Arbuscular mycorrhiza in mini-mycorrhizotrons: first contact of Medicago truncatula roots with Glomus intraradices induces chalcone synthase

The association of plants with arbuscular mycorrhizal (AM) fungi is widespr ead in nature, but little is known about molecular aspects of this symbiosi s. Particularly during the early stages of the AM symbiosis, it is difficul t to monitor growth of the two partners, to dissect gene expression pattern s and to correlate them with plant, fungal or symbiosis development. A new system, the 'mini-mycorrhizotron', was established to cultivate seedl ings of Medicago truncatula in mycorrhizal symbiosis with Glomus intraradic es under gnotobiotic conditions. This system allows natural growth of the s ymbiotic partners and permits the continuous noninvasive observation of the development of plant and fungus under a microscope. The mini-mycorrhizotron was used to determine the stage of induction of a m ycorrhiza-related gene detected by differential display-reverse transcripti on-PCR, namely a novel chalcone synthase (Mf-chs1). The gene is induced in roots at the stage of the first fungal contact. The mini-mycorrhizotron allowed identification and cloning of a symbiosis-r elated gene, and the correlation between its expression and the development al stage of the symbiosis was established. This provides a useful tool for molecular and developmental studies of the early stages of AM symbioses. (C ) New Phytologist (2001).