pst DC3000. Transcript levels of the defense response genes, including SlPR1a (pathogenesis related proteins), SlPR- Q′ b (β- 1, 3- glucanase), SlGST (Glutathione S-transferase) and peroxidase were up-regulated in the resistant cultivars Orange Strawberry and Amishpaste. The PTI marker genes SlPTI5 (encode a pathogen-inducible ethylene response element-binding protein-like transcription factor) and SlFLS2 (a leucine rich-repeat receptor like kinase) were strongly up-regulated in the incompatible reaction to resistant cultivar Orange Strawberry. On the other hand, transcripts from all tested genes were down-regulated in the compatible interaction in susceptible cultivars Yellow Pear and Brandywine. The induction of defense response and PTI marker genes occurred in the early infection process at 3 days post-inoculation (dpi) and consistent with lower levels of disease severity in resistant cultivars was observed than in susceptible cultivars. Our results contribute to the body of information on bacterial speck disease resistance in an economically important crop tomato. [ABSTRACT FROM AUTHOR]

Defense response genes in higher plant species are involved in a variety of signal transduction pathways and biochemical reactions to counterattack invading pathogens. In this study, a total of 366 non-redundant defense response gene homologs (DRHs), including 124 unigenes/expressed sequence tags, 226 tentative consensuses, and 16 DRH contigs have been identified by mining the Maize Genetics and Genomics and The Institute for Genomic Research maize databases using 35 essential defense response genes. Of 366 DRHs, 202 are mapped to 152 loci across ten maize chromosomes via both the genetic and in silico mapping approaches. The mapped DRHs seem to cluster together rather than be evenly distributed along the maize genome. Approximately half of these DHRs are located in regions harboring either major resistance genes or quantitative trait loci (QTL). Therefore, this comprehensive DRH linkage map will provide reference sequences to identify either positional candidate genes for resistance genes and/or QTLs or to develop makers for fine-mapping and marker-assisted selection of resistance genes and/or QTLs. [ABSTRACT FROM AUTHOR]

Barley powdery mildew resistance (PM-R) genes control different infection phenotypes against avirulent strains of Blumeria graminis f. sp. hordei (Bgh). A subset of seven Pallas isolines, containing the PM-R genes Mla1, Mla12, Mlg, Mlk, Mlp and mlo5, revealed fast-, intermediate- and slow-acting infection phenotypes. Scanning electron microscopy revealed the extent of Bgh development on each genotype through 72 hai. Quantitative RNA blot analysis of chitinase IIb, phenylalanine ammonia lyase and peroxidase transcription at 0–24 hai revealed similar patterns and levels of transcripts in all isolines including the susceptible parent Pallas. At 36–72 hai transcript accumulation was suppressed on the susceptible parent Pallas, where Bgh grew unimpeded. In resistant isolines transcript accumulation varied according to whether their PM-R genes were fast, intermediate or slow-acting. Transcript accumulation decreased at 36–72 hai in isolines with fast-acting PM-R genes (Mla1, Mlg, and mlo5), and this corresponded with arrested Bgh development. Transcript accumulation at 36–72 hai in isolines with intermediate or slow-acting PM-R genes (Mla12 and Mlk, Mlp) remained elevated and correlated with continued Bgh development and contact. These results suggest that defense response genes are transcriptionally activated by Bgh contact, which probably involves a general elicitor(s) from Bgh. Thus, PM-R genes appear to only modulate defense response gene transcription indirectly by limiting fungal development and contact. Fast-acting PM-R genes halt Bgh development before 24 hai, while slow-acting R genes allow Bgh development throughout 72 hai.Differences in infection phenotypes due to differing PM-R genes may be due to temporal differences in interactions between R genes and avirulence gene product(s); alternatively, slow-acting PM-R genes or required partner genes may be non-constitutive and need time to be induced. [Copyright &y& Elsevier]