Apple Scion Cultivars Regulate the Rhizosphere Microbiota of Scion/rootstock Combinations

In grafted apple combinations, rootstocks can increase the stress tolerance of plants, influence scion growth, and harbor a rich microbial community. However, whether scions modify plant growth, rootstock phenotypes and rhizosphere bacterial communities has not been clearly investigated. Here, 14 different combinations of two scion cultivars grafted on 7 rootstock genotypes were used as research materials, and we analyzed the plant traits, rhizosphere bacterial community, and potential functionalities across these plants in the same orchard. We found that apple scion cultivars influenced the tree height and trunk circumference, and the sugar concentration in roots varied significantly between scion cultivars, especially fructose and sucrose. Apple scion cultivars were the largest source of variation in the rhizosphere bacterial diversity and taxonomic composition of grafted combinations, and exerted a greater impact on the rhizosphere bacterial community than the rootstock genotype. The rhizosphere bacterial community networks were also significantly influenced by the scion cultivars. The dominant rhizosphere bacterial taxa were correlated with the root sugar concentration, especially sucrose. Further potential functional analysis of the bacterial community revealed that rhizosphere bacteria contained fructose and sucrose metabolism and had plant growth-promoting traits. Additionally, the predicted metabolism of the rhizosphere-associated bacterial communities varied depending on the scion cultivar. These results indicated that apple scion varieties regulate the rhizosphere bacterial community and function in grafted combinations through controlling the soluble sugar content, especially sucrose.