It is of great significance to understand the regulatory mechanisms by which plants deal with drought stress. Two EST libraries derived from rapeseed (Brassica napus) leaves in non-stressed and drought stress conditions were analyzed in order to obtain the transcriptomic landscape of drought-exposed B. napus plants, and also to identify and characterize significant drought responsive regulatory genes and microRNAs. The functional ontology analysis revealed a substantial shift in the B. napus transcriptome to govern cellular drought responsiveness via different stress-activated mechanisms. The activity of transcription factor and protein kinase modules generally increased in response to drought stress. The 26 regulatory genes consisting of 17 transcription factor genes, eight protein kinase genes and one protein phosphatase gene were identified showing significant alterations in their expressions in response to drought stress. We also found the six microRNAs which were differentially expressed during drought stress supporting the involvement of a posttranscriptional level of regulation for B. napus drought response. The drought responsive regulatory network shed light on the significance of some regulatory components involved in biosynthesis and signaling of various plant hormones (abscisic acid, auxin and brassinosteroids), ubiquitin proteasome system, and signaling through Reactive Oxygen Species (ROS). Our findings suggested a complex and multi-level regulatory system modulating response to drought stress in B. napus.

Many studies have been performed to identify regulatory circuit underlying plant stress tolerance. However, the reliability of some findings has been criticized because of exclusive use of stress sensitive plant species such as Arabidopsis thaliana. Sensitive plant species often harbor narrow defensive mechanisms and have relatively low capacity for adaptive responses. Therefore, it is useful to employ tolerant model plants, such as Eutrema salsugineum, to provide comprehensive insights into various mechanisms involved in response to abiotic stresses. In this study, comparative transcriptome and regulatory network analysis of stress-sensitive (A. thaliana) and -tolerant (E. salsugineum) model plants uncovered regulatory hierarchies underlying response to abiotic stresses and suggested the transcription factor genes, MYB44 and VIP1 as the candidate hub genes to perform molecular analyses on their Brassica napus homologs, BnMYB44 and BnVIP1. The full-length coding sequence of BnMYB44 and BnVIP1 with 891 and 969 bp long were cloned and sequenced. They shared high similarity with their counterparts in other plants at nucleotide and amino acid levels. The expression patterns of BnMYB44 and BnVIP1 genes of the two B. napus cultivars under drought and salt stress conditions coupled with the data obtained from the physiological measurements as well as analysis of the BnMYB44 and BnVIP1 promoters suggested that BnMYB44 and BnVIP1 genes may contribute to responses to drought and salt stresses in B. napus.

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