Systematic analysis of the basic/helix-loop-helix (bHLH) transcription factor family in pummelo (Citrus grandis) and identification of the key members involved in the response to iron deficiency

Abstract

Background As the second largest superfamily of transcription factors (TFs), the basic/helix-loop-helix (bHLH) proteins participate in regulating of a series of biological and developmental processes in plants. Although several bHLH genes of citrus have been identified and previously characterized, a large number of bHLH members are still unknown.Results In this study, we genome-widely identified a total of 128 CgbHLHs from pummelo (Citrus grandis) that were classified into 18 subfamilies by phylogenetic comparison with Arabidopsis thaliana bHLH proteins. All of these CgbHLHs were randomly distributed on nine known (125 genes) and one unknown (3 genes) chromosomes, and 12 and 47 of them were identified to be tandem and segmental duplicated genes, respectively. Sequence analysis showed detailed characteristics of their intron-exon structures, bHLH domain and conserved motifs. Gene ontology (GO) analysis suggested that most of CgbHLHs were annotated to the nucleus, DNA-binding transcription factor activity, response to abiotic stimulus, reproduction, post-embryonic development, flower development and photosynthesis. In addition, 37 CgbHLH proteins were predicted to have direct or indirect protein-protein interactions. Based on GO annotation, RNA sequencing data in public database and qRT-PCR results, 22 of CgbHLHs were identified as the key candidates that respond to iron deficiency.Conclusions In total, 128 CgbHLH proteins were identified from pummelo, and their detailed sequence and structure characteristics and putative functions were analyzed. This study provides comprehensive information for further functional elucidation of CgbHLH genes in citrus.