Analysis of Medicago truncatula polyribosomal mRNAs revealed selective translational regulation of early nodulation genes

REYNOSO, MAURICIO; BLANCO, FLAVIO; BAILEY-SERRES; CRESPI, MARTÍN; ZANETTI, MARÍA EUGENIA

Sainte Maxime

Congreso; Model Legumes 2011; 2011

Transcriptome analyses are used to characterize the steady-state levels of mRNAs at different stages of nodule formation. However, the steady-state level of a given mRNA does not necessarily reflect its level of translation. We aim to characterize global changes in polyribosome-associated mRNAs (referred to as translatomes) in roots of the model legume Medicago truncatula inoculated with its symbiotic partner Sinorhizobium meliloti. Purification of polyribosome was achieved either by conventional fractionation using sucrose density gradients or by immuno-purification using root tissues expressing a FLAG-tagged ribosomal protein (RP) L18 (Zanetti et al. 2005. Plant Physiol. 138:624). Rhizobia inoculation did not induce a major global change in the root translatome.  However, RT-qPCR experiments using total RNA and conventional- or immuno-purified polyribosomal RNA from mock and inoculated root revealed major translational regulations of individual mRNAs. Transcripts of CRE1 showed only a slight increase (~ 1.3 fold) in response to S. meliloti on total RNA as shown by Lohar et al. 2006 (Plant Physiol.140:221). In contrast, the abundance of CRE1 transcripts associated with polyribosomes of S. meliloti inoculated roots was about four times higher than that of mock-inoculated roots, indicating a change in the translational status of this transcript in response to rhizobia. On the other hand, ENOD40 mRNAs showed a much higher increase in the non-polyribosomal fraction (~7 folds) than in the polyribosomal fraction (~2.5 folds) in inoculated versus non-inoculated roots, revealing that the percentage of ENOD40 mRNAs engaged in translation decreased in response to S. meliloti. These results indicated that changes in translational status of individual mRNAs species in M. truncatula roots occur at early stages of nodule formation. Current experiments are being conducted to explore translational changes at whole genome scale combining the immunopurication of translatomes with RNA sequencing technology.