Function of MIOX genes in syncytia

Transcriptome analysis of syncytia induced by the cyst nematode Heterodera schachtii has shown that genes coding for myo-inositol-oxygenase (MIOX) are strongly expressed in syncytia. This enzyme is involved in a pathway that leads to the production of UDP-glucuronic acid but other possible functions have also been proposed. UDP- glucuronic acid is an important precursor for cell wall polysaccharides but can also be produced by a second pathway containing the enzyme UDP-glucose-dehydrogenase. In this project we will study the involvement of the MIOX pathway in the induction and maintenance of syncytia with the help of mutants and miRNA lines. The effect of downregulation of the MIOX pathway will be analysed with resistance tests, cell wall analysis (HPLC) and electron microscopy.

UDP-glucuronic acid, an important precursor for cell wall polysaccharides in Arabidopsis can be produced by two different pathways, the MIOX (myo-inositol oxygenase) and the UGD (UDP-glucose dehydrogenase) pathway. In syncytia induced by the beet cyst nematode Heterodera schachtii in Arabidopsis roots, all 4 MIOX genes are strongly expressed. A quadruple mutant with strongly reduced expression of all 4 MIOX genes is more resistant to H. schachtii. However, these plants are partly rescued by UGD since the expression of 2 of the 4 UGD genes is upregulated in miox quadruple mutants. This explains why we did not see any effect on syncytial cell walls in the miox quadruple mutant.For the production of cell walls in syncytia the UGD pathway seems to be more important. A ??ugd23 double mutant was unable to produce cell wall ingrowths in syncytia connected with female nematodes which leads to a reduced host suitability for H. schachtii, resulting in smaller syncytia, lower number of developing nematodes, and smaller females.The importance of the MIOX pathway for syncytium development, as indicated by the strong expression of all 4 MIOX genes in syncytia, on the other hand, is the removal of excess myo-inositol from syncytia. Myo-inositol is also a precursor for galactinol and we could show that higher levels of galactinol in syncytia are the reason for the higher resistance of the miox quadruple mutant.