# The ATP Binding Cassette (ABC) Transporter Gene Family in Lotus (Nelumbo Adans.): Genome-Wide Survey, Characterization and Gene Expression Profile

## TL;DR

This study identifies and characterizes ABC transporter genes in lotus, focusing on their role in carotenoid transport in stamens.

## Contribution

The discovery of NnABCG25 as a stamen-specific ABC transporter involved in carotenoid transport in lotus.

## Key findings

- 122 ABC transporter genes were identified in the lotus genome.
- NnABCG25 is stamen-specific and shows stable affinity for carotenoids like lycopene and β-carotene.
- Phylogenetic analysis grouped lotus ABC transporters into eight subfamilies, evolving under purifying selection.

## Abstract

Carotenoids are a class of important lipophilic pigments widely distributed in nature, which are C40 or C30 terpenoid compounds possessing an isoprene skeleton. Although extensive studies have been conducted on the biosynthetic pathways and regulatory mechanisms of carotenoids, their transport mechanisms remain poorly understood. In this study, we focused on the mutant cultivar ‘Yindu Zhimi’ with orange-reddish stamen to unravel ABC transporter function in carotenoid transport. In total, 122 NnABC transporters were identified in the lotus genome and NnABCG25 was revealed as a stamen-specific protein to mediate carotenoid transport. These findings expand our understanding of the role of ABC transporters in the transport of carotenoids, as well as providing a valuable reference for research on the ABC transporter gene family in other plants.

ABC transporters (ATP-binding cassette transporters) constitute one of the largest known protein families and are widely distributed in plants. Their primary function involves utilizing energy derived from ATP hydrolysis to transport substrates across membranes against concentration gradients. These transporters play crucial roles in the translocation and accumulation of metabolites, stress tolerance, disease resistance, and plant defense. Lotus is an important traditional Chinese medicinal herb and contains active ingredients primarily composed of secondary metabolites, whose transport and accumulation require the involvement of ABC transporters. However, the function of these ABC transporters remains unexplored in lotus. In this study, 122 ABC transporter genes were predicted within the lotus genome. We identified 1~15 conserved motifs among the NnABC proteins and most of them were stable proteins predominantly located on the plasma membrane with ExPASy-ProtParam, ProComp and WoLF PSORT analysis. Phylogenetic tree analysis revealed that the lotus ABC transporter gene family could be divided into eight subfamilies, from ABCA to ABCI, and the evolution was predominantly driven by purifying selection. Comparative transcriptome analysis between the cultivar ‘Yindu Zhimi’ with orange-reddish stamen and ‘Weishan Hong’ with yellowish stamen, along with quantitative real-time PCR results, showed that the NnABCG25 gene is highly specifically expressed in the orange-reddish stamen. Molecular docking demonstrated that NnABCG25 has a stable affinity for lycopene, β-carotene and β-apocarotenal, suggesting its potential involvement in the transport of carotenoids in the stamen. These findings expand our understanding of the role of ABC transporters in the transport and accumulation of carotenoids, as well as providing a valuable reference for research on the ABC transporter gene family in other plants.