Agricultural intensification has led to an increased yield of staple crops (Tilman, 1999; Godfray et al., 2010), but an intensive supply of chemical fertilizers has brought a series of adverse environmental impacts and sustainability issues (Foley et al., 2005; Bender et al., 2016). Arbuscular mycorrhizal (AM) fungi are widely distributed in natural and agricultural ecosystems and the most recognized benefit of AM fungi is enhanced plant uptake of immobile nutrients, especially phosphorus (P). Exploitation of the function of AM fungi to increase plant nutrient uptake offers one of the few untapped opportunities to reduce P fertilizer application and improve the sustainability of agriculture (Toju et al., 2018; Zhang et al., 2019). Whether or not inoculated, AM fungi can successfully establish and enhance plant growth that depends on a series of factors such as native AM fungal communities, pesticides, and cultivars (Bethlenfalvay et al., 1996; Busse et al., 2004; Muok et al., 2009). Hence, this approach is not always stable and may even have a neutral effect on crop production (Jeffries et al., 2003; Corkidi et al., 2004; Lekberg et al., 2008; Cely et al., 2016). Besides inoculating nonindigenous AM fungi, using the indigenous AM fungal community to enhance plant growth could be an alternative. P availability is considered to be the major driving force of the mycorrhizal responsiveness (MR) of plants (Johnson et al., 2010). High soil P availability causes a decrease in AM fungal colonization and mycorrhizal benefits (Mader et al., 2000). However, a rich community of AM fungi in agricultural ecosystems has been observed with intensive crop management (Oehl et al., 2004), regardless of the application of fertilizers, pesticides, and even high soil available P (Thomson et al., 1992; Kaeppler et al., 2000; Hao et al., 2008; Gai et al., 2010; Vestberg et al., 2011). Moreover, the indigenous AM fungal communities can play a role in promoting growth of maize and cotton (Liu et al., 2016). According to breeding practices and release date, a recent study divided maize varieties into three varietal groups, including old landraces, modern hybrids, and inbred lines, and compared MR of these varietal groups (Wang et al., 2020b). The results showed that there were no significant differences among varietal groups for MR, confirming that the ability to associate with and benefit from AM fungi has been maintained in modern crops (Wang et al., 2020b). In addition, the varietal group of modern hybrids inoculated with indigenous AM fungi exhibited the higher P acquisition and higher P use efficiency under field conditions (Wang et al., 2020b). Thus, maximizing the benefits of indigenous AM fungi could be a useful way to enhance crop growth, thereby reducing the dependence of crop production on the addition of P chemical fertilizers.

Mycorrhizal responsiveness reflects the degree of the response of plants to AM colonization and the capacity to benefit from the symbiosis (Janos, 2007; Sawers et al., 2010). MR is determined by the complexity of combinations of plant-funga environmental conditions, and the magnitude of MR exhibites great variation (Yao et al., 2001a,b; Hao et al., 2008; Berger and Gutjahr, 2021). Specifically, MR depends on root morphological, physiological, and mycorrhizal traits that are P uptake-related features [e.g., root length (RL) and root diameter (RD)] and the degree of MR differs widely among plant species and even among plant genotypes (Chu et al., 2013; Comas et al., 2014; Kramer-Walter et al., 2016). For mycorrhizal traits, mycorrhizal colonization might be a useful parameter for breeding maize varieties with high MR (Chu et al., 2013). Root external fungal structures and the internal to external hyphae ratio (IEHR) exhibited great variation among different varieties and these two indicators may also contribute to MR (Chu et al., 2013; Sawers et al., 2016). For root traits, poorly adapted varieties with limited ability to explore the soil (e.g., large diameter roots and poorly branched roots) often show a large performance increase with AM symbiosis (Hetrick et al., 1992; Newsham et al., 1995; Comas et al., 2014). Thus, breeding higher MR crop genotypes could be an important way to enhance mycorrhizal benefits. To explore the variation pattern of MR, based on greenhouse experiments of 20 wheat cultivars, a previous study firstly proposed that the release date of the cultivar could be a decisive factor (Hetrick et al., 1992) and some later studies also verified the findings of this study (Zhu et al., 2001; Chu et al., 2013). However, for other crops, such as onion, modern breeding practices did not change mycorrhizal responses (Galvan et al., 2011). A meta-analysis of studies from 1981 to 2010 found no evidence that new crop plant genotypes lost their ability to respond to mycorrhizae due to agricultural and breeding practices (Lehmann et al., 2012). Therefore, it is clear that previous mainstream study cannot draw a general conclusion about the effect of modern breeding on MR and, thus, study on specific species at different P levels is needed to better predict the MR of crop genotypes.

Cotton (Gossypium spp.) is an economically important crop worldwide, both in providing a significant amount of natural fiber for the manufacture of textiles and as an oilseed (Bao et al., 2019). Upland cotton (Gossypium hirsutum L.), which belongs to one of the four cotton species, currently accounts for more than 90% of global cotton fiber production. Xinjiang Province tops China in cotton planting areas and its cotton is famous for its long pile, good quality, and high output. In the past few decades, many cotton varieties have been cultivated in Xinjiang. AM fungi can colonize up to 90% of cotton root length and enhance cotton plant P uptake or fiber yield (Cely et al., 2016; Eskandari et al., 2016). Unlike other crops, such as maize, cotton plants have fewer root exudates and, thus, depend on root morphological characteristics to obtain nutrients (Wang et al., 2010). Therefore, a study was planned to identify cotton genotypes highly responsive to AM fungal inoculation. The primary objective of this study was to determine how MR on a set of cotton genotypes has been affected by their release date from 1950 to present (“old” vs. “modern” genotypes) at different P levels (low vs. high). The second objective was to identify the relationship between MR and functional traits that are related to P uptake efficiency. Such information will provide useful information for future cotton breeding aimed at delivering crop genotypes that are less dependent on the input of P fertilizers.

Extensive studies have revealed that MR and its trends based on release dates vary greatly among crop genotypes. For instance, the MR of old wheat cultivars was higher than that of modern wheat cultivars (Hetrick et al., 1992, 1996; Zhu et al., 2001). Nevertheless, MR for maize, tomato, and oats showed the opposite scenario (Koide et al., 1988; Bryla and Koide, 1998). Generally, the release date of a cultivar reflects the agricultural and breeding practices of its time (Lehmann et al., 2012). In the past few decades in Xinjiang, cotton breeding has mainly aimed to increase quality, fiber yield, and disease resistance, but has not considered how modern varieties interact with indigenous AM fungi. The data presented in this study showed that mycorrhizal colonization and the HLD of the modern cotton genotypes were relatively higher than those of the old genotypes at low and high P (Figure 1 and Supplementary Figure 1). The mycorrhizal colonization and the HLD of cotton genotypes developed between the 1980s and the 1990s exhibited an intermediate status between the old and modern genotypes (Figure 1 and Supplementary Figure 1). The 24 highly diverse cotton genotypes analyzed showed a positive response to AM fungi, irrespective of the initial soil P levels, indicating the ability of cotton to form a beneficial symbiosis with AM fungi (Figure 2 and Supplementary Table 3). The 24 cotton genotypes were divided into three groups according to release date, we noticed that MGR showed an increasing trend (Figure 3). Similar to maize, tomato, and oats (Koide et al., 1988; Bryla and Koide, 1998), the modern cotton genotypes benefitted more from plant growth in association with AM fungi than the old genotypes at both the P levels (Figure 3). Overall, the results indicated that release date had significant positive effects on the formation of AM symbiosis and on the MGR of modern cotton genotypes.

Enhancement of P uptake efficiency (PUpE) and PUtE is widely suggested as a target of crop breeding (Hetrick et al., 1992; Zabinski et al., 2002; Lynch, 2011; Leiser et al., 2016; Ros et al., 2018; Wang et al., 2020a). Increase in shoot P concentrations and decrease in PUtE (defined as the inverse of P concentration) for wheat, maize, and Centaurea after AM fungal inoculation were common phenomena in previous study (Hetrick et al., 1992; Zabinski et al., 2002; Wang et al., 2020a). This phenomenon has been described as luxury uptake in previous studies, but the term luxury uptake has no explanatory value (Hetrick et al., 1994; Zabinski et al., 2002). A further explanation is that P uptake is not perfectly regulated over time and “excess” P can be used at later stages (Wang et al., 2020a). Compared with previous results above, the results in this study showed an opposite trend: decrease in shoot P concentrations and increase in PUtE in most cotton varieties after AM fungi inoculation (Supplementary Figure 3 and Supplementary Table 4). Mycorrhizal plants had higher biomass than nonmycorrhizal plants (Supplementary Table 3). AM fungi may provide other nutrients to plants and, thus, P is more dilute in the tissues. The reason for the increase in PUtE after inoculation remains to be explored.

Plant genotypes with different root structures and functions are crucial for understanding the large variation in MR from positive to negative (Hetrick, 1991; Hao et al., 2008; Yucel et al., 2009). Traditionally, plant varieties with a confined and coarse root architecture, such as relatively low root hair density, large diameter roots, and short root hairs, obtain the greatest growth benefits from colonization by AM fungi (Brundrett, 2002; Ah, 2004; Smith and Read, 2008). Nonetheless, a meta-analysis combined previously published literatures indicated that the relationships between MGR and allocation to roots, RD, root hair length, and root hair density was statistically insignificant (Maherali, 2014). Similarly, a recent study indicated that magnitudes and directions of MR of 27 crops were diverse among crop species and were unrelated to the coordinated evolution with fine root traits (Martin-Robles et al., 2018). The particularity of the cotton root system is that it is mainly composed of fine roots: the abundance of fine roots (RD < 0.4 mm) occupied more than 70% (Figure 2). But, for other crops, such as maize, the proportion of fine root length was 10–20%, which was significantly lower than that of cotton (Wang et al., 2020a). When colonized with AM fungi, cotton plants had a notably higher abundance of roots with RD < 0.2 mm, but a decreased abundance of roots with RD > 0.4 mm, which was clear evidence that these root traits are functions of AM fungi (Figure 2). Plants need to invest more photosynthetically-fixed carbon to grow thick roots than thin roots (Kaschuk et al., 2009). When inoculated with AM fungi, the plants could then allocate more photosynthetically-fixed carbon to AM fungi and less photosynthetically-fixed carbon to roots (Smith and Read, 2008; Kaschuk et al., 2009). The reduction in carbon allocation to the root has led to the increased abundance of fine roots and the decreased abundance of thick roots in this study (Figure 2). Moreover, the MPR had a significant positive correlation with the abundance of root lengths with RD < 0.2 mm. Altogether, cotton plants in association with AM fungi had a unique root morphological trait and a greater abundance of fine roots was a predictor of MPR. However, when combined with the nonsignificant relationship between fine roots and MGR (Figure 5), further experiments are needed to verify the stability of the relationship between the PRL and MR.

It has been suggested that the variation of plant genetic factors may influence the degree of AM fungal colonization and, thus, determine the variation in growth response to AM fungi. Much emphasis has been invested in study on the development of root-internal fungal structures, as more arbuscules may transfer more mineral nutrients, such as P, to the host plant (Lehmann et al., 2012; Chu et al., 2013). The length of root-external hyphae may reflect the ability to explore mineral nutrients from the root free soil. The recent study showed that the apparent benefits of AM fungi for plant growth and P uptake were correlated with the increased abundance of root-external hyphae rather than root colonization (Sawers et al., 2016). It was tempting to speculate that the extension of mycelium, coupled with the ability of AM fungi to absorb mineral nutrients, determined the response to AM fungi (Jakobsen et al., 2001; Sawers et al., 2016). Considering that the relationship between MR and mycorrhizal colonization was nonsignificant, quantification of mycorrhizal colonization was not predictive of MR (Figure 5). Among all the 24 cotton varieties, the HLD was positively correlated with MGR and MPR in this study (Figure 5). The higher HLD of the modern cotton genotypes contributed to the increase in MR compared to the old genotypes. It is supposed that the variation of genetic factors between old and modern cotton genotypes may have a great impact on the supply of carbon to the AM fungi, which, in turn, affected the growth of root-external hyphae and even the magnitude of MR (Keymer and Gutjahr, 2018; Berger and Gutjahr, 2021). Overall, it could, thus, be suggested that the HLD is necessarily a good predictor of cotton growth response to AM fungal colonization.

The results of this study show that modern cotton genotypes are more intensely colonized and more mycorrhizae responsive than the old genotypes. It can be concluded that modern breeding in Xinjiang Province has improved the association of cotton varieties with indigenous AM fungi. Thus, harnessing indigenous AM fungi as beneficial microbes presents a promising strategy to strengthen the functionality of AM symbiosis, thereby reducing the dependence of crop production on adding P chemical fertilizers and optimizing agricultural sustainability. Moreover, the HLD can be used as a proxy for the MR of cotton genotypes based on its correlation with MGR. These results provide useful information for future plant breeding programs aimed at developing crops that benefit from AM fungi.

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

GF, LW, and XW designed the experiment. LW, XW, and BM conducted experiments and completed the measurement of the experimental indicators. LW conducted all analyses, conceived figures, and wrote the manuscript. GF, LW, XW, AK, and KK contributed to revisions. All authors contributed to the article and approved the submitted version of the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.