An ATP-Binding Cassette Transporter GhWBC1 from Elongating Cotton Fibers

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Abstract
We have isolated a cDNA (GhWBC1) from cotton (Gossypium hirsutum) that encodes an ATP-binding cassette transporter of the WBC (white/brown complex) subfamily. Members of this subfamily are half-sized transporters and are reported to mediate lipid and drug excretion in human (Homo sapiens). GhWBC1 is highly expressed in developing fiber cells, but transcripts were also detectable in other tissues except roots. The transcript level peaked in rapidly expanding fibers from 5 to 9 DPA and then decreased. The GhWBC1 expression was weak in fiber cells of an li (ligon-lintless) mutant, which is defective in fiber cell elongation. These data indicate that GhWBC1 gene expression correlates with cotton fiber elongation. Transient expression of enhanced green fluorescence protein-GhWBC1 fusion protein in onion (Allium cepa) epidermal cells revealed plasma membrane localization. The GhWBC1 cDNA driven by a constitutive 35S promoter was introduced into Arabidopsis. About 13% of the transformants produced short siliques (SSs), whereas others had normal siliques (long siliques [LSs]). In siliques of SS lines, most embryos were severely shriveled, and only several seeds per silique could be found at maturity. The transgene expression level was higher in SS lines than in LS lines. Expression of AtWBC11, the closest homolog of GhWBC1 in Arabidopsis, was not altered in either SS or LS transgenic plants examined. These data suggest that GhWBC1 interferes with substance translocation that is required for Arabidopsis seed and silique development. Characterization of Arabidopsis WBC members, particularly AtWBC11, will help to dissect the role of GhWBC1 in cotton fiber development and elongation.