The nematode resistance gene Mi of tomato confers resistance against the potato aphid

Abstract

Resistance against the aphid Macrosiphum euphorbiae previously was observed in tomato and attributed to a novel gene, designated Meu-1, tightly linked to the nematode resistance gene, Mi. Recent cloning of Mi allowed us to determine whether Meu-1 and Mi are the same gene. We show that Mi is expressed in leaves, that aphid resistance is isolate-specific, and that susceptible tomato transformed with Mi is resistant to the same aphid isolates as the original resistant lines. We conclude that Mi and Meu-1 are the same gene and that Mi mediates resistance against both aphids and nematodes, organisms belonging to different phyla. Mi is the first example of a plant resistance gene active against two such distantly related organisms. Furthermore, it is the first isolate-specific insect resistance gene to be cloned and belongs to the nucleotide-binding, leucine-rich repeat family of resistance genes.

Figure 1

Structure of Mi-1.2. (A) The 14.7-kb genomic clone carrying the Mi-1.2 primary transcript (cross-hatched bar) and flanking sequences (line). This fragment is sufficient to confer resistance to root-knot nematodes to susceptible tomato (7). (B) Mi-1.2 transcript with exons as solid bars and the two introns as angled lines. The arrow marked SM7 represents the approximate position of the primer used to isolate the 5′ ends of transcripts expressed in leaves. (C) Predicted protein structure of Mi-1.2. LZ, the position of a possible leucine zipper; NBS, a nucleotide binding site; LRR, leucine-rich repeat region (8, 9). The NBS-containing region that is highly conserved among many plant resistance-associated proteins is shown in gray.

Figure 2

Mi expression in leaves. RNA gel blot hybridized with probe 3–3, which contains the Mi-1.1 nucleotide binding site (7). Poly(A)⁺ RNA of aphid-resistant tomato line Sun6082 (R) and susceptible line CastlerockII (S) was loaded 5 μg per lane.

Figure 3

Survivorship of aphid isolates WU-3 and WU-4. Comparison of survivorship on a resistant line, Sun6082 (R), and a susceptible line, CastlerockII (S). (A) For aphid isolate WU-3, the survivorship was significantly higher on the susceptible than on the resistant tomato line (P = 0.0402). (B) For aphid isolate WU-4, there was no significant difference in survivorship on resistant and susceptible tomato lines (P = 0.3626).

Figure 4

Genetic complementation for aphid resistance in susceptible tomato after transformation with Mi-1.2. Comparison of the mean number of aphids per leaflet in independent experiments between two transgenic lines, 143–11 (A) and 143–31 (B), and the control lines Moneymaker (susceptible) and Motelle (resistant). Five plants per genotype were exposed to each of two different isolates of M. euphorbiae WU-3 (black bar) and WU-4 (cross-hatched bar). Significantly different means within each experiment are indicated by different letters according to Tukey HSD test (P < 0.05).