Valerie M. Williamson

The Root Knot Nematode Resistance Gene Mi from Tomato Is a Member of the Leucine Zipper, Nucleotide Binding, Leucine-Rich Repeat Family of Plant Genes

The Mi locus of tomato confers resistance to root knot nematodes. Tomato DNA spanning the locus was isolated as bacterial artificial chromosome clones, and 52 kb of contiguous DNA was sequenced. Three open reading frames were identified with similarity to cloned plant disease resistance genes. Two of them, Mi-1.1 and Mi-1.2, appear to be intact genes; the third is a pseudogene. A 4-kb mRNA hybridizing with these genes is present in tomato roots. Complementation studies using cloned copies of Mi-1.1 and Mi-1.2 indicated that Mi-1.2, but not Mi-1.1, is sufficient to confer resistance to a susceptible tomato line with the progeny of transformants segregating for resistance. The cloned gene most similar to Mi-1.2 is Prf, a tomato gene required for resistance to Pseudomonas syringae. Prf and Mi-1.2 share several structural motifs, including a nucleotide binding site and a leucine-rich repeat region, that are characteristic of a family of plant proteins, including several that are required for resistance against viruses, bacteria, fungi, and now, nematodes.

Nematode pathogenesis and resistance in plants.

Nematodes comprise a large phylum of animals that includes plant and animal parasites as well as many free-living species (Maggenti, 1981). Plant parasitic nematodes are obligate parasites, obtaining nutrition only from the cytoplasm of living plant cells. These tiny roundworms (generally 4 mm long and barely visible to the human eye) damage food and fiber crops throughout the world and cause billions of dollars in losses annually (Sasser and Freckman, 1987). Some plant parasitic nematodes are ectoparasites, living outside their host. These species cause severe root damage and can be important virus vectors (Brown et al., 1995). Other species spend much of their lives inside roots as migratory or sedentary endoparasites. Migratory parasites move through the root, causing massive cellular necrosis. However, it is the sedentary endoparasites of the family Heteroderidae that cause the most economic damage worldwide. This group is the focus of the review. The Heteroderidae can be divided into two groups: the cyst nematodes, which include the genera Heterodera and Globodera; and the root-knot nematodes (genus Meloidogyne). The soybean cyst nematode (Heterodera glycines) is the most economically important pathogen of soybean in the United States. Potato cyst nematodes (Globodera pallida and G. rosfochiensis) cause losses in potato-growing areas worldwide (Ross, 1986). Root-knot nematodes, so-called for the characteristic root galls or root knots that they form on many hosts (Figure lA), infect thousands of plant species and cause sevele losses in yield of many crops throughout the world (Mai, 1985). Symptoms of diseased plants infected by these groups of nematodes include stunted growth, wilting, and susceptibility to other pathogens. Nematodes in these three genera have complex interactions with their host plants that generally last more than a month and result in major morphological and developmental changes in both organisms. During the infection, elaborate developmental and morphological changes occur in host root cells, especially in those that become the feeding cells that provide the sole source of nutrients for the nematode. There has been a recent burst of activity in the investigation of the molecular changes that mediate the host-parasite interaction. This ac-

Sequence and genetic map of Meloidogyne hapla: A compact nematode genome for plant parasitism

We have established Meloidogyne hapla as a tractable model plant-parasitic nematode amenable to forward and reverse genetics, and we present a complete genome sequence. At 54 Mbp, M. hapla represents not only the smallest nematode genome yet completed, but also the smallest metazoan, and defines a platform to elucidate mechanisms of parasitism by what is the largest uncontrolled group of plant pathogens worldwide. The M. hapla genome encodes significantly fewer genes than does the free-living nematode Caenorhabditis elegans (most notably through a reduction of odorant receptors and other gene families), yet it has acquired horizontally from other kingdoms numerous genes suspected to be involved in adaptations to parasitism. In some cases, amplification and tandem duplication have occurred with genes suspected of being acquired horizontally and involved in parasitism of plants. Although M. hapla and C. elegans diverged >500 million years ago, many developmental and biochemical pathways, including those for dauer formation and RNAi, are conserved. Although overall genome organization is not conserved, there are areas of microsynteny that may suggest a primary biological function in nematodes for those genes in these areas. This sequence and map represent a wealth of biological information on both the nature of nematode parasitism of plants and its evolution.

The Root-Knot Nematode Resistance Gene Mi-1.2 of Tomato Is Responsible for Resistance Against the Whitefly Bemisia tabaci

The tomato gene Mi-1.2 confers resistance against root-knot nematodes and some isolates of potato aphid. Resistance to the whitefly Bemisia tabaci previously has been observed in Mi-bearing commercial tomato cultivars, suggesting that Mi, or a closely linked gene, is responsible for the resistance. The response of two biotypes of B. tabaci to tomato carrying the cloned Mi was compared with that of the isogenic untransformed tomato line Moneymaker. Our results indicate that Mi-1.2 is responsible for the resistance in tomato plants to both B- and Q- biotypes. Mi-1.2 is unique among characterized resistance genes in its activity against three very different organisms (root-knot nematodes, aphids, and whiteflies). These pests are among the most important on tomato crops worldwide, making Mi a valuable resource in integrated pest management programs.

Plant-nematode interactions.

Root-knot nematodes and cyst nematodes are obligate, biotrophic pathogens of numerous plant species. These organisms cause dramatic changes in the morphology and physiology of their hosts. The molecular characterization of induced plant genes has provided insight into the plant processes that are usurped by nematodes as they establish their specialized feeding cells. Recently, several gene products have been identified that are secreted by the nematode during parasitism. The corresponding genes have strong similarity to microbial genes or to genes that are found in nematodes that parasitize animals. New information on host resistance genes and nematode virulence genes provides additional insight into this complex interaction.

Evidence for a Role of the N Terminus and Leucine-Rich Repeat Region of the Mi Gene Product in Regulation of Localized Cell Death

The tomato Mi gene confers resistance against root-knot nematodes and potato aphids. Chimeric constructs of the functional gene, Mi-1.2, with a homolog, Mi-1.1, were produced, and their phenotypes were examined in Agrobacterium rhizogenes–transformed roots. Exchange of the leucine-rich repeat (LRR) region of Mi-1.1 into Mi-1.2 resulted in the loss of ability to confer nematode resistance, as did substitution of a 6–amino acid sequence from the Mi-1.1 LRR into Mi-1.2. Introduction of the Mi-1.2 LRR-encoding region into Mi-1.1 resulted in a lethal phenotype, as did substitution of the fragment encoding the N-terminal 161 amino acids of Mi-1.1 into Mi-1.2. Transient expression of the latter two chimeric constructs in Nicotiana benthamiana leaves produced localized cell death. The cell death caused by the N-terminal exchange was suppressed by coinfiltration with a construct expressing the N-terminal 161 amino acids of Mi-1.2. The phenotypes of these and other constructs indicate that the LRR region of Mi-1.2 has a role in signaling localized cell death and that the N-terminal 161 amino acids have a role in regulating this death.

A PCR-based marker tightly linked to the nematode resistance gene, Mi, in tomato

A PCR-based codominant marker has been developed which is tightly linked to Mi, a dominant genetic locus in tomato that confers resistance to several species of root-knot nematode. DNA from tomato lines differing in nematode resistance was screened for random amplified polymorphic DNA markers linked to Mi using decamer primers. Several markers were identified. One amplified product, REX-1, obtained using a pair of decamer primers, was present as a dominant marker in all nematode-resistant tomato lines tested. REX-1 was cloned and the DNA sequences of its ends were determined and used to develop 20-mer primers. PCR amplification with the 20-mer primers produced a single amplified band in both susceptible and resistant tomato lines. The amplified bands from susceptible and resistant lines were distinguishable after cleavage with the restriction enzyme Taq I. The linkage of REX-1 to Mi was verified in an F2 population. This marker is more tightly linked to Mi than is Aps-1, the currently-used isozyme marker, and allows screening of germplasm where the linkage between Mi and Aps-1 has been lost. Homozygous and heterozygous individuals can be distinguished and the procedure can be used for rapid, routine screening. The strategy used to obtain REX-1 is applicable to obtaining tightly-linked markers to other genetic loci. Such markers would allow rapid, concurrent screening for the segregation of several loci of interest.

Genetic and physical localization of the root-knot nematode resistance locus mi in tomato.

Abstract As part of a map-based cloning strategy designed to isolate the root-knot nematode resistance gene Mi, tomato F2 populations were analyzed in order to identify recombination points close to this economically important gene. A total of 21 089 F2 progeny plants were screened using morphological markers. An additional 1887 F2 were screened using PCR-based flanking markers. Fine-structure mapping of recombinants with newly developed AFLP markers, and RFLP markers derived from physically mapped cosmid subclones, localized Mi to a genomic region of about 550 kb. The low frequency of recombinants indicated that recombination was generally suppressed in these crosses and that crossovers were restricted to particular regions. To circumvent this problem, a population of Lycopersicon peruvianum, the species from which Mi was originally introgressed, that was segregating for resistance was developed. Screening of this population with PCR, RFLP and AFLP markers identified several plants with crossovers near Mi. Recombination frequency was approximately eight-fold higher in the Mi region of the L. peruvianum cross. However, even within the wild species cross, recombination sites were not uniformly distributed in the region. By combining data from the L. esculentum and L. peruvianum recombinant analyses, it was possible to localize Mi to a region of the genome spanning less than 65 kb.

Salicylic Acid Is Part of the Mi-1-Mediated Defense Response to Root-Knot Nematode in Tomato

The Mi-1 gene of tomato confers resistance against three species of root-knot nematode in tomato (Lycopersicon esculentum). Transformation of tomato carrying Mi-1 with a construct expressing NahG, which encodes salicylate hydroxylase, a bacterial enzyme that degrades salicylic acid (SA) to catechol, results in partial loss of resistance to root-knot nematodes. Exogenous SA was toxic to roots expressing NahG but not to control roots. This toxicity is most likely due to the production of catechol from SA, and we report here that 100 microM catechol is toxic to tomato roots. Benzothiadiazole, a SA analog, completely restores nematode resistance in Mi-1 roots transformed with NahG but does not confer resistance to susceptible tomato roots. The localized cell death produced by transient expression in Nicotiana benthamiana of Mi-DS4, a constitutively lethal chimera of Mi-1 with one of its homologs, was prevented by coexpression of NahG. These results indicate that SA is an important component of the signaling that leads to nematode resistance and the associated hypersensitive response.

Mapping a new nematode resistance locus in Lycopersicon peruvianum

Accessions of the wild tomato species L. peruvianum were screened with a root-knot nematode population (557R) which infects tomato plants carrying the nematode resistance gene Mi. Several accessions were found to carry resistance to 557R. A L. peruvianum backcross population segregating for resistance to 557R was produced. The segregation ratio of resistant to susceptible plants suggested that a single, dominant gene was a major factor in the new resistance. This gene, which we have designated Mi-3, confers resistance against nematode strains that can infect plants carrying Mi. Mi-3, or a closely linked gene, also confers resistance to nematodes at 32°C, a temperature at which Mi is not effective. Bulked-segregant analysis with resistant and susceptible DNA pools was employed to identify RAPD markers linked to this gene. Five-hundred-and-twenty oligonucleotide primers were screened and two markers linked to the new resistance gene were identified. One of the linked markers (NR14) was mapped to chromosome 12 of tomato in an L. esculentum/L. pennellii mapping population. Linkage of NR14 and Mi-3 with RFLP markers known to map on the short arm of chromosome 12 was confirmed by Southern analysis in the population segregating for Mi-3. We have positioned Mi-3 near RFLP marker TG180 which maps to the telomeric region of the short arm of chromosome 12 in tomato.