MicroRNA and target gene responses to salt stress in grafted cucumber seedlings

Grafting is widely applied to enhance cucumber growth and tolerance to various biotic and abiotic adversities. Except for regulating plant growth and plant development, microRNAs (miRNAs) respond to various adversities through negative mediation of target gene expression at the post-transcriptional level. In the present study, cucumber (Cucumis sativus) scions were grafted onto pumpkin (Cucurbita moschata) rootstocks to characterize the molecular mechanisms of miRNAs and target regulatory networks that confer increased salt stress tolerance. The expression levels of 17 miRNAs and their predicted target genes were determined by quantitative real-time PCR using auto-grafted (self-grafted cucumber seedlings) and hetero-grafted (cucumber scion grafted onto pumpkin rootstock) cucumber seedlings after 24 h of 100 mmol l(-1) NaCl treatment. Compared with that of auto-grafted cucumber seedlings, the expression level of most miRNAs in the leaves of hetero-grafted cucumber seedlings increased under normal conditions, decreased considerably after 2 h of salt stress, and increased after 6 and 24 h of salt stress. The expression level of most miRNAs in the roots of hetero-grafted cucumber seedlings increased under normal conditions and salt-stressed conditions. Compared with that of auto-grafted cucumber seedlings, the expression level of most target genes in the leaves of hetero-grafted cucumber seedlings decreased under normal and salt-stressed conditions. However, the expression level of most target genes in the roots of hetero-grafted cucumber seedlings increased significantly. These results provide useful information relevant to the functional analysis of miRNAs mediating graft-dependent salt tolerance.