Genome-wide identification of PbrbHLH family genes, and expression analysis in response to drought and cold stresses in pear (Pyrus bretschneideri).

Background: The basic helix-loop-helix (bHLH) transcription factors play important roles in many processes in plant growth, metabolism and responses to abiotic stresses. Although, the sequence of Chinese white pear genome (cv. 'Dangshansuli') has already been reported, there is still a lack of clarity regarding the bHLH family genes and their evolutionary history.

Results: In this work, a genome-wide identification of the bHLH genes in Chinese white pear was performed, and we characterized the functional roles of these PbrbHLH genes in response to abiotic stresses. Based on the phylogenetic analysis and structural characteristics, 197 identified bHLH genes could be well classified into 21 groups. Expansion of PbrbHLH gene family was mainly driven by WGD and dispersed duplication with the purifying selection from the recent WGD. The functional annotation enrichment showed that the majority of PbrbHLHs were enriched in the GO terms and KEGG pathways involved in responds to stress conditions as TFs. Transcriptomic profiles and qRT-PCR revealed that PbrbHLH7, PbrbHLH8, PbrbHLH128, PbrbHLH160, PbrbHLH161 and PbrbHLH195 were significantly up-regulated under cold and drought treatments. In addition, PbrbHLH195-silenced pear seedlings display significant reduced cold tolerance, exhibiting reduced chlorophyll content, as well as increased electrolyte leakage and concentrations of malondialdehyde and HO.

Conclusion: For the first time, a comprehensive analysis identified the bHLH genes in Chinese white pear and demonstrated that PbrbHLH195 is involved in the production of ROS in response to cold stress, suggesting that members of the PbrbHLH family play an essential role in the stress tolerance of pear.