Impact of mutualistic root fungi on crop quality and pest defense

The increasing global population brings major challenges to the human food supply, whereas modern agriculture is accompanied by several environmental problems and still leaves many people hungry or malnourished. A more sustainable production and a higher nutritional value of plant foods are therefore new agricultural paradigms. Part of the solution to enhance yield and nutritional value of crop foods in a more sustainable manner might be found in the overlooked rhizosphere, where ancient plant-microbe mutualisms are known to provide important ecological functions. These microbes interact actively with plant intrinsic regulators, which in turn have the potential not only to mediate plant-microbe interactions, but also to influence plant growth, health and quality. The present thesis had two main objectives: 1) to test novel effects of beneficial microbes on crop plants related with the new agricultural paradigms; and 2) to investigate the role of intrinsic plant regulators involved in microbial effects on crop plants. In chapter II, I tested whether the beneficial arbuscular mycorrhizal (AM) fungi can affect the aboveground oviposition of Lissorhoptrus oryzophilus (rice water weevil; RWW), a root-feeding insect of rice plants. Rice is one of the major staple foods worldwide. RWW is an important global pest of rice, whose adults feed aboveground and the larvae feed belowground. I found that AM fungi enhanced the aboveground oviposition by RWW, which is a novel aspect of agro-ecological interactions. This suggests that AM fungi can reduce rice resistance against RWW. Therefore, soil fungi that are generally considered beneficial in terms of nutrient uptake may not be beneficial in respect to protection against particular herbivores. In chapter III, I tested whether an endophyte can protect root against herbivory though gibberellic acid (GA) and jasmonic acid (JA) signaling in rice. In contrast to AM fungi, the Sebacinalean root endophyte Piriformospora indica attenuated the negative effects of RWW on growth through induced root tolerance, without affecting root resistance. This induced tolerance was mediated by induction of GA signaling and suppression of JA signaling. Thus, belowground plant-microbe mutualisms can enhance the tolerance of a globally important crop plant in response to the attack by an insect pest. These effects were at least partially mediated by plant intrinsic regulators and should be considered in future management practices. To explore the potential of AM fungi in improving the nutraceutical value of plant foods, in chapter IV I tested whether AM fungi can affect the bioactive compounds and mineral elements in edible leaves of Moringa oleifera, a high nutritional vegetable crop cultivated in the tropics and sub-tropics. AM fungi enhanced non-specifically the levels of glucosinolates, reduced species-specifically the levels of carotenoids, and increased the levels of two microelements in M. oleifera leaves. These results encourage research on other AM fungal species and their combinations to achieve general benefits on the nutraceutical value of M. oleifera. The role of cytokinin (CK) levels in roots and shoots in AM symbiosis is yet unclear. In chapter V, I tested whether plant CK status regulates the AM symbiosis between tobacco plants and the AM fungus Rhizophagus irregularis. The organ-specific CK status affected profoundly the performance of tobacco in response to AM symbiosis, and suggested that CK in roots and shoots contribute to balance the nutrient exchange between symbionts. Overall, the functions of plant-microbe mutualisms can vary considerably, while phytohormones can play a defining role in these mutualistic functions. These findings provide significant contributions to the field of plant-microbe interactions with potential for application in crop production.

Topic: Mycorrhizal Fungi and Plant Interactions
Subfield: Plant Science
Field: Agricultural and Biological Sciences
Domain: Life Sciences