Genome-Wide Identification of PIN and PILS Gene Families in Areca catechu and the Potential Role of AcPIN6 in Lateral Brace Root Formation

PIN-FORMED (PIN) and PIN-LIKES (PILS) are two families of auxin transporters that control the directional cell-to-cell transport and intracellular accumulation of auxin, thereby influencing plant growth and development. Most knowledge of PINs and PILSs was obtained from the dicot model plant Arabidopsis thaliana. Here, we focus on the distribution and expression of the PIN and PILS gene families in areca palm (Areca catechu), a monocot tree. The whole genomic dataset of areca palm was used to identify twelve AcPINs and eight AcPILSs, and a phylogenetic tree was constructed of PINS and PILS together with several other palm species, including the date palm (Phoenix dactylifera), oil palm (Elaeis guineensis), and coconut (Cocos nucifera). We further analyzed the expression patterns of AcPIN and AcPILS in areca palm, and found that AcPIN6 displayed an extremely high transcriptional abundance in the brace roots and was extremely stimulated in the lateral root primordium. This result implies that AcPIN6 plays an important role in the growth and formation of brace roots, especially in lateral root initiation. We also overexpressed AcPIN6 and AcPIN6–eGFP in Arabidopsis, and the results revealed that the PIN6 localized on the plasma membrane and affected auxin-related phenomena. Taken together, we analyzed the evolutionary relationships of PINs and PILSs in palm species, and the roles of PIN6 in areca palm root formation. The results will improve the understanding of root system construction in large palm trees.