Tomato and related species in the Solanum produce a variety of trichomes on the surface of leaves, stems, and reproductive structures (Fig. 1). Luckwill’s taxonomic survey of cultivated and wild tomato species documented four major types of SGTs (in addition to non-glandular trichomes) on various members of the genus (Luckwill, 1943). These include: type I trichomes characterized by a long multicellular stalk and a small glandular vesicle at the tip; type IV trichomes, which contain a shorter multicellular stalk and small gland at the tip; type VI trichomes consist of a 4-celled glandular head and a short multicellular stalk; and type VII trichomes, which contain a unicellular stalk and a multicellular glandular head. [Luckwill] Luckwill concluded that the diversity of trichome habit (type, density, etc.) within the genus is among the most valuable diagnostic traits for taxonomic classification of individual species. We anticipate that this variation will facilitate our long-term goal of understanding the evolution of SGT chemistry and function in Solanum species. The occurrence of multiple types of SGTs within a single species provides a unique opportunity to understand the regulation of development of each class of trichome, and identify the major biosynthetic pathways operating in each type.
All plants make natural products that serve specific roles in the interactions of the plant with biotic and abiotic agents found in its environment, and the Solanaceae includes many examples of such specialized metabolites. Well-known examples are atropine in nightshade (Atropa belladonna), scopolamine in Hyoscyamus species, and nicotine in tobacco (Nicotiana tabacum), three potent alkaloids that have had a long and significant human usage. Tomato plants contain steroidal glycoalkaloids of the tomatine class; these are believed to act as toxic defense compounds and are found everywhere in the plant, including the fruit and roots, and in SGTs. The metabolic pathways involved in the biosynthesis of most Solanaceae secondary compounds are not well understood. Some progress has been made in identifying some (but not all) of the enzymes involved in the ultimate steps of the synthesis of acylsugars, terpenoids, and methylketones. However, it is clear that each of these pathways, as well as the steroid alkaloids, requires multiple steps to produce the final products, and that several pathways of primary metabolism may converge in the production of these compounds.
Why study trichomes in the Solanum?
We are taking an integrated approach to lay the foundation for a longer-term ‘systems biology’ understanding of the entire network of genes and proteins involved in the development of each of the different types of SGT found inSolanum plants and the genes and enzymes responsible for their biosynthetic capacity. Tomato and other solanaceous species have strong advantages for the study of such complex biological processes. These are important domestic and international crops (examples include tomato, pepper, potato, eggplant, and tobacco), offering opportunities for translation of our basic research to economically important species and varieties. There is a robust and growing SOL community that uses tomato as a genomics reference species for the study of many related species. This cross-referencing of genomic and genetic resources is facilitated by strong genome conservation at the macro and micro levels, especially within the Solanaceae. Species within the Solanum are inter-fertile, and this has led to the creation of a variety of powerful resources for genetics and breeding. Functional genomics in the Solanum will greatly benefit from projects aimed at sequencing large regions of the tomato and potato genomes (http://www.sgn.cornell.edu/about/tomato_sequencing.pl). Finally, there is a community database, the SOL Genomics Network (SGN), which serves as a data repository and communication mechanism for the SOL community (http://www.sgn.cornell.edu/index.pl).
Our long term goal is to understand the regulation of development and metabolic activities of a wide variety of secreting and glandular trichomes within Solanum and rigorously test their biological functions.
Our specific objectives are to:
- Characterize the morphology and chemical composition of SGTs from cultivated and wild Solanum species. This will be enabled by development of rapid and accurate analytical chemistry methods for the major classes of metabolites, and their biosynthetic precursors. These studies will serve as a basis for all other activities.
- Use genetics to identify proteins that regulate development of distinct SGT types and/or specify the range of metabolites that accumulate in each type. Phenotypic analysis of these mutants and introgression lines will help create models for how these complex biological processes occur and allow map-based identification of genes that have central roles in SGT morphogenesis and regulation of metabolism.
- Exploit SGT cDNA sequencing, data mining and biochemistry to identify genes and enzymes involved in the biosynthesis of our target secondary metabolic pathways.
- Create databases and web interfaces that will allow tracking of samples and capture and analysis of data.
- Educate current and future plant scientists, and K-12 teachers, in integrative approaches to complex biological processes.
Luckwill, L. The genus Lycopersicon: historical, biological, and taxonomic survey of the wild and cultivated tomatoes; Aberdeen University Press, Aberdeen, Scotland, 1943