Ectopic expression of the mycorrhiza-specific chitinase gene Mtchit 3-3 in Medicago truncatula root-organ cultures stimulates spore germination of glomalean fungi.

Expression of Mtchit 3-3, a class III chitinase gene, is specifically induced by arbuscular mycorrhizal (AM) fungi in roots of the model legume Medicago truncatula and its transcripts accumulate in cells containing arbuscules. Agrobacterium rhizogenes-transformed roots and root-organ cultures of M. truncatula were used to study effects of Mtchit 3-3 on AM fungi. * This work provides evidence for enzymatic activity of the Mtchit 3-3 gene product and shows with promoter:gus fusions that a 2 kb fragment located 5' upstream from the translational start codon of Mtchit 3-3 is sufficient to confer arbuscule-dependent gene expression. By fusing the Mtchit 3-3 coding region to the CaMV 35S promoter the expression pattern was disrupted. Surprisingly, disruption stimulated spore germination of Glomus intraradices and Glomus constrictum, and in the case of G. intraradices resulted in a higher probability of root colonization and spore formation. However, no effect on the abundance of arbuscules within colonized roots became apparent. These observations demonstrate that disruption of the tight arbuscule-dependent expression pattern of Mtchit 3-3 has effects on the early interaction between roots and AM fungi. Copyright New Phytologist (2005).