Host plant root exudates reduce phytonematode cuticle aging to maintain Pasteuria endospore attachment

Abstract

Endospores of the nematode hyperparasite from the Pasteuria group of Gram-positive bacteria have the potential to suppress phytonematode populations and reduce root infection. Parasitism by the bacteria commences when endospores adhere to the nematode cuticle before they germinate and infect the developing nematode thereby prohibiting the nematode from producing eggs. Here we demonstrate that in a comparison between a Pasteuria endospore population that infects root-knot nematodes (Meloidogyne incognita) and an endospore population that infects pigeon pea cyst nematodes (Heterodera cajani), as the nematodes age there is a similar reduction in endospore encumbrance on both nematode species. To test the hypothesis that multitrophic evolutionary theory would suggest that plants might recruit bacteria that reduce phytonematode root parasitism, we exposed the two phytonematode species to root exudates of hosts, poor hosts and non-hosts plants and undertake standardised endospore encumbrance bioassays over a two-week aging period. The results showed that the normally expected reduction in endospore attachment due to aging was reduced when root-knot nematodes were exposed to root exudates from their host plants, cowpea and tomato, but not potato, whereas in the case of pigeon pea cyst nematode, the root-exudates from the homologous host plant, cowpea, also reduced the expected reduction in the decrease in endospore attachment due to aging. However, unexpectedly similar results were also produced by root exudates of the non-host, potato. These results are discussed in the light of the characterisation of the root-exudates by gas-liquid chromatography. It is concluded that signalling compounds present in root-exudates can affect cuticle aging both positively and negatively that in turn affects endospore attachment. However, the results are not strictly in line with what multitrophic interactions would have predicted.