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Identification of 14-3-3 proteins, putative pathogenicity factors from root knot nematodes

A large scale procedure was set up in order to purify stylet secreted proteins from M. incognita infective juveniles after secretion induction in vitro. The secreted proteins whose pI ranged from 5,5 to 7,0 were analysed by 2D-electrophoresis. Proteins were identified by trypsin digestion and mass spectrometry. A 14-3-3 protein was among the most abundant proteins.

The 14-3-3 protein identified in Meloidogyne incognita stylet secretions was further analysed. 14-3-3 proteins are involved in signal transduction (e.g. activation or inhibition of protein kinases), in the control of the cell-cycle (e.g. by sequestering cdc25 in the cytoplasm), stress responses and cytoskeleton organisation (e.g regulation of Rho GTPases). From the 14-3-3 peptide sequences, two coding sequences were obtained, which correspond to two 14-3-3 isoforms: Mi-14-3-3-a and Mi-14-3-3-b. The tissue localisation of Mi-14-3-3-a and b transcription was analysed by in situ hybridisation on freshly hatched L2s with specific DNA probes. Mi-14-3-3-a transcription was observed exclusively in primordium germinal cells, whereas Mi-14-3-3-b transcripts were detected specifically in the dorsal oesophageal gland cell.

The peptide sequence AFDDAIAE identified by mass spectrometry in the secreted 14-3-3 isoform was present in the deduced amino acid sequence of MI-14-3-3-B and absent from that of MI-14-3-3-A. This is consistent with the expression pattern of Mi-14-3-3-b in the dorsal oesophageal gland of infective juveniles.In order to investigate whether the secreted 14-3-3 could be targeted to a particular subcellular compartment of the plant cell or could play a role in the sequestering of plant cell factors, we analysed the localisation of the nematode 14-3-3s in tobacco BY2 cells. MI-14-3-3-A and MI-14-3-3-B fused at their N-terminal end to EGFP (enhanced green fluorescent protein) were expressed in BY2 cells and localised by confocal microscopy. Both EGFP-MI-14-3-3 fusion proteins exhibited a strong EGFP signal in the cytoplasm of the BY2 cells and a weak signal in the nucleus. The relative weakness of the signal observed in the nucleus as compared to the signal obtained with the diffusing free EGFP could result from the exclusion of the fusion protein from the nucleus.

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