Distribution of ABC Transporter Genes across the Plant Kingdom

The ATP-binding cassette (ABC) transporter gene superfamily is ubiquitous among extant organisms. ABC transporters act to transport compounds across cellular membranes and are involved in a diverse range of biological processes and functions including cancer resistance in humans, drug resistance among vertebrates, and herbicide resistance in weeds. This superfamily of genes appears to be larger and more diverse in the plant kingdom—yet, we know relatively less about ABC transporter function in plants compared with mammals and bacteria. Therefore, we undertook a plant kingdom-wide transcriptomic survey of ABC transporters to better understand their diversity.

We utilized sequence similarity-based informatics techniques to deduce likely ABC transporter genes from 1,301 phylogenetically-diverse plant transcriptomes. A total of 97,149 putative ABC transporter gene members were identified with an average of 88 gene members per RNASeq library and a standard deviation of 30 gene members. Of 97,149 ABC transporter sequences identified, 22,343 were full length. Fewer unique ABC transporter gene members appeared among algae compared with vascular and non-vascular land plants. Differences were also noted in abundance of certain ABC transporter subfamilies. Non-algae taxa had relatively more unique ABCB, ABCC, and ABCG transporter gene members on average (p < 0.005), and both algae and early non-vascular plants averaged significantly more ABCF transporter gene members (p < 0.005). Ferns had significantly fewer ABCA transporter gene members than most plant taxa (p < 0.005).

Our study provides an overview of ABC transporter protein gene members among many plant taxa including: algae, non-vascular land plants, lycophytes, ferns, conifers, and angiosperms. An increase in the number of gene family members present in the ABCB, ABCC, and ABCD transporter subfamilies may indicate greater variability of the ABC transporter protein superfamily among plant taxa since the divergence of non-vascular plants from algae. We conclude that the number of ABCD, ABCE, and ABCI subfamily proteins gene members has remained relatively unchanged throughout the plant kingdom. The striking difference between the number of ABCA subfamily transporter protein gene members between ferns and other plant taxa is surprising and merits further investigation.