Pyrus ussuriensis rootstocks improve scion potassium homeostasis by modulating lignin biosynthesis and ROS signaling under deficiency

Potassium (K), commonly known as the “quality element”, is critical for nutrient uptake in fruit trees. The nutrient homeostasis of grafted plants is greatly influenced by rootstock nutrition use efficiency. Pyrus ussuriensis and Pyrus betulaefolia are potassium-efficient (insensitive) and potassium-inefficient (sensitive) rootstocks, respectively. This study employed transcriptome analysis to examine the mechanisms of low-K stress responses in two grafting systems with different rootstocks. Field grafting experiments indicated that under potassium deficiency, the K content in the leaves of the scion ( Pyrus Sucui ) decreased by 32.1% when grafted onto P. ussuriensis and by 45% when grafted onto P. betulaefolia , with corresponding root K content decreases of 34.3% and 30.1%, respectively. The decline in potassium content under potassium deficiency was mitigated when P. ussuriensis was grafted as the rootstock. In vitro micrografting experiments demonstrated that potassium deficiency resulted in comparable reductions in K⁺ content, with rootstock-mediated decreases of 30% ( P. ussuriensis ) and 31.4% ( P. bretschneideri ), respectively. Transcriptome analysis suggest that significant responses to potassium deficiency are induced in rootstock systems, particularly through Differentially Expressed Genes (DEGs) associated with reactive oxygen species (ROS) signaling and lignin biosynthesis pathways. Under low-K treatment, a significant increase in POD content (18.5%) was detected in the roots of P. betulaefolia , whereas no significant changes were observed in P. ussuriensis. In particular, lignin content was reduced by 17.6% and 9.7% in the roots of P. betulaefolia and P. ussuriensis , respectively. Low-K stress resulted in a significant increase in POD expression in the roots of P. betulaefolia . In contrast, there were no significant changes in POD expression for the roots of P. ussuriensis , leading to divergent trends in POD content, contrary with the changes in lignin content. In conclusion, our study provides new insights into a potential correlation between ROS, lignin and potassium homeostasis of rootstock-induced trait alterations in scion.