Phosphate-solubilizing bacteria abate cadmium absorption and restore the rhizospheric bacterial community composition of grafted watermelon plants

The grafting of watermelon plants to rootstocks is common due to the strong capacity of rootstocks to adapt to abiotic and biotic stresses. However, the effect of phosphate-solubilizing bacteria (PSB) on grafted watermelon plant growth and bacterial structures in root soil is unclear. In this study, the growth and hormone levels of grafted plants were measured, and the bacterial communities under cadmium (Cd) stress and inoculation with PSB were sequenced in three treatments (S1, control; S2, 50 μmol Cd [CdCl2]; and S3, 50 μmol Cd plus inoculation with the Cd-resistant PSB strain 'N3'). The results showed that inoculation with PSB significantly (P < 0.05) improved the total dry weight of the grafted plants. Typically, inoculation with PSB significantly (P < 0.05) reduced Cd content in scions and roots. The level of the phytohormone jasmonic acid increased in treatment S2, but decreased in treatment S3 under inoculation with PSB. The functional annotation of prokaryotic taxa showed that Cd decreased the abundance of nitrogen respiration and chloroplast functional groups. Nevertheless, inoculation with PSB helped restore bacterial community structures. These findings provide a new understanding of the effect of PSB on the promotion of seedling growth and bacterial communities in grafted watermelon plants under Cd stress.