The other side of the coin: systemic effects of Serendipita indica root colonization on development of sedentary plant-parasitic nematodes in Arabidopsis thaliana.

MAIN CONCLUSION: Upon systemic S. indica colonization in split-root system cyst and root-knot nematodes benefit from endophyte-triggered carbon allocation and altered defense responses what significantly facilitates their development in A. thaliana. Serendipita indica is an endophytic fungus that establishes mutualistic relationships with different plants including Arabidopsis thaliana. It enhances host's growth and resistance to different abiotic and biotic stresses such as infestation by the cyst nematode Heterodera schachtii (CN). In this work, we show that S. indica also triggers similar direct reduction in development of the root-knot nematode Meloidogyne javanica (RKN) in A. thaliana. Further, to mimick the natural situation occurring frequently in soil where roots are unequally colonized by endophytes we used an in vitro split-root system with one half of A. thaliana root inoculated with S. indica and the other half infected with CN or RKN, respectively. Interestingly, in contrast to direct effects, systemic effects led to an increase in number of both nematodes. To elucidate this phenomenon, we focused on sugar metabolism and defense responses in systemic non-colonized roots of plants colonized by S. indica. We analyzed the expression of several SUSs and INVs as well as defense-related genes and measured sugar pools. The results show a significant downregulation of PDF1.2 as well as slightly increased sucrose levels in the non-colonized half of the root in three-chamber dish. Thus, we speculate that, in contrast to direct effects, both nematode species benefit from endophyte-triggered carbon allocation and altered defense responses in the systemic part of the root, which promotes their development. With this work, we highlight the complexity of this multilayered tripartite relationship and deliver new insights into sugar metabolism and plant defense responses during S. indica-nematode-plant interaction.

Arbuscular mycorrhizal fungi and their response to pesticides.

Arbuscular mycorrhizal fungi (AMF) form symbioses with the majority of plant species and can provide multiple benefits to the host plant. In agro-ecosystems, the abundance and community structure of AMF are affected by agricultural management practices. This review describes and discusses current knowledge on the effects of inorganic and organic chemical pesticides on AMF in the conflicting area between agricultural use and environmental concerns. Variable effects have been reported following chemical pesticide use, ranging from neutral to positive and negative. Moreover, a species-specific reaction has been documented. The reported effects of pesticides on arbuscular mycorrhizal symbiosis are very diverse, and even when the same substance is investigated, the results are often contradictory. These effects depend on many parameters, such as the active substance, the mode of action, the mode of application and the dosage. In the field, determinants such as the physico-chemical behavior of the active substances, the soil type and other soil microorganisms contribute to the fate of pesticides and thus the amount of active substances to which AMF are exposed. This review highlights that the fate of AMF following pesticide use needs to be addressed in a broader agro-ecosystem context. © 2018 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.