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A trade-off between high grain yield and high protein (or nitrogen, N) concentration is frequently observed for crop plants in agroecosystems and is difficult to resolve using conventional agricultural methods. Whether ecological strategies, such as exploring the interactions between plants and microbes, can be leveraged to tackle this problem remains unclear. Here, we used wheat as a model plant and focused on the interaction among plants, arbuscular mycorrhizal fungi (AMF) and hyphosphere bacteria. We conducted six independent but complementary experiments and demonstrated that AMF can enhance both wheat yield and grain N concentration, but only in specific varieties with high N conversion efficiency (grain yield per mean plant N at preanthesis). AMF can boost yield from increased grain weight via enhanced postanthesis photosynthetic carbon (C) assimilation through promoting plant phosphorus uptake, while elevating grain N concentration through stimulating postanthesis N uptake and assimilation. This dual effect arises from the enhanced postanthesis belowground C allocation, sustaining AMF hyphal network integrity and increasing soil nutrient availability via reshaping hyphosphere bacterial community structure. Our findings indicate that postanthesis plant–microbial interactions can potentially overcome an old dilemma between maximizing grain yield and N concentration in agricultural production.
An inverse relationship between grain yield and protein (or nitrogen, N) concentration is widely shown for grain/cereal crops in agroecosystems, which was first found in wheat and barley in the 1930s and later reported in other cereals and legumes (Neatby & McCalla, 1938; Simmonds, 1995; Zhong et al., 2024). Simultaneous enhancement of grain yield and N concentration is of prime importance regardless of if the target is human consumption or used for animal feeds. However, it is difficult to resolve this dilemma by conventional agricultural methods due to a negative genetic relationship between carbon (C) assimilation and N metabolism in the postanthesis stage (Munier-Jolain & Salon, 2005; Bogard et al., 2010). Starch synthesis in grains mainly uses carbohydrates from the current photosynthetic C assimilation during grain filling (Sanchez-Bragado et al., 2014). However, most grain N originates from senescence-induced remobilization of N in green parts, such as top leaves, involving the degradation of Calvin Cycle enzymes like RuBisCO (Kong et al., 2016; Tegeder & Masclaux-Daubresse, 2018). Photosynthetic C assimilation is therefore negatively coupled with N remobilization during grain filling. This inherent, physiologically based negative association has largely been responsible for the limitation to simultaneously maximize grain yield and N concentration using conventional breeding or cultivation strategies, leading to very few successful examples to shift this inverse relationship (Monaghan et al., 2001; Bogard et al., 2010). However, whether ecological strategies, such as leveraging plant–microbial interactions, are helpful to solve this dilemma is still unclear. Positive interactions often occur among plants, arbuscular mycorrhizal fungi (AMF) and bacteria in natural ecosystems, in which plants allocate C to AMF for maintaining hyphal growth, and further to hyphosphere bacteria for driving N and phosphorus (P) mineralization (Zhou et al., 2020; Zhang et al., 2022; Yao et al., 2026). In return, AMF acquire available N and P from soil mainly transformed by bacteria and transport these elements to plants for growth promotion (Duan et al., 2024). However, the direction of interactions among the three partners depends on crop growth stage in agroecosystems (Yang et al., 2024). For example, modern cereal cultivars which were developed under intensive fertilization regimes since the Green Revolution frequently exhibit attenuated AMF associations (Greenland, 1975; Sawers et al., 2008). This breeding legacy compromises their capacity to harness AMF-hyphosphere bacterial synergisms during vegetative stages, manifesting as mycorrhizal growth depression under high nutrient availability (Treseder & Allen, 2002). Nonetheless, while mycorrhizal growth is suppressed by conventional fertilization, the N and P plant demands during heading stage are exceedingly high. This results in a relief of the suppression, and plants could benefit from AMF–bacteria interactions during postanthesis (Yang et al., 2022). These previous findings suggest that beneficial interactions may occur among cereal crops, AMF and hyphosphere bacteria during the postanthesis stage. Theoretically, plant–AMF interactions during the postanthesis stage can co-enhance grain yield and N concentration of cereals. Once crop plants gain P through AMF during the postanthesis stage, this may stimulate photosynthetic C assimilation via facilitating energy transfer and maintaining metabolic balance (Wang et al., 2025). The enhanced photosynthesis can not only satisfy the C requirements to maximize grain sink potential, but also help to meet C demand in maintaining AMF hyphal integrity and hyphosphere bacterial nutrient-transforming activity. Mycorrhizal hyphal exudation during the postanthesis stage will reshape bacterial community structure in the hyphosphere, which can further enhance N and P transformation (Hodge et al., 2001; Govindarajulu et al., 2005; Duan et al., 2024; Zhou et al., 2025), and in turn increase hyphal growth of AMF (Zhang et al., 2022). Meanwhile, AMF can contribute to postanthesis N acquisition via extraradical hyphae extending beyond root depletion zones (Yang et al., 2022). Such postanthesis N uptake may theoretically compensate quickly for any reduced N remobilization to grains for protein synthesis, allowing functional N complexes, for example RuBisCO, to continue supporting photosynthesis (Kichey et al., 2007; Yang et al., 2022). Therefore, there is a great potential for plant–microbial facilitation in mediating co-improvement of grain yield and N concentration in crop plants. Nevertheless, direct experimental evidence is still scarce.