Root 1-aminocyclopropane-l-carboxylic acid (ACC) and rhizosphere ACC deaminase-producing bacteria affect apple root architecture under soil compaction stress

Purpose Soil compaction inhibits root growth by limiting ethylene diffusion and often occurs in apple orchards. This study explored the mechanism of ethylene accumulation around apple roots in compacted soil from a microbial perspective, as well as its effects on root architecture, to improve root growth. Methods One-year-old potted apple ( Malus domestica Borkh ‘Red Fuji’) trees whose rootstocks were Malus hupehensis (Mh) and Malus robusta (Mr) were used as materials. After maintaining soil compaction for one year, the root growth, root ethylene, and ethylene-modulating bacteria were detected, and correlation analysis was performed. This work isolated rhizosphere ACC deaminase-producing bacteria (ADPB), inoculated ADPB with Mh seedlings, and then measured ACC and ethylene contents, root growth, and acdS and efe copies. Results Soil compaction changed root architecture, decreased root activity, and inhibited root growth in both Mh and Mr. In addition, soil compaction enhanced root ACC and ethylene contents and reduced the acdS / efe ratio. Pearson’s correlation analysis indicated that rhizosphere ADPB was significantly correlated with ethylene production and root growth. After the inoculation of ADPB with Mh seedlings in compacted soil, the number of acdS copies and the acdS / efe ratio were increased, the root ACC and ethylene contents were decreased, and root growth was improved. Conclusion The ACC content in compacted soil was a decisive precursor of ethylene production in apple roots. Inoculating ADPB improved the root growth of Mh in compacted soil by down-regulating ACC and ethylene contents and recovering the ratio of acdS/efe , thereby alleviating compaction stress on apple roots.