Régulation de la Sénescence Des Nodules Chez Medicago Truncatula

Résumé du livre

Leguminous plants are able to associate with rhizobium to develop de novo a root organ, the root nodule. Root nodule can reduce atmosphere nitrogen into available nitrogen to plant host. Thus, the nitrogen-fixing symbiosis plays important roles in agriculture with direct nitrogen inputs to crops. In the first part of the thesis, we intended to characterize the impact of the bacterial GSH deficiency on the bacteroid differentiation and the nodule functioning during the symbiotic interaction between Medicago truncatula and Sinorhizobium meliloti. Physiological, biochemical, cellular and genetic markers were used to describe the nodule functioning at ten and twenty days after plant inoculation. Our results showed that the bacterial GSH deficiency does not affect bacteroid differentiation. However, it induces an early nodule senescence process in M. truncatula. During nodule senescence, proteolytic activities are increased terminated with the final degradation of bacteroids and plant cells. Hence, proteases are found to be the hallmarks of nodule senescence. At the onset of nodule senescence, a papain family cysteine protease, MtCP6, is a good molecular marker for the initiation of nodules senescence. In the second part of the thesis, serial promoter deletion analysis of a cysteine protease gene MtCP6 was conducted to identify cis regulatory element involved in the transcriptional regulation of nodule senescence of M. truncatula. Thereafter, identified cis-regulating region (67bp, NS) was validated to function in transcription activation in nodule zone III-IV. We have shown that tetramers of NS can increase the transcription into nodule senescence zone. In order to determine the significance of NS to the expression compared to the full promoter (-1,710bp), a functional analysis was conducted with deletion of NS from the full promoter and the analysis of a minimal promoter (-254bp). In addition, the potential roles of CAAT, WRKY and Dof motifs localized at 5' of NS sequence was validated by site-specific mutagenesis. Finally, yeast one hybrid experiment was performed to identify transcription factors interacting with NS DNA fragment. Preliminary resulrs are presented. Taken together, these data allow a better understanding of the regulation and the operation of the nodule senescence.