Molecular characterization of the basic helix-loop-helix (bHLH) genes that are differentially expressed and induced by iron deficiency in Populus.

KEY MESSAGE: Two Populus bHLH genes ( PtFIT and PtIRO ) were cloned and characterized. The iron deficiency tolerance may be regulated by the PtFIT -dependent response pathway in Populus. Five orthologs of eight Arabidopsis basic helix-loop-helix (bHLH) genes responding to iron deficiency in Populus were analyzed. Open reading frame (ORF) regions of two bHLH genes (PtFIT and PtIRO) were isolated from the iron deficiency tolerant (PtG) and susceptible (PtY) genotypes of Populus tremula 'Erecta'. Gene sequence analyses showed that each of the two genes was identical in PtG and PtY. Phylogenetic analysis revealed that PtFIT was clustered with the bHLH genes regulating iron deficiency responses, while PtIRO was clustered with another group of the bHLH genes regulating iron deficiency responses in a FIT-independent pathway. Tissue-specific expression analysis indicated that PtFIT was only detected in the root among all tested tissues, while PtIRO was rarely detected in all tested tissues. Real-time PCR showed that PtFIT was up-regulated in roots under the iron-deficient condition. A higher level of PtFIT transcripts was detected in PtG than in PtY. Pearson Correlation Coefficient calculations indicated a strong positive correlation (r = 0.94) between PtFIT and PtIRT1 in PtG. It suggests that the iron deficiency tolerance of PtG may be regulated by the PtFIT-dependent response pathway. The PtFIT-transgenic poplar plants had an increased expression level of PtFIT and PtIRT1 responding to iron deficiency. One PtFIT-transgenic line (TL2) showed enhanced iron deficiency tolerance with higher chlorophyll content and Chl a/b ratio under iron deficiency than the control plants, indicating that PtFIT is involved in iron deficiency response in Populus. The results would provide useful information to understand iron deficiency response mechanisms in woody species.