Additive effects of two quantitative trait loci that confer Rhopalosiphum maidis (corn leaf aphid) resistance in maize inbred line Mo17

Abstract

Plants show considerable within-species variation in their resistance to insect herbivores. In the case of Zea mays (cultivated maize), Rhopalosiphum maidis (corn leaf aphids) produce approximately twenty times more progeny on inbred line B73 than on inbred line Mo17. Genetic mapping of this difference in maize aphid resistance identified quantitative trait loci (QTL) on chromosomes 4 and 6, with the Mo17 allele reducing aphid reproduction in each case. The chromosome 4 QTL mapping interval includes several genes involved in the biosynthesis of DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one), a maize defensive metabolite that also is required for callose accumulation in response to aphid feeding. Consistent with the known association of callose with plant defence against aphids, R. maidis reproduction on B73×Mo17 recombinant inbred lines was negatively correlated with both DIMBOA content and callose formation. Further genetic mapping, as well as experiments with near-isogenic lines, confirmed that the Mo17 allele causes increased DIMBOA accumulation relative to the B73 allele. The chromosome 6 aphid resistance QTL functions independently of DIMBOA accumulation and has an effect that is additive to that of the chromosome 4 QTL. Thus, at least two separate defence mechanisms account for the higher level of R. maidis resistance in Mo17 compared with B73.

Keywords: DIMBOA; Rhopalosiphum maidis; aphid; benzoxazinoid; callose; maize; quantitative trait..

Fig. 1.

Mapping QTL for R. maidis resistance using B73×Mo17 recombinant inbred lines (RILs) and near-isogenic lines (NILs). (A) QTL for aphid progeny production were detected on chromosomes 4 and 6 by composite interval mapping. (B) Aphid progeny production on RILs that were separated according to whether they have the B73 or the Mo17 allele for the QTL on chromosomes 4 and 6, respectively. (C) Aphid reproduction on NILs with reciprocal insertions of the chromosome 4 QTL region into B73 and Mo17. Mean±SEM of n=6–10. (D) Aphid reproduction on NILs with insertions of the chromosome 6 QTL region of Mo17 into B73. Mean±SEM. of n=6–10. B=B73 genotype, M=Mo17 genotype, H=heterozygous, and shaded areas indicate the reciprocal introgressions into the other genetic background. Different letters above bars indicate significant differences, ANOVA followed by Tukey’s HSD.

Fig. 2.

DIMBOA content in B73×Mo17 RILs. (A) Location of QTL for DIMBOA content on maize chromosomes 4 and 5 using composite interval mapping and B73×Mo17 RILs. (B) Correlation of DIMBOA content and aphid reproduction on B73×Mo17 RILs. A best-fit line was placed by linear regression, r=–0.361, P<0.05, Pearson correlation.

Fig. 3.

DIMBOA content of and R. maidis reproduction on a W22 Bx1::Ds knockout mutant. (A) DIMBOA content in a segregating Bx1::Ds transposon insertion line relative to wild-type W22. (B) R. maidis reproduction on maize inbred line W22 and a segregating Bx1::Ds transposon insertion line. n=4 (wild-type W22), 12 (Bx1 segregants), and 8 (Bx1::Ds segregants). *P<0.05, Dunnett’s test relative to wild-type W22. ND=not detected.

Fig. 4.

Correlation of callose accumulation and R. maidis reproduction. (A) Callose accumulation in response to aphid feeding on B73×Mo17 RILs is positively correlated with DIMBOA content. A best-fit line was placed by linear regression, r=0.693, P<0.05, Pearson correlation. (B) Aphid reproduction is negatively correlated with callose accumulation in B73×Mo17 RILs. A best-fit line was placed by linear regression, r=0.649, P<0.05, Pearson correlation. (C) Callose accumulation in B73, Mo17, and selected chromosome 4 QTL NILs. Mean±SEM,*P<0.05, t-test comparing aphid-fed and control plants. (D) Benzoxazinoid content in B73, Mo17, and NILs with the chromosome 4 QTL allele of B73 introgressed into Mo17. Different letters indicate significant differences, ANOVA followed by Tukey’s HSD test.

Fig. 5.

Model of aphid defence regulation by benzoxazinoid metabolism in maize seedlings. Both synthesis and catabolism regulate the abundance of DIMBOA-glucoside. Genetic variation in the biosynthetic pathway affects DIMBOA-glucoside abundance (Butrón et al., 2010), and both Mo17 and B73 have a transposon insertion in Bx10c, which encodes a constitutively expressed DIMBOA-Glc catabolic enzyme (Meihls et al., 2013). DIMBOA-Glc is a precursor for DIMBOA, which is required to trigger callose formation and perhaps other aphid defence responses in maize.