Impact of crop improvement on cultivated sunflower mycorrhizal responsiveness and preferential associations : a comprehensive review and experimental study of sunflower response to arbuscular mycorrhizal fungi

The history of crop improvement has been shown to have reduced crop genetic diversity, altered various morphological and physiological plant traits, and impacted biotic interactions. For many crops, symbiosis with Arbuscular Mycorrhizal Fungi (AMF) has been found to be altered by crop domestication and improvement – yet few studies have assessed this question in cultivated sunflower (Helianthus annuus L.). Here, I provide a comprehensive review of the impact of crop improvement on the AMF association in cultivated sunflowers and concomitant shifts in plant responsiveness, followed by a manipulative experiment under controlled conditions. Meta-analysis of the Mycorrhizal Growth Response (MGR) of cultivated sunflowers showed an overall large positive effect of AM on plant growth, but results also shed light on a potential publication bias against negative-effect studies. Within the manipulative experiment, contrary to expectations we found reduced plant biomass under mycorrhization though the leaf nutrient status was improved. Comparing sunflower genotypes, inbred lines introduced after the Green Revolution showed reduced growth responsiveness which translated into significantly negative MGR values compared to the lines introduced before the Green Revolution. Pre-Green Revolution lines invested less biomass in their leaves, which minimized the plant total biomass loss induced by the AMF, suggesting that higher total plant photosynthetic capacity was positively correlated with the strength of the AMF carbon sink. Our results suggest little impact of the history of sunflower crop improvement on root AMF community composition, though AMF richness and evenness increased in sunflower root communities with ongoing plant growth and development. This study highlights the impact of the history of crop improvement on the responsiveness of cultivated sunflowers, with implications for modern crop improvement efforts.