Identification and characterization of resistance to cowpea aphid (Aphis craccivora Koch) in Medicago truncatula

Abstract

Background: Cowpea aphid (CPA; Aphis craccivora) is the most important insect pest of cowpea and also causes significant yield losses in other legume crops including alfalfa, beans, chickpea, lentils, lupins and peanuts. In many of these crops there is no natural genetic resistance to this sap-sucking insect or resistance genes have been overcome by newly emerged CPA biotypes.

Results: In this study, we screened a subset of the Medicago truncatula core collection of the South Australian Research and Development Institute (SARDI) and identified strong resistance to CPA in a M. truncatula accession SA30199, compared to all other M. truncatula accessions tested. The biology of resistance to CPA in SA30199 plants was characterised compared to the highly susceptible accession Borung and showed that resistance occurred at the level of the phloem, required an intact plant and involved a combination of antixenosis and antibiosis. Quantitative trait loci (QTL) analysis using a F2 population (n = 150) from a cross between SA30199 and Borung revealed that resistance to CPA is controlled in part by a major quantitative trait locus (QTL) on chromosome 2, explaining 39% of the antibiosis resistance.

Conclusions: The identification of strong CPA resistance in M. truncatula allows for the identification of key regulators and genes important in this model legume to give effective CPA resistance that may have relevance for other legume crops. The identified locus will also facilitate marker assisted breeding of M. truncatula for increased resistance to CPA and potentially other closely related Medicago species such as alfalfa.

Figure 1

Mean relative growth rate of cowpea aphid nymphs on 17Medicago truncatulaaccessions over four days. Values are mean and standard error of six replicates. The * for SA30199 is to denote that the MRGR value of this accession is significantly different from the other accessions by Tukey Kramer HSD test (P < 0.05) with the exception of SA3054

Figure 2

Settling of cowpea aphid alatae in a host choice test. Values are mean and SE of 12 biological replicates. Means labelled with * for M. truncatula accession SA30199 are significantly different from those for accession Borung (P < 0.05)

Figure 3

EPG showing representative waveform patterns produced when cowpea aphid apterae feed on susceptible accession Borung (A) or on resistant accession SA30199 (B). The horizontal axes represent a 1-h time period; the vertical axes represent voltage. Histological studies of plant-aphid interactions have correlated stylet positions in plant tissues with specific EPG waveforms [47]. “Non-penetration” indicates stylets are outside the plant. “Pathway” indicates mostly intramural probing activities between mesophyll or parenchyma cells. “Sieve element phase,” consisting primarily of sap ingestion with short periods of salivation into sieve elements, was frequently seen with plants of Borung and only rarely seen with plants of SA30199. Sharp, downward spikes, named potential drops, indicate cell puncture events by stylets, each lasting approximately 5 s. The percentage of time aphids spent in various activities on Borung or SA30199 during 9h exposure to the host plants is shown in C. Each value represents the mean and SE of 16 biological replicates. Means for the cultivars labeled with * are significantly different (P < 0.01)

Figure 4

Effects of plant genotype and shoot excision on the population growth rate of cowpea aphid. Each value represents the mean and SE of ten biological replicates. Means labelled with the same letter are not significantly different (P > 0.05)

Figure 5

Effects of plant prior infestation with cowpea aphid. Values represent the mean and standard error of ten biological replicates. Means labelled with the same letter are not significantly different (P > 0.05)

Figure 6

Frequency distribution of cowpea aphid colony weight per F₂plant 15 days after infestation with two apterous cowpea aphids. Parental medians (downward arrow) and range (brackets) are indicated

Figure 7

Genetic map position of the major QTL involved in cowpea aphid antibiosis based on phenotype data for the F₂individuals of the SA30199 x Borung population. Interval distances are listed in centiMorgans. The genomic location of the QTL is depicted on the right of linkage group 2 (LG2), with standard deviations depicted by lines either side