Genetic Mapping of a Major Resistance Gene to Pea Aphid (Acyrthosipon pisum) in the Model Legume Medicago truncatula.

Resistance to the Australian pea aphid (PA; Acyrthosiphon pisum) biotype in cultivar Jester of the model legume Medicago truncatula is mediated by a single dominant gene and is phloem-mediated. The genetic map position for this resistance gene, APR (Acyrthosiphon pisum resistance), is provided and shows that APR maps 39 centiMorgans (cM) distal of the A. kondoi resistance (AKR) locus, which mediates resistance to a closely related species of the same genus bluegreen aphid (A. kondoi). The APR region on chromosome 3 is dense in classical nucleotide binding site leucine-rich repeats (NLRs) and overlaps with the region harbouring the RAP1 gene which confers resistance to a European PA biotype in the accession Jemalong A17. Further screening of a core collection of M. truncatula accessions identified seven lines with strong resistance to PA. Allelism experiments showed that the single dominant resistance to PA in M. truncatula accessions SA10481 and SA1516 are allelic to SA10733, the donor of the APR locus in cultivar Jester. While it remains unclear whether there are multiple PA resistance genes in an R-gene cluster or the resistance loci identified in the other M. truncatula accessions are allelic to APR, the introgression of APR into current M. truncatula cultivars will provide more durable resistance to PA.

Characterization and genetic dissection of resistance to spotted alfalfa aphid (Therioaphis trifolii) in Medicago truncatula.

Aphids cause significant yield losses in agricultural crops worldwide. Medicago truncatula, a model legume, cultivated pasture species in Australia and close relative of alfalfa (Medicago sativa), was used to study the defence response against Therioaphis trifolii f. maculate [spotted alfalfa aphid (SAA)]. Aphid performance and plant damage were compared among three accessions. A20 is highly susceptible, A17 has moderate resistance, and Jester is strongly resistant. Subsequent analyses using A17 and A20, reciprocal F1s and an A17×A20 recombinant inbred line (RIL) population revealed that this moderate resistance is phloem mediated and involves antibiosis and tolerance but not antixenosis. Electrical penetration graph analysis also identified a novel waveform termed extended potential drop, which occurred following SAA infestation of M. truncatula. Genetic dissection using the RIL population revealed three quantitative trait loci on chromosomes 3, 6, and 7 involved in distinct modes of aphid defence including antibiosis and tolerance. An antibiosis locus resides on linkage group 3 (LG3) and is derived from A17, whereas a plant tolerance and antibiosis locus resides on LG6 and is derived from A20, which exhibits strong temporary tolerance. The loci identified reside in regions harbouring classical resistance genes, and introgression of these loci in current medic cultivars may help provide durable resistance to SAA, while elucidation of their molecular mechanisms may provide valuable insight into other aphid-plant interactions.

Identification of distinct quantitative trait loci associated with defence against the closely related aphids Acyrthosiphon pisum and A. kondoi in Medicago truncatula.

Aphids are a major family of plant insect pests. Medicago truncatula and Acyrthosiphon pisum (pea aphid, PA) are model species with a suite of resources available to help dissect the mechanism underlying plant-aphid interactions. A previous study focused on monogenic and relatively strong resistance in M. truncatula to PA and other aphid species. In this study a moderate resistance to PA was characterized in detail in the M. truncatula line A17 and compared with the highly susceptible line A20 and the more resistant line Jester. The results show that PA resistance in A17 involves both antibiosis and tolerance, and that resistance is phloem based. Quantitative trait locus (QTL) analysis using a recombinant inbred line (RIL) population (n=114) from a cross between A17 and A20 revealed that one locus, which co-segregated with AIN (Acyrthosiphon-induced necrosis) on chromosome 3, is responsible for the reduction of aphid biomass (indicator of antibiosis) for both PA and bluegreen aphid (BGA, A. kondoi), albeit to a lesser degree for PA than BGA. Interestingly, two independent loci on chromosomes 5 and 3 were identified for the plant biomass reduction (indicator of plant tolerance) by PA and BGA, respectively, demonstrating that the plant's tolerance response to these two closely related aphid species is distinct. Together with previously identified major resistant (R) genes, the QTLs identified in this study are powerful tools to understand fully the spectrum of plant defence against sap-sucking insects and provide opportunities for breeders to generate effective and sustainable strategies for aphid control.

[The analysis of differential expression and cloning of genes related to raised secondary lateral veins mutant of Lycoris aurea].

Lycoris aurea exhibits parallel venation, the main vein with many lateral veins in a longitudinal parallel arrangement. There are secondary lateral veins (SLV) between each longitudinal veins. In general, SLVs are not remarkable. In this paper, the material was one kind of Lycoris aurea mutant called Raised Secondary Lateral Veins mutant (RSLV), because many Raised Secondary Lateral Veins are in abaxial surface of its leaves. Its growing potential is weaker than that of wild type and its blades are very thin. Moreover, the stamens of RSLV degenerate completely. Two cDNA libraries were constructed from RSLV mutant and wild type (WT) leaves. From the libraries, 3,122 ESTs, which are longer than 100 bp each after vector sequence removed, were acquired by single-pass sequencing from the 5'end. Following a multistep selection, 512 70-mer oligo-DNA probes were designed for attachment on the microarray slide based on the ESTs. The gene expression profile of RSLV mutant and WT leaves was compared through the microarray at transcriptional level. The microarray experiment results were further confirmed by Quantitative Real-Time PCR (QRT-PCR). We identified 5 genes whose expressions changed more than 2-fold between RSLV mutant and WT leaves. They encode phloem protein 2 (PP2), ferritin, pectin methyl esterase (PME), chlorophyll a/b binding protein (CAB protein) and pyruvate decarboxylase (PDC), respectively. Furthermore, the full-length cDNA sequences of the 5 genes were separately obtained from RSLV and WT by RACE. The relationship between differential expressions of the genes and the formation of the RSLV mutant phenotype were discussed.