Overexpression of the LcPIN2 and LtPIN2 Gene in Arabidopsis thaliana Promotes Root Elongation

The auxin polar transporter, PIN-FORMED 2 (PIN2) plays an important role in root development. However, it remains unclear whether PIN2 genes form two Liriodendron species, L. chinense (LcPIN2) and L. tulipifera (LtPIN2), are both involved in root development and whether and to what extent these two genes diverge in func-tion. Here, we cloned and overexpressed LcPIN2 and LtPIN2 in Arabidopsis thaliana wild-type (WT) and Atpin2 mutant. Phylogenetic and sequence analysis showed a small degree of differentiation between these two Lirioden-dron PIN2 genes. Tissue-specific gene expression analysis indicated that both Liriodendron PIN2 genes were highly expressed in roots, implying a potential role in root development. Finally, heterologous overexpression of LcPIN2 and LtPIN2 in Arabidopsis both significantly increased the root length compared to wild-type and empty vector. Furthermore, the root length defect in Atpin2 was complemented both by LcPIN2 and LtPIN2. However, heterologous overexpression of LcPIN2 and LtPIN2 cannot rescue the defect in root gravitropism of Atpin2 mutants. Taken together, our findings unravel PIN2 genes from the magnoliids plant Liriodendron were functionally conserved with AtPIN2 in the dicotyledonous plant Arabidopsis in regard to the regulation of root length, but not root gravitropism. This study also provides a potential target for genetic improvement of the root system in these valuable forest trees Liriodendron.