Global analyses of the symbiotic cell programme in arbuscular mycorrhiza

Vivienne Gianinazzi-Pearson, Diederik van Tuinen, Laurent Brechenmacher, Silvio Gianinazzi
UMR 1088 INRA/Université de Bourgogne BBCE-IPM, INRA-CMSE, BP 86510, 21065 DIJON CEDEX, France

In arbuscular mycorrhiza, which represent the most widespread root symbiosis formed by terrestrial plant taxa, plant-fungal compatibility must have been established very early on since the arbuscule-forming fungi have been found within fossil plants dating back to the Devonian era. The fact that the reciprocal compatibility systems developed in these primitive land plants have been acquired by new plant species appearing during evolution means that arbuscular mycorrhiza interactions should be based on widespread molecular mechanisms within the plant kingdom. Analyses of the symbiotic genetic programme governing arbuscular mycorrhiza implies firstly identifying the symbiotic transcriptome, and then isolating the corresponding genes in order to determine their role in arbuscular mycorrhiza formation and function. We have adopted different approaches to identify that part of the functional genome in the symbiont partners which is active in arbuscule mycorrhiza interactions. A number of plant genes with modulated expression in the symbiosis have been identified by targeting genes active in other plant-microbe interactions or in other physiological processes1. These, together with those reported by other groups, belong essentially to categories related to defense-related functions, nodulation cell programmes and transmembrane transport processes.
In order to gain a more global insight into the functional genome in arbuscular mycorrhiza and to identify genes specific to the symbiotic cell programme, we have undertaken more extensive searches for fungal and plant gene expression profiles during symbiosis development and functioning using non targeted strategies. The first, based on differential RNA expression analyses using DDRT-PCR2, has enabled identification of five novel pea genes (Psam1 - 5) with modified expression in root-AM fungal interactions. More recently, we have adopted alternative methods of transcriptome analysis based on large scale sequencing of cDNA from symbiotic and non-symbiotic root tissues3, and on suppressive subtractive cDNA analysis of mycorrhizal and non-mycorrhizal roots4 at different stages of symbiosis development (research part of EU QLG2-CT-2000-00676 and INRA-ATS projects on Medicago). These strategies have led to a significant increase in the number of genes known to be up-regulated in AM interactions, and the majority represent novel plant or fungal genes. However, the majority of the genes are of plant origin and these different untargeted approaches have so far identified only a very limited number of expressed fungal genes.
1Gianinazzi-Pearson, V. (1996) The Plant Cell 8, 1871-1883.
2Martin-Laurent F., van Tuinen D., Dumas-Gaudot E., Gianinazzi-Pearson V., Gianinazzi, S. and Franken, P. (1997) Molecular General Genetics 256, 37-44.
3Journet E. P., Crespeau H., van Tuinen D., Gouzy J., Jaillon O., Niebel A., Carreau V,. Chatagnier O., Kahn D., Gianinazzi-Pearson V. and Gamas P. (2000). Molecular Genetics of Model Legumes. Impact for Legume Biology and Breeding, John Innes Centre, Norwich, UK.
4Brechenmacher L., Weidmann S., van Tuinen D., Chatagnier O., Franken P. and Gianinazzi-Pearson V. (2001). 10th International Congress on Molecular Plant-Microbe Interactions, University of Wisconsin, Madison, WI, USA.