Dynamic Expression of miRNAs and Their Targets in the Response to Drought Stress of Grafted Cucumber Seedlings

Grafting of cucumber is widely used to improve growth and tolerance to biotic and abiotic stresses. MicroRNAs (miRNAs) regulate plant growth and development, and respond to various stresses through negative, post-transcriptional regulation of the expression of their target genes. We grafted cucumber (Cucumis sativus) scions onto pumpkin (Cucurbita moschata) rootstocks to study the molecular mechanisms of miRNA-mediated grafting-induced responses to drought stress. The relative expressions of 17 selected miRNAs and their predicted target mRNAs were detected by quantitative real-time PCR (qRT-PCR) in leaves and roots of hetero-grafted cucumber seedlings (cucumber as scion and pumpkin as rootstock) and auto-grafted cucumber seedlings (cucumber as scion and rootstock) after 24 hours of 15% PEG6000 treatment. Compared with the expression in leaves of auto-grafted cucumber seedlings, the expressions of most miRNAs in the leaves of hetero-grafted seedlings changed dynamically: induced under normal conditions, and reduced after 3 h of drought stress, and then induced after 8 h and 24 h of drought stress. Similarly, compared with the expression in roots of auto-grafted cucumber seedlings, the expressions of most miRNAs in the roots of hetero-grafted cucumber seedlings changed dynamically: reduced under normal condition, and still reduced after 1 h, but induced after 3 h and 8 h, then reduced significantly after 24 h of drought stress. These results are useful for the functional analysis of miRNAs in the mediation of grafting-dependent drought tolerance.