Analysis of the Role of Galactinol in Nematode Induced Syncytia

Cyst nematodes are important pathogens on many crop plants and can result in considerable crop loss. The interaction between Arabidopsis thaliana roots and the beet cyst nematode Heterodera schachtii has been established as a model system. In a previous project we found that all 4 genes coding for myo-inositol oxygenase are strongly expressed in syncytia. Myo- inositol is a precursor for galactinol and reducing the level of myo-inositol also leads to a reduced level of galactinol. In this project we want to study the role of galactinol for the resistance of plants against cyst nematodes. Our hypothesis is that a high level of galactinol in syncytia leads to enhanced resistance against cyst nematodes. We will produce transgenic Arabidopsis lines with high or low levels of galactinol and study their resistance. In parallel we will perform a transcriptome analysis of syncytia with a high galactinol level. This should indicate which genes might be responsible for the enhanced resistance of plants with a high level of galactinol in syncytia. Our work might provide new a new approach for breeding nematode resistant plants or for the development of agrochemicals against plant pathogenic nematodes. Furthermore, plants with a high or low level of galactinol would also be interesting tools to study the role of galactinol for abiotic stress resistance.

In this project we studied the role of galactinol for the resistance of plants against cyst nematodes. The interaction between Arabidopsis thaliana roots and the beet cyst nematode Heterodera schachtii has been established as a model system. Our hypothesis was that a high level of galactinol in syncytia leads to enhanced resistance against cyst nematodes. We thus produced transgenic Arabidopsis lines with high or low levels of galactinol to study their resistance. In addition, we also analysed their reaction against abiotic stress. We constructed a double mutant for 2 galactinol synthases that are expressed in syncytia which had a low level of galactinol. Lines with a higher galactinol level were obtained in two ways. A double mutant for 2 raffinose synthases had no raffinose but a very high galactinol level. By expressing myo-iositol phosphate synthase (MIPS-OE) we obtained lines with higher levels of myo-inositol and galactinol. A metabolome analysis of syncytia, roots and seedlings in cooperation with Prof. Weckwerth (University of Vienna) confirmed the above mentioned metabolite levels. In addition to that no obvious increase or decrease in specific metabolites was found. A transcriptome analysis with RNAseq revealed that in MIPS-OE lines PR-genes including a plant defensin were upregulated. These lines were more resistant to Botrytis cinerea. If these lines are also more resistant to nematodes has to be further studied because of a high variation in myo-inositol and galactinol levels in these lines. We also found a better germination with NaCl (125 mM), NaHCO3 (125 mM) or sorbitol (350 mM). The raffinose synthase double mutant showed a delayed germination in the dark but not in the light which needs to be further studied. This mutant also had an increased resistance against nematodes. As expected, the galactinol synthase double mutant had a lower level of galactinol. However, it also was more resistant against nematodes which is in contrast to the galactinol level. An explanation might be the differences in gene expression. A few genes were significantly downregulated which might be involved in phytoalexin production. This has to be further studied in the future.