Citrus is one of the most economically important fruit crops around world. Drought and salinity stresses adversely affected its productivity and fruit quality. However, the genetic regulatory networks and signaling pathways involved in drought and salinity remain to be elucidated. With RNA-seq and sRNA-seq, an integrative analysis of miRNA and mRNA expression profiling and their regulatory networks were conducted using citrus roots subjected to dehydration and salt treatment. Differentially expressed (DE) mRNA and miRNA profiles were obtained according to fold change analysis and the relationships between miRNAs and target mRNAs were found to be coherent and incoherent in the regulatory networks. GO enrichment analysis revealed that some crucial biological processes related to signal transduction (e.g. ‘MAPK cascade’), hormone-mediated signaling pathways (e.g. abscisic acid- activated signaling pathway’), reactive oxygen species (ROS) metabolic process (e.g. ‘hydrogen peroxide catabolic process’) and transcription factors (e.g., ‘MYB, ZFP and bZIP’) were involved in dehydration and/or salt treatment. The molecular players in response to dehydration and salt treatment were partially overlapping. Quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) analysis further confirmed the results from RNA-seq and sRNA-seq analysis. This study provides new insights into the molecular mechanisms how citrus roots respond to dehydration and salt treatment.

In plants, CAMTA (Calmodulin-binding transcription activator) proteins have been widely analysed, due to their vital roles in developmental regulation, environmental stress response, and hormone crosstalk. However, in citrus, this gene family, to date, has not been systematically characterised. In this study, 9 CitCAMTA genes have been identified from the citrus (Citrus sinensis and Citrus clementina) genomes. Their phylogenetic relationship, protein domain, exon–intron structure, and so forth were investigated in detail, revealing the divergence in their functions. Tissue-specific expression analysis showed that these CitCAMTA genes – with the exception of CitCAMTA4 – could be expressed in at least 1 tissue, indicating their roles in the development of root, stem, leaf, cotyledon, seed, and fruit peel. Under stress and hormone treatments, the expression patterns of each CitCAMTA remarkably varied, which showed a promising for improving citrus adaptation to stresses. Unexpectedly, the expression level of CitCAMTA4 was undetectable under all treatments. Taken together, our work lays a foundation for further analysis on citrus CAMTA genes, and provides a basic understanding of their roles in stress responses.