Tomato: a crop species amenable to improvement by cellular and molecular methods

Tomato is a crop plant with a relatively small DNA content per haploid genome and a well developed genetics. Plant regeneration from explants and protoplasts is feasable which led to the development of efficient transformation procedures. In view of the current data, the isolation of useful mutants at the cellular level probably will be of limited value in the genetic improvement of tomato. Protoplast fusion may lead to novel combinations of organelle and nuclear DNA (cybrids), whereas this technique also provides a means of introducing genetic information from alien species into tomato. Important developments have come from molecular approaches. Following the construction of an RFLP map, these RFLP markers can be used in tomato to tag quantitative traits bred in from related species. Both RFLP's and transposons are in the process of being used to clone desired genes for which no gene products are known. Cloned genes can be introduced and potentially improve specific properties of tomato especially those controlled by single genes. Recent results suggest that, in principle, phenotypic mutants can be created for cloned and characterized genes and will prove their value in further improving the cultivated tomato.

The parasitic plant genome project: New tools for understanding the biology of <em>Orobanche</em> and <em>Striga</em>.

The Parasitic Plant Genome Project has sequenced transcripts from three parasitic species and a nonparasitic relative in the Orobanchaceae with the goal of understanding genetic changes associated with parasitism. The species studied span the trophic spectrum from free-living nonparasite to obligate holoparasite. Parasitic species used were Triphysaria versicolor, a photosynthetically competent species that opportunistically parasitizes roots of neighboring plants; Striga hermonthica, a hemiparasite that has an obligate need for a host; and Orobanche aegyptiaca, a holoparasite with absolute nutritional dependence on a host. Lindenbergia philippensis represents the closest nonparasite sister group to the parasitic Orobanchaceae and was included for comparative purposes. Tissues for transcriptome sequencing from each plant were gathered to identify expressed genes for key life stages from seed conditioning through anthesis. Two of the species studied, S. hermonthica and O. aegyptiaca, are economically important weeds and the data generated by this project are expected to aid in research and control of these species and their relatives. The sequences generated through this project will provide an abundant resource of molecular markers for understanding population dynamics, as well as provide insight into the biology of parasitism and advance progress toward understanding parasite virulence and host resistance mechanisms. In addition, the sequences provide important information on target sites for herbicide action or other novel control strategies such as trans-specific gene silencing

Chemical signalling between plants: mechanistic similarities between phytotoxic allelopathy and host recognition by parasitic plants

Parasitic plants in the Orobanchaceae use chemicals released from host-plant roots to direct developmental processes crucial to their heterotrophic lifestyle. An illustrative example is the development of haustoria; parasite root organs that function in host attachment and penetration, and in the establishment of a physiological conduit through which host resources are robbed. The facultative parasite Triphysaria develops haustoria only in the presence of host roots or host root factors. An in vitro assay was used to identify several phenolic derivatives that induce haustorium formation; the activity of multiple signalling molecules is consistent with a redundancy of active molecules in the rhizosphere triggering haustorium development. Haustorium-inducing factors are structurally related to phytotoxic allelochemicals released by some plants to inhibit the growth of neighbouring plants. We used genomic approaches to demonstrate that similar genetic pathways are up-regulated in parasitic roots upon contact with host plants as are regulated in response to allelochemical exposure. A parasite quinone oxidoreductase was identified that has properties suggesting that it functions in both allelochemical detoxification and haustorium signal transduction. These and other mechanistic similarities between allelopathic toxicity and haustorium signal transduction support the hypothesis that parasitic plants have recruited allelotoxin defence mechanisms for host-plant recognitio

Use of the maize transposons activator and dissociation to show that phosphinothricin and spectinomycin resistance genes act non-cell-autonomously in tobacco and tomato seedlings

Cell-autonomous genes have been used to monitor the excision of both endogenous transposons in maize and Antirrhinum, and transposons introduced into transgenic plants. In tobacco and Arabidopsis, the streptomycin phosphotransferase (SPT) gene reveals somatic excision of the maize transposon Activator (Ac) as green sectors on a white background in cotyledons of seedlings germinated in the presence of streptomycin. Cotyledons of tomato seedlings germinated on streptomycin-containing medium do not bleach, suggesting that a different assay for transposon excision in tomato is desirable. We have tested the use of the spectinomycin resistance (SPEC) gene (aadA) and a Basta resistance (BAR) gene (phosphinothricin acetyltransferase, or PAT) for monitoring somatic excision of Ac in tobacco and tomato. Both genetic and molecular studies demonstrate that genotypically variegated individuals that carry clones of cells from which Ac or Ds have excised from either SPEC or BAR genes, can be phenotypically completely resistant to the corresponding antibiotic. This demonstrates that these genes act non-cell-autonomously, in contrast to the SPT gene in tobacco. Possible reasons for this difference are discussed