Submitted to: Molecular Biology and Evolution
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/14/2008
Publication Date: 10/1/2008
Citation: Craig, J.P., Bekal, S., Hudson, M., Domier, L.L., Niblack, T., Lambert, K.N. 2008. Analysis of a Horizontally Transferred Pathway Involved in Vitamin B6 Biosynthesis from the Soybean Cyst Nematode Heterodera glycines. Molecular Biology and Evolution. 25(10):2085-2098. doi: 10.1093/molbev/msn141. Interpretive Summary: Soybean cyst nematodes are small parasitic worms that cause one of the most damaging diseases of soybean. The worms burrow into soybean roots and induce formation of specialized cells that synthesize nutrients for the nematodes to the detriment of the host plants. If the interactions between nematodes and soybean roots could be disrupted, the nematodes would not be able to cause disease. Unfortunately, the interactions that lead to the formation of the specialized feeding cells in soybean roots are not known. To begin to identify nematode genes that might be involved in these interactions, the sequence of the genome of soybean cyst nematode was determined and compared to sequences of plants, bacteria and non-parsitic nematodes. These analyses identified two genes in soybean cyst nematodes for the synthesis of vitamin B6 that appear to have come from bacteria and are absent in non-parasitic nematodes. The function of the genes in the life cycle of soybean cyst nematodes remains to be determined, but vitamin B6 participates in over 140 different enzymatic reactions and also could have a role in suppressing soybean defense responses that allow the nematodes to invade and seize control soybean roots unscathed. This research will be of interest to other scientist working to limit the impact of soybean cyst nematodes on soybean production and those interested in studying the interactions of plant and disease causing agents.
Technical Abstract: Heterodera glycines is an obligate plant parasite capable of biochemically and developmentally altering its host's cells in order to create a specialized feeding cell. Although the exact mechanism of feeding cell morphogenesis remains a mystery, the nematode's ability to manipulate the plant is thought to be due in part to horizontally transferred genes (HGT). A bioinformatic screen of the nematode genome has revealed homologues of the genes SNZ and SNO, which comprise a metabolic pathway for the de novo biosynthesis of pyridoxal 5’phosphate (PLP), the active form of vitamin B6. Analysis of the two genes, HgSNZ and HgSNO, show that they contain nematode-like introns, generate polyadenylated mRNAs, and map to the SCN genetic linkage map, indicating they are part of the nematode genome. However, gene synteny, protein homology, and phylogenetic evidence suggest prokaryotic origin. This would represent the first case of the horizontal gene transfer of a complete pathway into a nematode or terrestrial animal. Vitamin B6 acts as a cofactor in over 140 different enzymes and recent studies point towards an important role as a potent quencher of reactive oxygen species (ROS). With H. glycines’ penchant for acquiring parasitism genes through HGT along with the absence of this pathway in other land-based animals suggests a specific need for vitamin B6 which may involve the parasite-host interaction.