Tomato Ve disease resistance genes encode cell surface-like receptors

Abstract

In tomato, Ve is implicated in race-specific resistance to infection by Verticillium species causing crop disease. Characterization of the Ve locus involved positional cloning and isolation of two closely linked inverted genes. Expression of individual Ve genes in susceptible potato plants conferred resistance to an aggressive race 1 isolate of Verticillium albo-atrum. The deduced primary structure of Ve1 and Ve2 included a hydrophobic N-terminal signal peptide, leucine-rich repeats containing 28 or 35 potential glycosylation sites, a hydrophobic membrane-spanning domain, and a C-terminal domain with the mammalian E/DXXXLphi or YXXphi endocytosis signals (phi is an amino acid with a hydrophobic side chain). A leucine zipper-like sequence occurs in the hydrophobic N-terminal signal peptide of Ve1 and a Pro-Glu-Ser-Thr (PEST)-like sequence resides in the C-terminal domain of Ve2. These structures suggest that the Ve genes encode a class of cell-surface glycoproteins with receptor-mediated endocytosis-like signals and leucine zipper or PEST sequences.

Figure 1

Schematic genetic and physical representation of L. esculentum linkage group (A) bordering the Ve gene. Analysis of populations segregating for Ve identified closely linked codominant random-amplified polymorphic DNAs and allele-specific SCARs that map to the region of RFLP GP39. Identification of contiguous λ-genomic clones (B) facilitated the subcloning of genomic DNA (C) containing the Ve locus. Vertical lines indicate the location of the AUG initiation codon in the subgenomic clones. Expressed sequences were cloned into λ, and arrowheads depict the direction of transcription for the cDNAs (D) identified by using the genomic clone pG1Ve. Potato plants transformed with the genomic subclones pG2Ve and pG3Ve or cDNAs pC1Ve and pC2Ve exhibited in vivo complementation and resistance (R) when challenged with V. albo-atrum race 1. Potato plants transformed with the genomic pG1Ve and the binary vectors pBIN19 and pBI121 were susceptible (S) to infection.

Figure 2

Genetic complementation in potato plants transformed with Ve. Disease symptoms were recorded 3 weeks after plants containing pG2Ve∷pBIN19 (left) (resistant) or pBIN19 (right) (susceptible) were inoculated with V. albo-atrum race 1. Similar disease resistance was observed in plants transformed with pG3Ve∷pBIN19, pC1Ve∷pBI121, and pC2Ve∷pBI121.

Figure 3

Primary structure of the Ve1 (A) and Ve2 (B) proteins deduced from cDNA sequence. The polypeptides have been divided into domains A–G as described in the text. A dashed line occurs above the putative N-terminal leucine zipper in domain A of Ve1 and below the endocytosis signals in domain E of Ve1 and domain G of Ve2. Highlighted are the hydrophobic amino acids (purple) of the putative signal peptide domain A and membrane-associated domain D; conserved L/I (red), G (green), and potential N-glycosylation sites (blue) within the LRR domain B; neutral and acidic amino acids (gray) of domain C; and neutral and basic amino acids (yellow) of domain E. The PEST sequence of Ve2 is shown in domain F.