A truss of vine-ripened tomatoes advertises itself with an earthy perfume that says “fresh” better than any words. It emanates from tiny hair-like projections on the stems of the fruit, called trichomes.

Trichomes adorn many species in the tomato-potato family, the Solanaceae, and function as biochemical factories that synthesise and variously secrete or store a huge variety of defensive compounds to deter leaf-chewing insect pests and pathogens.

Around a third of the world’s vascular plants have trichomes – they have evolved independently in multiple families. Some of the best known are the aromatic mint family, the Lamiaceae, of rosemary and thyme fame; the Boraginaceae (comfrey, borage, echium), beloved of herbalists; and the Geraniaceae, whose leaves emit scents redolent of roses, apples, lemons, oranges, cinnamon and peppermint.

Trichomatous plants fascinate Professor Rob Last, a biochemist at Michigan State University and a lead researcher in the Solanum Trichome Project. Last will give a plenary lecture on the project at ComBio 2008 in Canberra from September 21 to 25.

The Solanum Trichome Project is focused around the tomato, Solanum lycopersicon, and four of its wild relatives from the same region of the Andes where the tomato was domesticated. Solanum habrochaites, S. pennellii, S. pimpinellifolium, and S. peruvianum have all contributed genes to the multi-hued cornucopia of tomatoes available to consumers today.

The project is employing a genetic approach to investigate trichome development and the biochemical pathways that operate in them.

Tomatoes were selected for their convenience as research subjects, rather than for potentially exciting biochemistry. “It’s not that tomatoes have the most interesting secondary or specialised chemistry – it that was our interest, we would have picked artemisins,” Last says.

[The genus Artemisia (Asteraceae) includes wormwood, source of the once popular but toxic liqueur absinthe. The traditional Chinese pharmacopoeia has long recognised wormwood’s anti-malarial properties; Western researchers are now interested in artemisin as a model for a new class of antimalarial drugs].

Tomatoes were the subject of choice because the Tomato Genome Project is making good progress, the emerging chromosomal map and gene catalogue simplifies the task of identifying the genes involved in synthesising the compounds in trichomes, and dissecting the metabolic pathways that generate them, he says.

The five Solanum species chosen for the trichome project are conveniently compatible, and their hybrids produce viable seeds. Researchers can study patterns of inheritance, and import genes of interest. Tomatoes are also amenable to experiments involving substituting chromosomal segments, importing transgenes, or silencing genes in situ.